# A Boring Primer



## Aukai

How come this isn't a sticky yet


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## francist

Wow, guess how Mikey spent his Christmas vacation?

Great effort Mikey, and a worthy study guide. Guaranteed to help lots of new as well as not-so-new guys out.  Awesome job!

-frank


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## JimDawson

Aukai said:


> How come this isn't a sticky yet



It is now

Thank you @mikey, great job


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## ttabbal

That wasn't boring at all!! FAIL!  

Now I have more stuff to blame @mikey for! I could even use a nice boring bar.


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## thomas s

Mikey I like it thank you.


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## P. Waller

That is a lot of words just for boring operations.

A shorter version

DO use the largest boring bar that will fit in the hole yet allow the chips out, controlling chips is very important
DO keep the bar as short as possible
DO use tooling that has the least nose radius possible if forced to hang the boring bar far out of the holder
DO if deep boring use thru coolant bars if possible

DO NOT if deep boring baby the feed rate or DOC
DO NOT if deep boring use negative rake tooling unless it has a positive geometry
DO NOT if a blind bore allow the chips to accumulate at the bottom of the bore, this often ends in tears and broken tools, controlling chips is very important
DO NOT run the tool hard into the bottom of a blind bore, this will also end in tears and broken tools, this needs no further comments I presume

That about sums it up


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## tweinke

Thank You Mikey!


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## bill70j

Thank you, Mikey.  I have cut and pasted in Word so I can print and study (old school on paper).  Lots to learn!!  Bill


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## rwm

Excellent! I will study this.
Robert


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## mikey

Just got back from the dentist - man, I hate going to the dentist!

Thank you all for your kind words. Unfortunately, the forum software did not allow me to use the same format I wrote the paper in but at least you got the gist of it.

I really hopes this helps you guys. I know that there is a good chance that some projects are sitting on a shelf, waiting for the skills to catch up so the bore can be made. Now, maybe those projects will be done. 

I know this is a long discussion but boring is complicated, far more than any other lathe operation. And you can't just wing it if you need to come in on size accurately. I needed to tell you what, when, why, how and how much in order to make sense of the whole thing. That is not easy to do when there are hundreds of bars and tens of thousands of inserts out there!

Anyway, I hope this makes boring more fun for you. I would enjoy hearing of any experiences, corrections or criticisms. Nothing written is ever good enough or really ever finished, at least in the author's mind.


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## mikey

P. Waller said:


> That is a lot of words just for boring operations.
> 
> A shorter version
> 
> DO use the largest boring bar that will fit in the hole yet allow the chips out, controlling chips is very important
> DO keep the bar as short as possible
> DO use tooling that has the least nose radius possible if forced to hang the boring bar far out of the holder
> DO if deep boring use thru coolant bars if possible
> 
> DO NOT if deep boring baby the feed rate or DOC
> DO NOT if deep boring use negative rake tooling unless it has a positive geometry
> DO NOT if a blind bore allow the chips to accumulate at the bottom of the bore, this often ends in tears and broken tools, controlling chips is very important
> DO NOT run the tool hard into the bottom of a blind bore, this will also end in tears and broken tools, this needs no further comments I presume
> 
> That about sums it up



Yeah, you're right - lot of words but I don't have the luxury of standing in front of a class and speaking it, and I cannot show them how to do it so I have to use words. Its harder than it might seem to do that. This actually represents only 40% of the original document. I cut out everything that absolutely did not have to be included and its still a long discussion. Not sure how to get it shorter without sacrificing clarity.

Our new guys are starting from zero, with no training, no other guidance and no mentors to show us how it all really works. New guys sort of need more guidance to succeed; I'm sure you understand.


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## higgite

One item on my Christmas wish list was a Tormach TTS boring head arbor. My daughter gifted me one and proceeded to ask me what it’s for. After a brief explanation, she said boring is a good name for it… on multiple levels. I’m not sure what she meant by that. 

Great primer, Mike. At least, what I’ve read so far. It’ll take me days to absorb it all, but I will. Thanks for posting it.

Tom


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## Bob Korves

Excellent practical treatise, Mike.  THANK YOU!
-Bob


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## Rata222

Mikey
I really appreciate this write up - as well as the one on grinding HSS cutters.  They provide a wealth of information and insights that have helped me.   Thank you for sharing your knowledge  and the amount of work put into these "Primers".  This forum is a valuable resource that helps us home hobbyists (non-professional machinists) to gain knowledge that would otherwise be unavailable.
Jim


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## mikey

Thank you all for your very kind words, I really mean that. Proof is in the pudding, though. If it helps you bore a hole then I'll REALLY be happy!


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## Z2V

Mickey, great reading material, thank you. Glad that @JimDawson made it a sticky.


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## mikey

higgite said:


> One item on my Christmas wish list was a Tormach TTS boring head arbor. My daughter gifted me one and proceeded to ask me what it’s for. After a brief explanation, she said boring is a good name for it… on multiple levels. I’m not sure what she meant by that.
> 
> Great primer, Mike. At least, what I’ve read so far. It’ll take me days to absorb it all, but I will. Thanks for posting it.
> 
> Tom



Hey Tom, if you want to see her eyes go round and round, try to explain that Centripetal force is a real force and Centrifugal force is a sensation that the boring bar would feel if it could feel. She will hate you for weeks for making her sit through that!


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## Jubil

Thanks. I had no idea there was so much to learn about boring. 

Chuck


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## mikey

Rata222 said:


> Mikey
> I really appreciate this write up - as well as the one on grinding HSS cutters.  They provide a wealth of information and insights that have helped me.   Thank you for sharing your knowledge  and the amount of work put into these "Primers".  This forum is a valuable resource that helps us home hobbyists (non-professional machinists) to gain knowledge that would otherwise be unavailable.
> Jim



You most welcome, Jim. I find that knowing why something works and then learning how to control it makes the process so much more enjoyable. I'm just sharing what I've learned about that stuff and I hope it helps to make our hobby more enjoyable for you.


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## mikey

Jubil said:


> Thanks. I had no idea there was so much to learn about boring.
> 
> Chuck



Chuck, there is far more to boring than I put in that article. 

Any idiot can bore a hole in a model canon. To bore a hole dead on size or bring it in within a few tenths, now that takes skill and skill requires knowledge. All the experienced guys here will tell you that boring accurately isn't easy, and it is even more not easy if you don't understand what you're doing.


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## Aaron_W

Thank you for taking the time to put that all together. I've read a little and have watched some youtube videos, but internal turning whether threads or boring is still really hazy for me. I've been watching for the set of Borite bars you recommended, but currently just have a cheap set of Chinese Carbide bars. I have tried boring one time, and it was a success but scared the heck out of me and while it did come out ok, I don't think I was doing it right based on the noises it made.

I still need to bore the cylinder for my little steam engine so this will be quite handy.


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## Aaron_W

bill70j said:


> Thank you, Mikey.  I have cut and pasted in Word so I can print and study (old school on paper).  Lots to learn!!  Bill




I'm with you Bill. For something like this it helps a lot if I can print it out and follow along while I'm at the machine.


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## mikey

Aaron_W said:


> I'm with you Bill. For something like this it helps a lot if I can print it out and follow along while I'm at the machine.



Aaron, you should consider using carbide shanked inserted tip bars. Have a read and see if that interests you; for lathe boring, they are a better choice. For use in a boring head, Borite cobalt bars work a treat; they are what I prefer and use.

Once you get the hang of this boring thing, it isn't all that difficult to do. Most of the issues are because we don't understand the tools and how they are used. I hope this helps you.


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## mikey

As I mentioned earlier, the forum software did not allow me to format my post as intended. Therefor, I am attaching the original article as written in pdf form here. I hope this helps to make it easier to read.


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## Bob Korves

mikey said:


> As I mentioned earlier, the forum software did not allow me to format my post as intended. Therefor, I am attaching the original article as written in pdf form here. I hope this helps to make it easier to read.


Downloaded.  Nice to have my own archive copy...  Thanks again, Mike!


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## HBilly1022

Thank you Mike for taking the time and I'm sure it took a lot, to write this article and thanks for sharing your knowledge so freely. I too have downloaded the files and read a good part of them so far. I'm sure it will take a few reads for me to start to grasp it. 

I struggle with boring. Sometimes it goes well but most often it doesn't. This should help to get me on the right track.

Downside to this article is .......... it's going to cost me more money  because I now _have_ to get some proper boring bars.


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## Bob Korves

The only carbide boring bar I have was found in a tool cart that was gifted to me, buried in a pile of chips in the back of one of the drawers.
Looks like this, a bit shorter OAL:


That photo is close to full real size on my ~16" diagonal computer screen
https://www.travers.com/kyocera-mbe...o-boring-bars-150mm-mbe-0591394/p/20-518-067/
Solid carbide, no markings on it
1.5mm (~.060") minimum bore size
6.0mm (~1/4") maximum bore depth
3.0mm (~1/8") diameter shank
28mm (~1.2") overall length
Over $30.00  (!)
I have it Scotch taped to a piece of paper (so I don't lose it) and put in my boring bar drawer as a conversation piece.  It is highly unlikely I will ever even think about using it, and it would probably get broken or lost before I got it set up.

I have been looking for ordinary sized solid carbide bars at fire sale prices, no luck yet...


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## mikey

HBilly1022 said:


> Thank you Mike for taking the time and I'm sure it took a lot, to write this article and thanks for sharing your knowledge so freely. I too have downloaded the files and read a good part of them so far. I'm sure it will take a few reads for me to start to grasp it.
> 
> I struggle with boring. Sometimes it goes well but most often it doesn't. This should help to get me on the right track.
> 
> Downside to this article is .......... it's going to cost me more money  because I now _have_ to get some proper boring bars.



HB, we all struggle with boring if we're honest about it. Unless you really understand the stuff in the article, you are going to have issues. I think of boring as sort of a rite of passage for a hobby guy and until you can bore an accurate hole with control, you aren't quite up there in your own mind, you know? This should help you get over that hump, and it is a very important hump because being able to bore accurately will open up all sorts of possibilities for you. 

Yup, it's going to cost you to buy good boring tools but if you need to bore accurately then you need good tools. Please don't waste money on junk. Think hard on what you need, figure out which tools will give you that capability and wait for it to come up on ebay. My suggestion to you is to get one bar and learn how to use it while you wait for the other bars you need to come up.


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## mikey

Bob Korves said:


> I have been looking for ordinary sized solid carbide bars at fire sale prices, no luck yet...



Bob, may I suggest Micro 100 solid carbide boring bars? They are truly excellent if you need small boring bars, thread relief bars or internal threading bars. They come up on ebay at excellent prices if you are patient. Look for lots; they are cheaper that way. Money well spent because they will last for many years and can be resharpened easily.


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## Bob Korves

Thanks, Mike.  You already recommended them many times, and I put them to memory.  I am also looking for bigger bars, up to 3/4", though they get pretty pricey...  I am not in any hurry, if I get in a jam for one, I will borrow or buy one.


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## Rata222

mikey said:


> If I were going to set up my hobby shop today and had to choose a good combination of bars for general use, I would choose these three Circle Machine bars:
> 
> ¼”: *CCBI-250-4-5R*
> 3/8”: * QCMI-375-6-5R*
> 1/2*": QCMI-500-8-5R *



Searching Ebay for "CCBI-250-4-5R".  There are 9 used Circle Machine boring bars that were just recently listed listed $50 - $60.   Verify for yourself- but they look to be the 1/4" bar that Mikey is recommending.


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## mikey

Nice find, Jim. The bar you want is 4" long, RH and undamaged. I cannot tell the condition of the bars from the pics so might be a good idea to contact the seller before buying them. These things sold for about $250 new, without inserts. 

These bars take flat topped inserts with a positive axial rake. The CDCD500 insert has a 0.002" NR, while the CDCD505 has a 0.007"NR. I would avoid the CDCD51 insert with a 0.015"NR. Here is the grade info:




Good luck!


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## WallyEmm

mikey said:


> These bars take flat topped inserts with a positive axial rake. The CDCD500 insert has a 0.002" NR, while the CDCD505 has a 0.007"NR. I would avoid the CDCD51 insert with a 0.015"NR.



I haven't deciphered insert coding yet - are these bars limited to CDCD inserts or are other styles usable?

Thanks,
Wally


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## ddickey

You must use C, 80° diamond, the first letter stands for the shape. and the correct size of course.


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## mikey

WallyEmm said:


> I haven't deciphered insert coding yet - are these bars limited to CDCD inserts or are other styles usable?
> 
> Thanks,
> Wally



They take CDCD (flat top of varied grades/coatings) but they also accept CDCG with a ground in chip breaker or CDCT with a pressed in chip breaker. I have tried both chip breaker inserts and do not care for them myself but others may have different experiences with them. Widia and Kennametal also make CDXX inserts for these bars but you will have to cross reference them. You cannot use inserts of any other configuration on these bars.

I normally will give an opinion on an insert only if I've tried them personally. I have tried all three of these inserts and can tell you that the CDCD insert works really well, at least for me. It is, by far, the most accurate insert of the three and lasts the longest in my shop. If I had to choose which chip breaker insert to buy, I would go with the CDCG. There is a tiny land between the cutting edge and the ground groove and that thing cuts well. In harder materials, it does not last as long as a flat insert but while it lasts, it cuts pretty good.

The other thing with the chip breaker inserts is that they are only available with a 0.007" or 0.015" nose radius. These are okay for general work but for fine precision boring, I prefer a 0.002" NR so one more strike against these chip breaker inserts for me.


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## ddickey

Mikey,
So you won't reverse your bar without moving the tool towards centerline? This is hard on the insert? Do you take another spring pass then, or?


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## mikey

ddickey said:


> Mikey,
> So you won't reverse your bar without moving the tool towards centerline? This is hard on the insert? Do you take another spring pass then, or?



No, I don't reverse my tool while it is in contact with the work. It dulls the insert prematurely and typically will cut on the way out so now I don't know what the bore ID is. Try it and see.

Measure the bore, dial in a cut and make a pass, then move the cutter away from the wall and measure it to see what the change in ID is. Repeat this with the same cut and measure it; the change in ID should be the same. Now make a pass but leave the cutter in contact as you reverse. The change in ID will not be the same and you will have to make a clean up pass to get rid of irregularities in the bore because the cut on the way out is inconsistent.

Two things that really mess up accuracy: Taking too deep a cut without keeping up with the feed and leaving the cutter in contact with the bore on the way out. Both leave irregularities in the bore so that it is no longer a consistent ID all the way down. The common advice to deal with this is to take a "spring pass" to clean it all up. Since you are the cause, it is better to clean up your technique than clean up the bore.

I haven't taken a spring pass in over 20 years so I dunno', this works for me.

EDIT: A cutter is meant to cut in only one direction. Dragging it out in reverse dulls the edge. This is true for any single point cutting tool and most edged tools in general. I don't drag my lathe turning tools in reverse, either, be they carbide or HSS. A boring bar cuts clean and consistently in one direction only ... on the way in.


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## mikey

Duane, I just re-read my response above and it sounds really dogmatic to me. I apologize, I didn't mean it that way. A little back story might explain my attitude.

Back when I was a baby hobby guy (yeah, I kept a journal way back then so I have notes to refer to) and was trying to sort out this boring thing I did drag my bar out with the tip still in contact with the bore. I didn't know any better but I did notice several things. One, the cutter showed a lot of flank wear prematurely, which is a sign that the cutter is dulling. Two, the bore was really messed up and inaccurate; I had tight and loose spots in the bore. Three, the finish was messed up.  Four, I broke more inserts than I thought was normal; turns out this is because I was dragging it out of the bore.

Naturally, instead of trying to figure it out first, I looked for advice. I was told to take spring passes to clean up the bore, and this came from a pro machinist and several "advanced hobbyists". I began taking spring passes and the bores were even more inconsistent. I didn't know it at the time but this is because a spring pass is taken without any more depth of cut. Since we know that radial forces are very high when we take super-light passes, the tool was deflecting and not cutting so the inconsistencies in the bore were not surprising; I just didn't know why this was at the time.

It bugged me that this was happening and the advice I got from several supposedly reliable sources was not working so I began to experiment. The results were as I alluded to in my previous post. By cutting only on the way in I no longer had premature flank wear, I no longer broke inserts, the tool actually cut consistently on the way in and the finish greatly improved. Most importantly, the bore ID was consistent.

It wasn't until I learned about radial cutting forces that their effect on a spring pass made sense to me but now it is really clear. A spring pass will cut but you will notice that it will cut inconsistently. The idea is that it is cutting off the high spots, right? What is actually happening is that the cutter is skating over the leveler spots due to high radial forces and digging into the high spots; that digging in is not to a consistent depth because you're essentially ramming the cutter into that high spot. The result is that the tip of the tool is oscillating as it proceeds down the bore. The result of all of this is that the bore no longer has a consistent ID; some spots will be tight, some loose, some in the middle. If you're boring a model canon, this may not matter. If you're shooting for a precision bore then this matters a lot.

This is one of those things that you don't see in an article. A single sentence or two can represent months of confusion trying to sort out why something is happening. When I suggest to you that you should cut only on the way in, it is because I now understand what the geometry is doing to the cutting forces in the bore and have confirmed it in numerous trials, at least to my personal satisfaction.

So, spring passes? No, I don't do spring passes. I find a depth of cut and feed rate that works and make repeated passes until I get to where I can dial in a finish pass or two. Every roughing pass produces a consistent cut and change in ID so I can predict the result. The bore remains round, straight and has clean finishes so that my finish pass can also be consistent.

When I said to find a depth of cut and feed rate that works and then stick with it, I really mean it is the fastest way to bore on a lathe.


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## middle.road

Wow @*mikey, *that is one very nice write up! Perhaps now I might get the hang of boring...


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## WallyEmm

Thanks for the detailed replies, I appreciate the background information.  

Wally


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## mikey

middle.road said:


> Wow @*mikey, *that is one very nice write up! Perhaps now I might get the hang of boring...



Thank you! I really hope it helps you bore better.


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## mikey

WallyEmm said:


> Thanks for the detailed replies, I appreciate the background information.
> 
> Wally



You're welcome, Wally. That detail is what I cut out of the article to make it shorter. If I didn't do that, you'd still be reading it next week!


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## ddickey

Thanks for the response Mikey.
I was really just curious.
Great article by the way.


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## mikey

ddickey said:


> Thanks for the response Mikey.
> I was really just curious.
> Great article by the way.



You're welcome, Sir, and thanks for the compliment. Now you know what a bumbling idiot I was at the start. Seems to be a habit of mine to be stupid from the get go. At least I got there in the end, though.


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## Titanium Knurler

Great post Mikey!  Just did a whole bunch of boring putting a headstock handwheel on my PM 1236-T. I did a bunch wrong but fortunately it turned OK.  I am also going to print your Primer and re-read it; great reference! 
Thanks again, Mike


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## mikey

Well, Howdy there, stranger! Glad to see you're still with us. Man, that is one beautiful lathe you have there. 

Lemme know if you have questions, okay?


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## Titanium Knurler

Will do and thanks again for a terrific post!


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## mikey

Titanium Knurler said:


> Will do and thanks again for a terrific post!



I was running out the door just after I responded. I meant to say that it looks like your handwheel project turned out really nice. You didn't say what the purpose of it is, though. Are you using it to thread stuff under manual power? If so, good idea. I have to make one for my lathe, too.


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## Titanium Knurler

Mikey, I don’t want to clutter-up your wonderful thread with my Boring project.  I posted the images to illustrate that I recently spent hours boring, did a whole bunch of things incorrectly according to your treatise but thankfully ended up with a nice result. It was my first effort at boring and had to get the two pieces, the aluminum handwheel and stainless adapter collar, to within .004” so that I could shrink fit the two together. TIR of adapter collar when on the lathe turned out to be .0001”.  How the hell that happened, I don’t know; I guess every dog has it’s day.

I wanted a handwheel because I always seemed to be grabbing the chuck for one reason or another: threading, getting the gears to mesh, moving the 4JC chuck back and forth while adjusting for runout.  The 10” wheel gives me plenty
	

		
			
		

		
	





	

		
			
		

		
	
 of leverage and makes it much easier to turn the chuck manually; I use it all the time.  

I am planning on a series of posts(see attached photo) about setting up my PM1236-T soon. I will have info on the handwheel project there.

Thanks again for your wonderful post on Boring!

Mike


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## Titanium Knurler

Damn, hopefully I will get posting an image correctly soon. Hit the “full image” button several times because it didn’t seem to do anything. Sorry about that.


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## Hobby-Machinist

Mikey, this is a WONDERFUL primer!
Thanks so much for posting it.


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## higgite

This is now a sticky! Thank you, H-M! 

Tom


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## mikey

Hobby-Machinist said:


> Mikey, this is a WONDERFUL primer!
> Thanks so much for posting it.



You're most welcome. Now we have to wait and see if it helps.


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## mikey

Titanium Knurler said:


> Mikey, I don’t want to clutter-up your wonderful thread with my Boring project.  I posted the images to illustrate that I recently spent hours boring, *did a whole bunch of things incorrectly according to your treatise* but thankfully ended up with a nice result. It was my first effort at boring and had to get the two pieces, the aluminum handwheel and stainless adapter collar, to within .004” so that I could shrink fit the two together. TIR of adapter collar when on the lathe turned out to be .0001”.  How the hell that happened, I don’t know; I guess every dog has it’s day.
> 
> I wanted a handwheel because I always seemed to be grabbing the chuck for one reason or another: threading, getting the gears to mesh, moving the 4JC chuck back and forth while adjusting for runout.  The 10” wheel gives me plenty of leverage and makes it much easier to turn the chuck manually; I use it all the  time.
> 
> View attachment 284665
> 
> 
> I am planning on a series of posts(see attached photo) about setting up my PM1236-T soon. I will have info on the handwheel project there.
> 
> Thanks again for your wonderful post on Boring!
> 
> Mike



Mike, you may not have done anything wrong, especially if you got the result you needed. As I said in the article, this is my way, not the way. If what you are doing works for you then go with it.

I am looking forward to seeing your posts about your lathe and to see what you've been up to.


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## mcdanlj

Thanks @mikey! Soon after I first read this thread, I was boring brass with a cheap chinese boring bit and was getting a bad finish; I was able to see what was wrong and fix it from your explanations. I ended up with a smooth clean bore, precisely sized. Then further reading helped me when I did some boring operations for a new Z crank for my mill. Today I used a Micro 100 boring bar for the first time. I see what the fuss is about... ☺


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## mikey

mcdanlj said:


> Thanks @mikey! Soon after I first read this thread, I was boring brass with a cheap chinese boring bit and was getting a bad finish; I was able to see what was wrong and fix it from your explanations. I ended up with a smooth clean bore, precisely sized. Then further reading helped me when I did some boring operations for a new Z crank for my mill. Today I used a Micro 100 boring bar for the first time. I see what the fuss is about... ☺



It isn't until we use a good tool the right way that we realize how simple things really can be. Micro 100 makes some of the finest small boring tools on the market. Circle Machine made small solid carbide tools of similar quality. They are not known for their solid carbide tools in the hobby machining circles so they can be cheaper on eBay vs Micro 100. 

Happy things are working out for you.


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## mcdanlj

The tool I used was Micro 100 BB-1801500 — 0.18" by 1.5" — amazon prime caught me in its clutches for under $30 delivered in two days, so could be worse. A 0.18" from Circle Machine seems to be max 1" and cost more and be out of stock online new and I'd be lost trying to sort through the options on ebay.

I expect most of my boring to be larger and I have multiple insert bars, and thanks to you I know more about how to mount them on my QCTP for best results. Sleeves to insert into my holders are now on my list to make!


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## Z2V

@mcdanlj, if you do a search on Amazon for Micro 100 and browse through all the listings ( there’s pages of them) you can often find some crazy low prices for these boring bars and threading tools, like 75% or so off. I have accumulated a good variety of different sizes over the last year or so from Amazon. You just have to look periodically, the deals are out there.

Glad to hear that you are getting the hang of how cutting tools work and how to make them work for you in the way that you need them to.


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## tjb

What a great article!  Okay, this is now a 'sticky'.  Pardon my ignorance, but how do I access a sticky?

Regards,
Terry


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## mcdanlj

"Sticky" just means that if you go to the parent forum, in this case A BEGINNER'S FORUM (Learn How To Machine Here!), you will see this post at the top, in a section that doesn't age out as people make new posts. It is done particularly for posts with high long-term value for new visitors to the forum, but also for anything that makes a good reference that the community wants to be able to find more easily.


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## tjb

mcdanlj said:


> "Sticky" just means that if you go to the parent forum, in this case A BEGINNER'S FORUM (Learn How To Machine Here!), you will see this post at the top, in a section that doesn't age out as people make new posts. It is done particularly for posts with high long-term value for new visitors to the forum, but also for anything that makes a good reference that the community wants to be able to find more easily.


Thanks for the education.
Regards,
Terry


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## mikey

Thank you. The sticky threads are on the lead page of the Beginner's Forum.


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## docRoot

Thank you Mikey!
I am the rankest of beginners in metal and haven't turned on my new to me lathe yet (it has an out-of-true nose).  I want to learn this machine and this discipline by fixing it and making as much of the tooling myself as possible, so a primer on not just boring but with a clear explanation of cutting in general is perfect.  I write instructions and make tutorials myself and know how much time, energy, experience, and angst goes into them, and this one is exceptional!


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## mikey

Well, thank you, doc! What a nice way to start off my day.

Yup, writing something like this is hard to do and only a fellow writer would really understand that. On the other hand, it enables one guy to reach many and in the case of the HM family, these guys are worth the effort.

Welcome to HM. If you need help with your lathe or anything else, start a thread and the guys will bend over backwards to help you.

Mike


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## mikey

I was discussing boring speeds with @gun410 in his thread here and I know this is an area I left un-discussed in this Primer. I left it out because the passage where I discussed it in my draft was too detailed and confusing so I dumped it. However, it might be useful to discuss it here.

Boring is not like external turning. It differs in many ways that, at least in my opinion, make it an entirely different operation. One important difference is how we use our speeds and feeds and I wanted to discuss it here and perhaps you guys can either correct, modify or contribute your methods so we can all learn.

I'm sure most of us know how to calculate RPM but for the benefit of our newer guys, I'll go over it again. Every material has a recommended cutting speed; these speeds are found all over the net in tables and these cutting speeds are listed in Surface Feet per Minute (SFM). This varies with the cutter being used, and typically the cutting speed will roughly double to quadruple when using carbide vs HSS. Please note that this cutting speed is not the RPM you run the lathe at. To calculate your RPM, a convenient formula is: *RPM = CS X 3.82 / D*, where CS is the cutting speed of your material listed in the table for the type of cutter you're using in SFM; 3.82 is a constant, and D is typically the Diameter of the cutter or work piece. 

For example, say we are turning 1.5" OD bar of 12L14. Using a HSS cutter, it has a cutting speed of 200 sfm; this quadruples to about 800 sfm with carbide. So, if we use a carbide cutter then RPM = 800 X 3.82 / 1.5 = 2037 rpm. When external turning, we would set our lathe to this speed or as close to it as we can get and go for it. Feeds will vary based on which operation and gear set you have available; you feed faster for roughing and slower for finishing. 

However, when we bore, our cutter is not seeing that 1.5" OD. It is seeing the starter hole, or ID. Say we only have a 3/8" boring bar, the largest drill we have is 1/2" and we need to bore the hole out to 1.0000" ID. Our starting calculated speed would be: 800 X 3.82 / 0.5" = 6112 rpm. As the bore enlarges, speed requirements fall so that by the time we get to 1.000", our speed should be about half our starting speed. Get it?

So, is it necessary or practical to alter speeds as you bore? For softer materials, there is an upper limit that our lathes can run and for smaller work pieces we will be running at top speed anyway, right? However, when boring larger hole in harder materials, this speed thing can matter because of one really important thing - work hardening. Run high carbon steel, tool steel or stainles steel at too high a speed and fail to keep up with your feeds and that bore will get harder than a ... well, it will get really hard on the surface. Subsequent attempts to take fine finishing cuts will result in failure, I assure you. The point is that your speed does matter and you should pay attention to it when boring materials that are known to work harden. 

Related to our speeds is our feeds, and our feeds are related to our nose radius. Recall that a good roughing feed rate is about 1/2 NR, while about .25 NR works for finishing (both in Inches Per Minute, or IPM). I already know the feed rate that works well for every one of my bars when used with all the materials I normally work with, and I know it because I've done practice bores in those materials and wrote the results down. Its not that I'm all that anal. Well, okay, maybe a little bit but mostly its because when I have a project to bore, the time to find out this kind of important information is not when I have the project ready to go; it's before.

So, what happens if you have to bore something that you never bored before? What feed rate do you use? Well, for me, I calculate my speed and then feed manually to get an idea of what my feed needs to be. I've bored enough holes that I can estimate my gearing needs pretty quickly this way. If the bore is shallow, maybe under 2" deep, I will just feed manually to get the coiled chips I want and ignore power feed. 

Is your depth of cut related to feeds and speeds in any way? Yup, it sort of is. If you take a honking deep cut of 0.040" in aluminum with a 0.008" NR cutter then you might have trouble with accuracy if you try to rough at 0.020 IPM. The cutter may or may not chatter but the bar may deflect more than you would like. This is why I suggest restraint in taking really big roughing cuts unless you can use a reasonable feed rate to keep up with that cut. If you cannot keep up, then bar deflection can muck up the bore and you'll have to clean it up. Best to keep your ego under control and leave the huge depths of cut to the YouTube guys. We are after dead on accuracy in the bore. It may take a little longer to get there but you will hit it.

The last thing I wanted to emphasize is that when boring, calculations are fine but it is important to understand that they don't take your boring bar, lathe, tool post or experience into account. If you use a little Sherline lathe, it is nowhere near as rigid as a PM1340GT, and a carbide bar in both lathes will have totally different results if we use the same numbers. So, sure, use numbers but it is really, really, really important to learn to adjust to the cut. If your numbers work, fine, use them. If they don't work then change them until they do. Slow down the speed, increase the feed or do whatever it takes to respond to that cut. Boring is complicated because we have to adjust to what the lathe and bar demands; once we figure out what works for our set up, boring becomes easy.

Okay, that's my contribution. I welcome any discussion or criticism you may have.


----------



## ttabbal

I picked up a QCBI bar a while back and thought I'd try it out along with a Chinesium steel bar. The usual sort with CCMT inserts. 

I was just practicing on a bit of 6061 aluminum. The bore started at about 1/2". Steel bar was 10mm, carbide 3/8. Stick out was a bit high, about 1.5".

I was surprised to see that the steel bar was deflecting with light cuts of 0.005". Surface finish wasn't great as a result, and of course size was off. Deeper cuts and different feed rates didn't improve things. 

Switching to the Circle bar was night and day. I was able to cut what I dialed in, no deflection, and finish was very good. I even had it sticking out further. The difference was eye opening to say the least. Roughing and finish cuts were very good and I didn't notice any issues. I didn't push it hard, but was quite impressed. 

The Circle insert showed no visible difference, the other insert seemed to lose some of the coating, but the shape and size was the same and I didn't feel any sign of chipping, so I'm not sure what to make of it. Both were new inserts.


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## mikey

ttabbal said:


> I picked up a QCBI bar a while back and thought I'd try it out along with a Chinesium steel bar. The usual sort with CCMT inserts.
> 
> I was just practicing on a bit of 6061 aluminum. The bore started at about 1/2". Steel bar was 10mm, carbide 3/8. Stick out was a bit high, about 1.5".
> 
> I was surprised to see that the steel bar was deflecting with light cuts of 0.005". Surface finish wasn't great as a result, and of course size was off. Deeper cuts and different feed rates didn't improve things.
> 
> Switching to the Circle bar was night and day. I was able to cut what I dialed in, no deflection, and finish was very good. I even had it sticking out further. The difference was eye opening to say the least. Roughing and finish cuts were very good and I didn't notice any issues. I didn't push it hard, but was quite impressed.
> 
> The Circle insert showed no visible difference, the other insert seemed to lose some of the coating, but the shape and size was the same and I didn't feel any sign of chipping, so I'm not sure what to make of it. Both were new inserts.



I have QCMI bars, which are the same as your QCBI except that they accept grooving and threading inserts as well as boring inserts. My 3/8" bar with a 0.007"NR will routinely take 0.020" deep roughing passes without even slowing down the lathe, and this is on a Sherline lathe; my Emco lathe will go much deeper. And this bar will do this at max extension, about 3.75" sticking out. And it will hold its accuracy at that depth.

Those cheap steel SCLCR boring bar sets that use CCMT/CCGT inserts will work but they cannot approach the quality and reliability of a good carbide bar. I am saying this not to disparage these cheaper bars; I am saying this from experience with them and with good bars - there is just no comparison. The mini-lathe guy I wrote this boring article for actually owned some cheap bars and was really having trouble boring accurately. In order for me to explain what was happening in the bore and then explain how a good carbide bar worked, I had to write a whole article that was 60% longer than the one you see here. 

Boring is not hard to do. Heck, guys have been boring for a hundred years without understanding what a cutting force is, and thousands of hobby guys bore holes all the time with only experience to guide them and they do fine. What is difficult is to bore a precision hole with control, and hit it the first time. That is hard. That takes knowledge and a good bar.


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## mikey

I just re-read my post above and it occurred to me that the Primer suggests that you rough with the NR + 0.005-0.010" deep cuts. This is not true. You can rough at whatever depth of cut you prefer provided it is deeper than the NR. However, when doing so your feed must be able to keep up with the speed you set or you will have all sorts of defects in the bore. 

In soft materials, like aluminum, you can take 0.020 - 0.040" deep cuts if the bore is within the extension range of the bar and your lathe is rigid enough to handle the cutting loads this imparts to the lathe. This is assuming you have a good bar holder because a junk one will not tolerate heavy depths of cut; it will contribute to those bore defects significantly. I take heavy cuts like this all the time in softer materials but I focus on getting my feed right by shooting for coiled chips when doing so. This correct feed thing assures me that my bore diameter remains consistent and the finish doesn't suffer. In harder materials, I prefer to lower my depths of cut because I need to avoid generating a lot of heat to avoid work hardening.

Just like with turning tools, how deep a cut a tool can take is not nearly as important as how *small* a cut that bar can take because that determines how accurate the bar is. In a hobby shop, taking big cuts to rough the bore quickly is less important than it is in a job shop. If my cutter cannot bring me in on size precisely then it doesn't matter how fast I miss the bore; I still miss the bore. So, my suggestion is to leave the ego-boosting testosterone depths of cut to the YouTube guys and focus on using roughing depths of cut that you can keep up with so that your bore diameter remains consistent. Then hone your skill at coming in precisely on size, with control. As with all cutting operations, it is the small end of things that really matters.


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## theKbStockpiler

mikey said:


> *Radial forces* can be accommodated by assessing the change in inside diameter (ID) a given depth of cut (DOC) produces. For example, if you dial in a 0.005” DOC and find it produces only 0.0095” change in ID then the impact of Radial forces on your setup is that 0.0005”; adjust your DOC to get the change in ID you need. You may need to dial in a 0.0055” DOC to get a 0.010” change in ID. Doing this to determine a setting for finish cuts is very useful in planning your approach to final size. I know of no other practical way to directly offset the effect of Radial forces but this actually does work rather well.


I'm new to this but does  this make sense?  ' if you dial in a 0.005” DOC and find it produces only 0.0095” ' ,it's supposed to be a .001 cut and the radial force consumes .0005 so the net cut is actually .00095. correct?


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## mikey

Correct. You have to adjust as you go. Over time, you learn what your cutter wants and then boring gets easier. I have found that the bigger the nose radius, the greater the impact radial forces have so I try to stay with a 0.007" nose radius or smaller.


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## mikey

Just a heads up for the guys who have read this post. There is a used Circle Machine 3/8" solid carbide QCMI bar on ebay for a good price. It is used and looks to be in okay shape but I cannot vouch for its actual condition. These bars are expensive but this one is listed for a fair price.

There is also a brand new bar of the same make and model plus 5 inserts on ebay. I haven't seen a brand new offering of this bar for 3 or 4 years now so they are becoming more rare. If you need a bar that can bore, groove and thread to almost 4" deep, this is a very good one. I personally own and use this bar a lot. It is one of the best carbide bars I've used and if you need a bar of this size, I can recommend it. 

Just know that you also need to find the inserts. They come up on ebay but you'll have to be patient. If one of you buys one of these bars, let me know and I can tell you which inserts to look for.


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## Rata222

Mikey
I have printed and  read ( several times) this post, your post on grinding lathe tools and  many others.    I really appreciate the detailed information you give to help us understand -"the physics " of whats going on.  It is obvious that a tremendous amount of work goes in writing them.
Just wanted to say an overdue  Thank you.  You have helped me a lot to improve - and therefore enjoy more- this hobby.

And thank you to all the excellent  contributors to the group  - I have learned a lot from this forum.
Jim


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## mikey

Rata222 said:


> Mikey
> I have printed and  read ( several times) this post, your post on grinding lathe tools and  many others.    I really appreciate the detailed information you give to help us understand -"the physics " of whats going on.  It is obvious that a tremendous amount of work goes in writing them.
> Just wanted to say an overdue  Thank you.  You have helped me a lot to improve - and therefore enjoy more- this hobby.
> 
> And thank you to all the excellent  contributors to the group  - I have learned a lot from this forum.
> Jim



Thank you, Jim. I appreciate your comments and I really hope whatever I write does help you. 

I lost my best friend not that long ago and I think about him every day. I realize that I do not know how long I have before I join him so I've been trying to resurrect and clean up some articles I wrote some time ago so I can get the information out there. That's the motivation for these articles. 

So thank you, Jim, for making my day!


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## thomas s

Mikey sorry for your loss.


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## mikey

He went about 5 years ago Tom, so not that recent. It just feels that way.


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## ttabbal

Well Mike, if I can somehow get good enough to contribute half of what you have to the community, I'll be thrilled. Great stuff man. I'm still learning from this post in particular. I can almost bore a decent hole now!


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## mikey

Thank you, Travis! This is one of those skills that really opens up possibilities for us, and I hope it helps you to continue to grow. I appreciate your kind words, Sir!


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## GunsOfNavarone

The Bubba analogy... super helpful! I must be stupid, but whatever works!


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## mikey

GunsOfNavarone said:


> The Bubba analogy... super helpful! I must be stupid, but whatever works!



You would be surprised how many people were helped by that analogy so not stupid. Cutting forces are sort of nebulous because you cannot see them but they are there and affect every single cut we take. The more we know about them and how to counter them, the better our results. It took a long time for me to sort out how these forces work but it helps me to understand how the different machining processes work; hope it helps you, too.


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## Janderso

I read through this boring treatise last night and some this morning.
Mikey, I just learned more in the last few hours about the process of boring than all the practice, video watching and reading put together.
Very well done sir.
I will refer back to it often. I like the Bubba Examples.
I never considered adjusting above center height to allow for deflection. I don’t think I have read about it either.
A machinist would learn these things from a journeymen or in a class.
I’m glad you mentioned the Micro 100 solid carbide tools, I have had very good results with them.
Us hobby guys learn by trial And error.
Thank you very much for your time and effort putting this together.


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## Janderso

GunsOfNavarone said:


> The Bubba analogy... super helpful! I must be stupid, but whatever works!


What's that saying, "keep it simple stupid"


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## mikey

Janderso said:


> I read through this boring treatise last night and some this morning.
> Mikey, I just learned more in the last few hours about the process of boring than all the practice, video watching and reading put together.
> Very well done sir.
> I will refer back to it often. I like the Bubba Examples.
> I never considered adjusting above center height to allow for deflection. I don’t think I have read about it either.
> A machinist would learn these things from a journeymen or in a class.
> I’m glad you mentioned the Micro 100 solid carbide tools, I have had very good results with them.
> Us hobby guys learn by trial And error.
> Thank you very much for your time and effort putting this together.



You're welcome, Jeff, and I hope it helps to make boring more fun for you. 

Don't forget that Bubba is critical to boring. If you don't own a boring tool holder like the 4D then buy or make one. Of all the revelations I had when learning about boring, the importance of that holder was number 1. 

I hope now you can see why those cutting forces matter. It is totally counter-intuitive to know that radial forces get larger at smaller depths of cut. You would think they would rise with deeper cuts but it ain't so. Tangential forces rise with increasing depths of cut, though, so at least that makes sense. But then bar diameter and composition take on more importance, too, so as I said, there are nuances upon nuances when boring. I hope the article makes it all a little less confusing for you.


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## Executioner

Thanks. Well worth the read.


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## Chris John

I know what I need to do in my free time now thanks for sharing...


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## snorky

As a newbie to machining -thanks Mikey for thinking of us and making it easier to understand.


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## mikey

Yup, you're welcome.


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## rock_breaker

Excellent presentation, as others have done this will be printed  and  put my 3 ring note book. I am also going to try to summarize for reference to be kept over my lathe. 
Thank you! 
Have a good day
Ray


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## mikey

Thanks, Ray. Hope it comes in handy.


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## AmericanMachinist

Would a boring bar be the preferred tool to bore an ID with a keyway cut in it?

I have a 0.875" bore that I need to enlarge to 1.100".   There is a keyway approx 3/16" x 3/16".  

I started to bore it (and get past the keyway) with a Horrible Freight 15/16" bit, and the bit flexed noticeably when it hit the keyway.  It also ended up spinning the MT spindle in the tailstock due to the forces perpendicular to the bed axis.

I have a Micro100 boring bar but it's a pretty long and narrow model - around 1.5" cut with a 1/4" shank.  I don't want to risk breaking it.  

I'm not sure how much it would help (if necessary...) but I could put the part on the mill, bore aligned with the spindle, and use a 3/4" EM to create a smooth and oblong cut extending outside of the existing 7/8" bore, where the square keyway is now.

Any suggestions?

Thanks!


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## mikey

Wait, I'm confused. You said you tried to "bore" a 0.875" ID hole with a 15/16" bit. Did you mean you tried to drill it with a 15/16" drill bit? 

That aside, a boring bar should have no real issues boring a hole with a keyway in it. That is just an interrupted cut that most bars will take in stride. 

You also do not need a huge diameter bar provided the bore depth does not exceed the theoretical depth capacity of the bar material. To remind you, steel bars can go to a depth of about 4 times the diameter of the bar, while carbide can go 8-10 times the bar diameter. So, how deep is the bore?


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## AmericanMachinist

Thanks Mikey!   Yes, I was trying to "drill" the bore first to get it close to size, before applying the boring bar.

I'll get out the boring bar one night this week and give it a shot.  It's a narrow shank bar, so I was nervous about breaking it, but will take light cuts.

Would it be plenty safe to take ~ 0.010" off the radius, at 420 RPM = 96 SFM at the current bore size and 120 SFM at the ending bore size.

Or would you recommend speeding up to "carbide" speed - very fast, since it's a solid carbide bar?   I'm just nervous about hitting that keyway wall and wrecking my $55 tool!  

Edit:  I missed your question about bore depth.  It's ~0.750".   I was - maybe overly - concerned about the keyway more than the pure depth of the bore, given the tool has a 0.25" x 1.5" or so shank.


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## mikey

Okay, I still don't know what the bore depth is but that aside, let me give you what might seem to be some stupid advice. Don't risk your Micro 100 bar. Go and buy a cheap brazed carbide set like this one and practice first. Find some mild steel and drill a hole, then enlarge the hole with these cheap bars until you figure out how boring works. You need to know what speed, feed and depth of cut the bar can handle on your lathe with you in control. I know this might not be what you expected but this is how I learned to bore - with this kind of set, on mild steel, for months until I figured out how to bore with them. As cheap and junk as these sets are, in my hands, I can use these bars to bore a hole with very good precision in almost any material.

The reason for suggesting this set is because brazed carbide will take an interrupted cut without issues. Once you learn to bore with these bars, boring your thing with the keyway in it (that may not be easily replaceable) will be a routine job for you. YES, inserted carbide bars are more versatile and I much prefer them but for a beginner hobby guy do not underestimate what these cheap brazed bars can do. Take your diamond stone and lightly hone the top edge so it is dead flat and that will sharpen it adequately. Then practice until you can control the bar and hit your size every time. When you can do that, then go and bore your project.


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## AmericanMachinist

mikey said:


> Okay, I still don't know what the bore depth is but that aside, let me give you what might seem to be some stupid advice. Don't risk your Micro 100 bar. Go and buy a cheap brazed carbide set like this one and practice first. Find some mild steel and drill a hole, then enlarge the hole with these cheap bars until you figure out how boring works. You need to know what speed, feed and depth of cut the bar can handle on your lathe with you in control. I know this might not be what you expected but this is how I learned to bore - with this kind of set, on mild steel, for months until I figured out how to bore with them. As cheap and junk as these sets are, in my hands, I can use these bars to bore a hole with very good precision in almost any material.
> 
> The reason for suggesting this set is because brazed carbide will take an interrupted cut without issues. Once you learn to bore with these bars, boring your thing with the keyway in it (that may not be easily replaceable) will be a routine job for you. YES, inserted carbide bars are more versatile and I much prefer them but for a beginner hobby guy do not underestimate what these cheap brazed bars can do. Take your diamond stone and lightly hone the top edge so it is dead flat and that will sharpen it adequately. Then practice until you can control the bar and hit your size every time. When you can do that, then go and bore your project.


Thanks! I initially missed your question on bore depth and edited my post to add it, likely while you were typing your reply.  0.750"


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## mikey

Okay, thanks for the bore depth. I still think that those brazed bars are a good way to learn to bore. If you keep them sharp they will cut almost as well as a HSS tool and that will allow you to get really accurate really fast, and that builds confidence. I run them fast, very close to turning speed and they will take a much heavier depth of cut than an insert will. 

If you rotate the tip of the bar up a few degrees from horizontal this will reduce tangential forces, which is good because the steel they use in these bars is of questionable quality and anything that reduces deflection is a good thing. If you also turn the tip of the bar a few degrees (like 5 degrees) towards the wall of the bore, this will reduce radial forces and enhance your accuracy. Increasing your speed reduces cutting forces as well. Taken together, these things will enable those cheap bars to cut much better than you might think.

In spite of what I've written in this primer, learning the basics of boring with solid bars is a good way to learn to bore.


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## rock_breaker

In what ever you do you will have an impact cut the length of the key way, boring thereafter. Having HSS boring bars only, I would use the largest boring bar possible (1/2"), slowest rotation speed and DOC variable, starting @ 0.010" perhaps getting up to 0.025", trying for the least impact (abuse) to the machine but yet still removing material for feed rate. Mikey has thoroughly covered the subject and in my book has the right answers.
Have a good day
Ray


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## AmericanMachinist

mikey said:


> Okay, I still don't know what the bore depth is but that aside, let me give you what might seem to be some stupid advice. Don't risk your Micro 100 bar. Go and buy a cheap brazed carbide set like this one and practice first. Find some mild steel and drill a hole, then enlarge the hole with these cheap bars until you figure out how boring works. You need to know what speed, feed and depth of cut the bar can handle on your lathe with you in control. I know this might not be what you expected but this is how I learned to bore - with this kind of set, on mild steel, for months until I figured out how to bore with them. As cheap and junk as these sets are, in my hands, I can use these bars to bore a hole with very good precision in almost any material.
> 
> The reason for suggesting this set is because brazed carbide will take an interrupted cut without issues. Once you learn to bore with these bars, boring your thing with the keyway in it (that may not be easily replaceable) will be a routine job for you. YES, inserted carbide bars are more versatile and I much prefer them but for a beginner hobby guy do not underestimate what these cheap brazed bars can do. Take your diamond stone and lightly hone the top edge so it is dead flat and that will sharpen it adequately. Then practice until you can control the bar and hit your size every time. When you can do that, then go and bore your project.


Mikey - would you advise against a bar similar to this Ultra-Dex as an alternative to the brazed set you linked?









						Ultra-Dex Usa Indexable Boring Bar, S12Q SCLCL3, 7 in L, High Speed Steel, 80 Degrees Diamond Insert Shape S12Q SCLCL3 | Zoro
					

Order Ultra-Dex Usa Indexable Boring Bar,  S12Q SCLCL3,  7 in L,  High Speed Steel,  80 Degrees  Diamond Insert Shape, S12Q SCLCL3 at Zoro.com. Great prices & free shipping on orders over $50 when you sign in or sign up for an account.




					www.zoro.com
				




I'd prefer to invest in a quality American made tool rather than buy a cheap chinese import.   with a 3/4" bar, looks like I should be able to bore up to a few inches deep... and the large diameter of the bar would seem to make it very durable.  Biggest risk is probably chipping the inserts.

Note:  That bar in the link was just an example... I haven't done a lot of research yet on which insert would be preferable. 

Thanks!


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## mikey

AmericanMachinist said:


> Mikey - would you advise against a bar similar to this Ultra-Dex as an alternative to the brazed set you linked?
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Ultra-Dex Usa Indexable Boring Bar, S12Q SCLCL3, 7 in L, High Speed Steel, 80 Degrees Diamond Insert Shape S12Q SCLCL3 | Zoro
> 
> 
> Order Ultra-Dex Usa Indexable Boring Bar,  S12Q SCLCL3,  7 in L,  High Speed Steel,  80 Degrees  Diamond Insert Shape, S12Q SCLCL3 at Zoro.com. Great prices & free shipping on orders over $50 when you sign in or sign up for an account.
> 
> 
> 
> 
> www.zoro.com
> 
> 
> 
> 
> 
> I'd prefer to invest in a quality American made tool rather than buy a cheap chinese import.   with a 3/4" bar, looks like I should be able to bore up to a few inches deep... and the large diameter of the bar would seem to make it very durable.  Biggest risk is probably chipping the inserts.
> 
> Note:  That bar in the link was just an example... I haven't done a lot of research yet on which insert would be preferable.
> 
> Thanks!



The bar you linked to is a Left Hand bar so unless you plan to bore in reverse, you might want to go with the Right Hand version, the SCLC*R*.

The SCLCR bar is a decent one and the inserts are commonly used on turning tools as well so you don't have to stock so many different kinds of inserts if your turning tools are also SCLCR. Moreover, there are inserts with very positive rake geometry that works well when boring aluminum, stainless and plastics; these are the CCGT-AK inserts. The only thing I don't like about this geometry is that lack of really small nose radii so it can be a bit finicky to hit precise inside diameters; for general use they work fine. 

Let me introduce some food for thought. We know that steel bars have an extension capability of 4 times the diameter. For a 0.75" depth of bore, a 1/4" steel bar (1" depth capacity) would be able to go deep enough and would be far cheaper than the 3/4" bar will be. A 1/4" carbide bar will go more than double that depth, out to 2.5" deep. Just remember that a boring bar is limited by bore depth, not bore diameter, and the bar you choose must be big enough to go deep enough. 

The other reason for considering a smaller diameter bar is because the bigger the bar, the bigger the available nose radii will be on inserts that fit it. This has an impact on accuracy. The converse is generally true; smaller bars will usually have inserts with smaller nose radii available and smaller nose radii can take finer cuts so accuracy is easier to achieve. 

Okay, let me bottom line this. Bigger bars need bigger holders, bigger starter holes, bigger nose radii and generally will cost more. For shallow bores, like 0.75", big bars don't make sense. I would go with a smaller bar that you will use far more often, will be cheaper to buy and will likely have nose radii that will allow more accuracy. If cost is a concern, I suggest a 3/8" steel SCLCR bar. It will do the job easily while also being far more useful to you in the shop. 

If you insist on US-made bars then look at Kennametal, Circle Machine/Widia and Valenite. I suggest you not overlook the better import bars from Seco, Iscar, Sandvik, Mitsubishi - they make some of the finest bars in the world, possibly better than what US makers produce.


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## kiwi_007

Mikey, thank you for the information you provided in your article, muddy water is now looking clearer.

Do you have an article/plans or write up on making the exchangeable tip live centre?

Thanks


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## mikey

Yup, it's already on the site. I'll link it later when I get home or you can search for it.


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## mikey

kiwi_007 said:


> Mikey, thank you for the information you provided in your article, muddy water is now looking clearer.
> 
> Do you have an article/plans or write up on making the exchangeable tip live centre?
> 
> Thanks



Here is the thread.


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## LeChuck

Thanks for this very informative post! 

I'm looking for a new boring bar and having difficulties finding something to fit my needs. I make pens and work with plastics. I've been using a small Micro 100 solid carbide bar and that's been working very well but I need something that can go deeper while still being able to handle the small, delicate stuff, almost all the way down the barrel of a pen. Small bites and finishing cuts are more important, not hogging material. I'd like to be able to use only one bar to limit tool setup time and it looks like a 3/16 bar would fit the bill. Those CCBI Circle bars look good. The cost is high though and I think I'd like a zero lead angle, which seems to be less available at discount prices, the other thing is I really like those uncoated inserts as they make a really nice cut in my turning tool. I get a nice cut quality with no tool marks with them.

I know there are many Chinese alternatives out there. In fact, I already have a set that uses good inserts, but the issue with it is it has a negative radial rake angle, and I want to find one with zero rake angle. I think it would be better for what I do and facilitate setup. Mine have flats on each side and don't fit well in my holder, and most out there seem to have at least one flat, so angle can't be changed in a regular holder. I could probably find bushings to use my actual BXA boring bar holder, but that's meant for 1 inch bars and way overkill. I'd also need a smaller bar than the 7mm one I have.

Any leads for a good quality insert boring bar that can go as deep as 78-80mm for finish cuts, 3/16 shank, can use the right inserts for plastics preferably, and with zero radial rake? The Circle bars might leave a finish that's good enough for me though.

Thanks for any tips


----------



## mikey

LeChuck said:


> I'd like to be able to use only one bar to limit tool setup time and it looks like a 3/16 bar would fit the bill. Those CCBI Circle bars look good. The cost is high though and *I think I'd like a zero lead angle*, which seems to be less available at discount prices, the other thing is I really like those uncoated inserts as they make a really nice cut in my turning tool. I get a nice cut quality with no tool marks with them.
> 
> I know there are many Chinese alternatives out there. In fact, I already have a set that uses good inserts, but the issue with it is it has a negative radial rake angle, and *I want to find one with zero rake angle*. I think it would be better for what I do and facilitate setup. Mine have flats on each side and don't fit well in my holder, and most out there seem to have at least one flat, so angle can't be changed in a regular holder. I could probably find bushings to use my actual BXA boring bar holder, but that's meant for 1 inch bars and way overkill. I'd also need a smaller bar than the 7mm one I have.
> 
> Any leads for *a good quality insert boring bar that can go as deep as 78-80mm for finish cuts, 3/16 shank, can use the right inserts for plastics preferably, and with zero radial rake?* The Circle bars might leave a finish that's good enough for me though.
> 
> Thanks for any tips



Zero lead angle carbide bars in 3/16" size are hard to find, especially those that have ground, uncoated inserts available for them. Even Circle Machine zero lead bars that use triangular inserts bed the insert at a 5 deg lead angle. Kennametal might have something but you'll have to search for them.

As for "zero rake angle", I am assuming you mean zero radial rake. Many bars have an orienting flat ground into them that puts the insert at the angle the maker intends that insert to be used at. The relief angles on these inserts are intended to work with the bar at that angle so you usually cannot just turn the bar so the insert is flat; the relief angles under the cutting edge will very likely rub so you need to buy bars that are meant to be used with the insert held flat across the top. CCBI bars are meant to be used with the insert flat.

You want a 3/16" diameter bar that can go over 3" deep and bore accurately. Given that a carbide bar has an extension capability of 8-10 times the diameter, a 0.187" bar can go 1.875" deep and cut accurately but to go 3" and expect to hold tight tolerances might be reaching a bit. This is theoretical and I admit that I have gone beyond this 10:1 range multiple times so you might get away with it if you know what you're doing.

Bottom line: the bar you are looking for probably does not exist. If I was going to try to do what you intend, I would buy a Circle Machine CCBI-187-6-5R. This is a 3/16" diameter solid carbide bar that is 6" long and has a 5 degree positive lead. It will take 0.007" and 0.002" nose radius uncoated inserts with a ground edge (CDCD inserts) that will cut plastic, aluminum, steel, stainless steel or wood. You will need a really solid boring bar holder but I bet you can get that 3" depth with this bar. I don't know that you can hold tenths tolerances at that depth but I suspect it will cut accurately enough for a pen.

I don't see any of these bars on ebay right now but if you wait for it, it will show up. If you are able to find this exact bar, PM me and I will tell you exactly which insert to buy.

Hope this helps. Let me know if any of this is unclear.


----------



## Aukai

I was going to say you'd be along, but didn't know how busy you were...


----------



## LeChuck

Thank you, I really appreciate you taking the time to answer. That is the bar I have been looking at after reading your post, aside from the 5R bit, but that can still probably work fine for me, I just have to bore a tiny bit deeper. I actually had found one on eBay and just ordered it. So I will have to find inserts and a good holder for it. Maybe the BXA holder I have can work with a proper sleeve, but it's kind of a hunk to handle on and off constantly.

I do not need tenths tolerance down at 3 inches (or anywhere else, for that matter). At that point it's mostly a cleanup to get a better surface than with a drill bit for translucent pens. With a good narrow boring bar, I might also be able to only use one smaller drill bit for everything, then go to desired bore size with the bar, saving time (drilling deep into a piece of acrylic manually with larger bits is not the most fun part of the process, and I have broken barrels or even almost melted one or 2 staying in a few more seconds than I should have). It will also be used more often in a cap, which is shorter, more like 2 inches. The areas where I need better tolerances are for inside threads, and that means a precision of 0.05 to 0.1mm, so it's not for the space shuttle, just enough to get parts that fit smoothly together, caps that aren't too loose or too tight, small relief shoulder for a nib housing and so on. I realize that if I setup the bar to go down to three inches then it might have to be extended all the time, but not more than I really have to.

I also noted that because of the angle of the inserts I suppose, the 5R can make a slightly smaller minimum bore than a 0R. The smallest one I need currently is 7.5mm so the 3/16 bar will fit.


----------



## mikey

You're welcome.

Okay, if you found a CCBI-187-6-5R then you need good inserts. I suggest Circle Machine CDCD-500 (0.002" nose radius) and CDCD-505 (0.007" nose radius) in whichever grade you can find - CG5, CG6, TM1 or CM1. The CM1 works at really low speeds in hard materials and is one of my favorite grades; I have them all and all work well. Here is an ebay listing for 4 each CDCD-505-CM1 inserts. In plastics, 4 will likely last you the rest of your life. When you search, look for "cdcd inserts" and other brands will also come up. Watch for the CDCD-500 inserts. Avoid the CDCD-51 inserts; the nose radius is too big.

These inserts are positive axial rake inserts with ground edges that cut very similar to HSS - very sharp, great chip clearance if you keep up the feed rate and are really, really accurate. The 0.002" nose radius will take a 0.001" depth of cut and actually cut it so I think you will find these inserts to be satisfactory. You should also know that if you use them properly each insert will likely last you for decades on pen barrel material so they will prove to be economical in the end.

All boring bars benefit from a solid holder but the smaller the bar and the deeper you push it, the more important a solid holder becomes. If I were you, I would make a holder from aluminum specifically for this bar that will fit your QCTP. In the boring primer, I showed one for my Sherline lathe with the details on how to make it. That holder will easily hold this bar very solidly. Most hobby guys don't realize the role the holder plays and I cannot emphasize enough how important it is. You may not need to hold tenths tolerances but if you ever do you will find that that accuracy is just as much in the holder as it is in the bar.

I have found that plastics are really sensitive to speeds and feed. Too much of either increases heat and that melts the material so getting good finishes is difficult. The trick, at least for me, is to slow down the speed and increase the feed. You will need to experiment until you find the combination that works for you. With these positive axial rake inserts, the rake acts like back rake on a turning tool. This essentially means the rake increases chip flow rates and also thins the chip. This is good for plastics because much of the heat from boring will go out with the chip and reduce the tendency to melt the material so I think you made a good choice with this bar; it is the only bar that uses this positive axial rake geometry that I am aware of. Just remember to keep up the feed rate and use a carriage stop if doing a blind bore.

Please keep us posted on your experiences with this bar. Of all my bars, the CCBI bars are the most accurate that I own and I suspect they will become favorites of yours, too.


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## LeChuck

Great, I ordered the inserts, and will keep an eye out for the 0.002 radius version. I'll spend some time figuring out the holder situation. Unfortunately, without a mill, I can't make something like that with any decent level of accuracy. Making a T-nut by hand for the quick change toolpost back when I got my lathe was already an adventure. Can't wait to try the boring bar and see how it improves my process. Making pens is not the most accurate thing you'll do on a metal lathe, or the highest production, but it's an interesting mix of both, so good accuracy matters as well as process and avoiding making X unnecessary setup steps or tool changes. As for speeds and feeds, it's just manual feed for me (often using a stop, or keeping my eye ont he DRO) except for the initial dimensioning of the whole blank, and I turn at around 1000rpm for all turning and facing, and switch down to about 300 for drilling. That is really when things heat up big time.

Your posts have been very informative, thanks!


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## mikey

You will want to slow the lathe down for boring plastics. Manual feed is fine; I do it all the time. 

Yeah, you need to sort out how to make or obtain a good tool holder. What kind of lathe are you using and which size tool post?


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## LeChuck

My metal lathe is a PM1236-T from Precision Matthews, with their BXA QCTP as well. It's been working very well aside from the processes I'm still working out for what I do. Just turned a couple new mandrels today as I've bought a little Taig setup to dedicate to sanding (currently shaping on my large Jet wood lathe but I'm trying to optimize all of this) and needed mandrels that fit the tiny little collet.


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## mikey

Okay, good lathe. Now you need a mill!


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## LeChuck

Ha, yeah, that would be fun, but hard to justify business wise. One of these days maybe!


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## mikey

LeChuck said:


> Ha, yeah, that would be fun, but hard to justify business wise. One of these days maybe!


Wait ... you think a mill is optional or something? No, no, no ... not having a mill is like being a one-legged track runner. You get there in the end but it will take you a while.    We are definitely have to work on you!


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## LeChuck

I would have all the things if I could


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## Brento

Is there any information on hss boring bars talked in this thread?


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## mikey

Brento said:


> Is there any information on hss boring bars talked in this thread?



There are many makers of cobalt HSS boring bars, although not as many as there used to be. Borite, Bokum and Criterion used to make the best ones. Unfortunately, Borite is gone. Allied Machine took over Criterion but I don't know if they still make HSS bars. Bokum is still in business.

You can also get import cobalt bars; LMS and others carry them in various shank sizes. You can find them on LMS, ebay, MSC and other industrial supply houses.

I use these in my boring heads, rarely on the lathe. They work well on the lathe, though, and are easy to sharpen. My Borite sets must be over 20+ years old by now and are just as good as they were when new. They have the same limitation as any boring bar does - they typically cannot bore beyond a 4:1 depth to diameter ratio. Within that parameter, they are very accurate and will take micro cuts better than a carbide bar will.


----------



## Weldingrod1

If you are shooting for transparent you might want to test reaming after boring. I assume you've already discovered the special drill bits for plastics? If not, you are in for a treat!!

Sent from my SM-G892A using Tapatalk


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## LeChuck

Would this be a reasonably good adapter for a 3/16 bar in a large 3/4 inch BXA holder?






						Micro100
					






					www.micro100.com
				




That's about all I found. It seems to hold the bar itself with 2 set screws in the front collar.


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## LeChuck

Oooh, actually, these smaller ones will fit in a standard holder with a v-groove at the bottom, and have a flat on top for securing, so no need to adapt to the large/heavy boring bar holder:






						Micro100
					






					www.micro100.com


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## Brento

Im gonna be grinding my boring bars shortly my lathe isnt the best for using carbide.


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## mikey

LeChuck said:


> Oooh, actually, these smaller ones will fit in a standard holder with a v-groove at the bottom, and have a flat on top for securing, so no need to adapt to the large/heavy boring bar holder:
> 
> 
> 
> 
> 
> 
> Micro100
> 
> 
> 
> 
> 
> 
> 
> www.micro100.com



That will work ... but if you had that mill I was telling you about, it would be very simple to make one yourself. In fact, there are simpler, more effective sleeves than this but you gotta have a mill. 

See? We be working on you!


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## Aukai

Don't forget about the tool holder being part of the rigidity when you can step up,,,, and you need a mill, just sayn'


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## ttabbal

You guys are like drug pushers...

But yeah, they have a point. Lol


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## mikey

ttabbal said:


> You guys are like drug pushers...



Okay, let's take away your mill and see how that works!


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## LeChuck

Believe me, I've often been looking at mills


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## LeChuck

To think of it, there are other things and parts I could make with a mill.....the PM-728VT is very tempting. But I have to save money for a CNC lathe some day


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## mikey

We're just messing with you ... mostly.


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## DavidR8

mikey said:


> We're just messing with you ... mostly.


DO NOT BELIEVE THIS @LeChuck!
We are absolutely out to spend your money.


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## LeChuck

Oh you guys


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## LeChuck

FYI, the holder mentioned earlier is a dud. It has a flat on 4 sides, which is stupid and wasn't really visible on photos, and the flats are not all the way along the cylinder so it can't really lay properly in a regular holder. I'm sure it could be modified but I don't want to bother. Looking inside, it only has about 3/4 of actual support for a 3/16 bar, the rest of the bore being larger, so it seems like poor support anyway. I will probably just use the bar directly in the V quick change holder. It should have 3 screws for support, which is more than I've been using for my Micro 100 bar.


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## mikey

Personally, I would go buy an Aloris 4D and make sleeves to fit it. It is the best commercial boring bar holder that I know of. Sleeves can be made on your lathe and you can use a hacksaw to cut slots instead of a slitting saw, you being mill-less and all. This will be the most rigid way to hold your tiny boring bar and will also rigidly hold your other bars, too. 

Or you can make a holder like this? It would be better than clamping down on your little bar in a #2 holder and will fit your standard tool holders.


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## LeChuck

Yeah, my boring bar holder is similar to that one but I just haven't been able to find sleeves for it. I might try to make that holder eventually. I'm just not really setup for that type of work right now and time is super limited, already working 7/7, 60+ hour weeks. I'll see if I can make that simply enough.


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## mikey

Similar is not the same. If you're using an import holder then it likely uses two cotters to clamp the bar; no bueno. 

Sorry you're working so hard.


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## LeChuck

Yeah, it doesn't have a slit, just a couple set screws that are off to the side.


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## LeChuck

This is what I have. Seems to grip pretty well on that 3/4 aluminum bar. I see some Aloris holders that are designed this way. The 4D does have a slit, with 1 inch opening.


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## LeChuck

I can try it when I get a chance. I have some 3/4 inch brass rod on hand, I can stick it in my collet chuck, drill a 3/16 hole through it, make a somewhat straight slit with a saw, and see how it works out


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## mikey

Your holder is the import type that uses two cotters to pinch the bar at only two points. It is not solid enough to resist heavy boring forces, although it might be okay for small bars. An Aloris 4D is a vastly superior holder. 

Instead of using brass, why not just order some cheap 12L14 from ebay? 

When making sleeves for boring bars, do not rely on the accuracy of a drill bit. You really need to use a reamer the same size as the bar because an inaccurate holder leads to chatter in the bore. Buy only new reamers. Here is one. 

Basically, you must choose two drills. The key drill is one sized to leave about 0.003-0.005" for the reamer to take off. The drill you use just before using that pre-reamer drill is the one that is one size smaller than the pre-reamer drill. Just spot the work piece and use that pre-pre-reamer drill, then use the pre-reamer drill, then use the reamer. Trust me; this will give you the best chance at a straight hole that is close enough in size to hold the boring bar solidly. I am attaching more info on choosing reamer drills for your edification.


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## LeChuck

Thanks for all the tips! I doubt my needs will involve the kind of boring forces you are referring to though 

Will check out the reamers.


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## DavidR8

@mikey what boring bars do you recommend for boring heads?


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## mikey

I prefer cobalt HSS bars for most work. My personal choice is Borite cobalt bars but they are no longer made. Luckily, I have full sets in 3/8" and 1/2" shanks and boring heads for both sizes. 

LMS has these in 1/2" shank. 

Here is one in 3/8" shank. 

When I have to go really deep, I use Micro 100 solid carbide bars. 

When I rough in garbage materials like A32 or something similar, I use those cheap Chinese brazed bars; they are not that good but for rough work, they do a fair job. You have to sharpen them, though. 

When I bore a hole that is not too large, I sometimes use inserted carbide bars (also from Borite) but once I get out to around 2-2.5" or so I have to worry about vibration so I don't use them all that often. The reason for the vibration issue is that inserts need speed to finish well and as you know, centripetal forces increase fast with increases in speed and that causes vibration at higher speeds. Inserts do work at lower speeds but the finish tends to suck. Moreover, it is harder to bore to very tight tolerances with them because you have deal with that nose radius. HSS, on the other hand, will pretty much cut what you dial in.

Bottom line: go with cobalt HSS for most of your work. They are easy to sharpen (just hone the top flat with a diamond stone) and work well for most stuff we deal with in a hobby shop. For harder materials, try brazed carbide bits. Good brands are Criterion, Circle Machine, Borite. For solid carbide, look for Micro 100 and Circle Machine. Try to stay away from inserts unless you are boring smaller holes that allow you to use higher speeds.


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## DavidR8

Thank you sir!


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## 9t8z28

I case anyone is interested there are a few carbide Circle Machine CCBI boring bars on Ebay right now for $60 each.  Thats a great price for brand new bars.  Just in case anyone is wondering these are the bars Mikey highly recommends.  I can post links if anyone is interested or if I can in this section.


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## LeChuck

Mine snapped while boring inside a piece of acrylic (on CNC lathe). I'm not sure if I'll replace it at all but I might if the price is low. Even with a carbide bar, there was too much chatter at the extension I need.


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## Brento

Does anyone make their own hss boring bars?


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## 9t8z28

LeChuck said:


> Mine snapped while boring inside a piece of acrylic (on CNC lathe). I'm not sure if I'll replace it at all but I might if the price is low. Even with a carbide bar, there was too much chatter at the extension I need.


What bar do you need to replace?


----------



## LeChuck

In this case it was a CCBI-187-6-5R. I could have used a shorter one here though, I had to cut off a good chunk of it to fit my turret.


----------



## 9t8z28

LeChuck said:


> In this case it was a CCBI-187-6-5R. I could have used a shorter one here though, I had to cut off a good chunk of it to fit my turret.


Theres 3 on Ebay right now.  $60 each, new.


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## LeChuck

Only one for the width I need but keeping my eye on them.


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## 9t8z28

LeChuck said:


> Only one for the width I need but keeping my eye on them.


How much stickout and how much grip do you have on the bar?  Didn't you say you cut a 6" bar shorter?


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## Aukai

CCBI-187-6-5R, the ones I found on ebay were 175.00, and 375.00 for the 6", the 4" were 60.00.


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## mikey

Max stickout for a 1/4" bar is only 2.5". A 4" bar is enough.


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## rwm

I'm confused (again). Are those boring bars carbide with a carbide insert or steel bars with a carbide insert?
Robert


----------



## LeChuck

I need a ton of stick-out because I need to bore to about 74mm. I actually got some decent results in the softer material after tuning the feeds and speeds, and doing several passes (and discovering that taking really small passes is actually worse, you need more engagement) but the harder acrylic was just too hard, chattered, and eventually got hot in there and completely locked the bar with swarf and it snapped. This material is nasty to work with but manually you can feel before it goes wrong or too hot. I need to work out different settings for it or only work that material manually. This was my very first attempt at this material on the CNC. Funny thing is the carbide bar snapped but the (rare) material was intact. I finished drilling it to size on the manual lathe.

I originally got the bar (per advice in here) for my manual lathe but then got a small CNC lathe and used it in there so I had to cut it to clear the turret. For this lathe, 4 inches is fine, anything extra has to be cut off anyway.

The problem is not really the bar here. I expected it to chatter a bit, and I reduced the stick out as much as I could. Once I got under 76mm of stick out, the vibrations got down by a lot. The problem is the lathe. A turret is not the best thing for me as it limits the tools I can use. With gang tooling, I would use a couple extra drill bits and wouldn't need the boring bar so much, or I could use a much shorter bar for the really small diameters and then I could get this bar thicker than 3/16 for the deeper holes, but as it is, I need a single boring bar that can go really deep, go into small 1/4" holes. Making pens requires strange drill sizes, like 6.9mm. On a manual lathe you can get all the drill bits you want. On the CNC you gotta use a couple drill bits then go up to the each final dimension with a bar. 

Actually, if I can figure out a way to hold a 9mm drill bit in there instead of a 3/8, I might be able to get away with not using a boring bar at all for the deepest hole, and shorten the stick out by 20mm. But finding sleeves to go from imperial (5/8 turret bore) to metric....I have some ER16  collets but this lathe being small, it probably wouldn't fit with a drill bit long enough to go that deep. Gotta try.


----------



## mikey

rwm said:


> I'm confused (again). Are those boring bars carbide with a carbide insert or steel bars with a carbide insert?
> Robert


Carbide bar, carbide inserts.


----------



## rwm

OK that makes more sense with the stickout you guys are talkin about! I may need one of those. What insert do they accept?
R


----------



## mikey

rwm said:


> OK that makes more sense with the stickout you guys are talkin about! I may need one of those. What insert do they accept?
> R



See post #33 of this thread.


----------



## 9t8z28

rwm said:


> OK that makes more sense with the stickout you guys are talkin about! I may need one of those. What insert do they accept?
> R


They use CDCD500 (.002" radius) - CDCD505 (.007" radius) inserts.  The flat inserts work great but they do offer chip breaker inserts.


----------



## 9t8z28

Mikey, I have developed an obsession with Circle Machine carbide boring bars over the last several years and have quite a collection now.  I am still looking for a 5/8" QCMI bar.  Here's what I got so far along with a nice selection of boring, grooving, and threading inserts for the applicable bars that I snagged up whenever they came up on Ebay. 

QCBI-500-7-5R (1/2”)
QCMI-500-8-5R 1/2”)
QCMI-375-6-5R (3/8”)
QCBS-3631-6-5R (3/8”)
CCBI-312-6-5R (5/16”)
CCBI-250–4-5R (1/4”)
CCMI-250-4–0R (1/4”)
CCBI-180/187-6-5R (3/16")
CCBI-180/187–4-5R (3/16”)
CCMI-187-4-0R (3/16”)
CCBI-203/625-2-5R (13/64")
CCBI-165/156-6-7R (5/32”)


----------



## mikey

9t8z28 said:


> Mikey, I have developed an obsession with Circle Machine carbide boring bars over the last several years and have quite a collection now.  I am still looking for a 5/8" QCMI bar.  Here's what I got so far along with a nice selection of boring, grooving, and threading inserts for the applicable bars that I snagged up whenever they came up on Ebay.
> 
> QCBI-500-7-5R (1/2”)
> QCMI-500-8-5R 1/2”)
> QCMI-375-6-5R (3/8”)
> QCBS-3631-6-5R (3/8”)
> CCBI-312-6-5R (5/16”)
> CCBI-250–4-5R (1/4”)
> CCMI-250-4–0R (1/4”)
> CCBI-180/187-6-5R (3/16")
> CCBI-180/187–4-5R (3/16”)
> CCMI-187-4-0R (3/16”)
> CCBI-203/625-2-5R (13/64")
> CCBI-165/156-6-7R (5/32”)



Wow, you da' man!!!

If I see a 5/8" QCMI, I'll try to remember to alert you. 

I have one!


----------



## xyz

mikey said:


> *Boring on the Lathe for (New) Hobby Guys
> 
> A Primer*
> 
> View attachment 284096
> 
> ​Being able to bore an accurate hole on the lathe is an important skill every hobby guy should have. We may not need to bore a precision hole all that often but when we need to, we should be _able_ to.
> 
> As we all know, that is easier said than done.
> 
> I don’t know about you guys but when I started I didn’t really understand how a boring bar actually worked. I certainly didn’t know how to choose them and I didn’t know how to use them because nobody ever told me. Even the manufacturers of our bars had not put out any educational material that covered the basics of boring. Has anybody found the equivalent of “Boring for Dummies” and “How a Boring Bar Actually Works” yet? I haven’t, and I’ve looked hard!
> 
> The information I needed was actually out there but it was scattered about in the technical, academic and manufacturer’s literature. Without any other recourse, I decided to sort this boring thing out for myself. Being a little slow, it took a long time for me to tumble onto the idea that if boring bars and inserts have geometries then those features are meant to accommodate or mitigate cutting forces. Duh! It took even longer to figure out how these geometries do that, and then I had to sort out how our usage of those tools augments or modifies their impact on tool performance. Eventually, I developed at least a rudimentary understanding of this process and was then able to bore a stupid hole on size, on purpose and without relying on luck … much.
> 
> Sometime in 2005, a mini-lathe guy asked me about which boring bar to buy. This discussion was my answer to him. As you will see, the answer to his question is a little complicated and became what is essentially a Primer, the basic elements, on boring on the lathe. Over the years, I have shared it with some folks and some of them have encouraged me to share it with the HM membership so here it is, rewritten for the final time. You will see references to mini-lathes but since most of the deflection that happens while boring happens to the bar and not the lathe, this stuff should apply to most lathes found in a hobby shop. Just keep in mind that this is MY way of boring on the lathe, not THE way.
> 
> I am approaching this subject from the same perspective that I believe the makers of our boring tools use – cutting tool geometry – because, as I hope to show you, it is the only approach that makes sense. As we develop this discussion, keep the following in mind:
> 
> *The key to boring accurately lies in understanding the cutting forces inherent in the boring process. Then you can see how the different bar and insert geometries alter them and how your usage of those tools refines their effect on these forces. Only then does boring become the simple process it really is.*
> 
> We will look at cutting forces, basic boring bar and insert geometry and then go through a checklist to show you how it all ties together. I promise, no math!
> 
> Let me say up front that I cannot really tell you which boring bar to buy; there are just too many variables that go into that decision and you need to choose a bar based on your own needs. Later on, I will tell you what I use and why.
> 
> You will also find that no boring bar does it all and you may as well accept the fact that you will own more than one bar. What I hope to do is give you the information you need to choose those bars wisely because boring bars are potentially one of the most expensive kinds of tooling we encounter. A single (good) carbide boring bar and pack of inserts can retail for almost as much as an entire mini-lathe costs so mistakes here have significant financial consequences.
> 
> I should also tell you that nothing qualifies me to write this. I am just a hobby guy like you. The content here is what I have learned over the years, with a perspective that I have not seen anywhere else to date. That doesn’t mean it is correct, complete or anything resembling an authoritative work. Boring is a complicated process with many nuances. In fact, even the nuances have nuances, so I am just going to shoot for the basic elements and you can take it from there.
> 
> If you can already bore accurately then just ignore me.
> 
> Finally, I have read that boring is just like turning, except it is done on the inside instead of the outside. While I disagree with this statement, the cutting forces encountered with external turning are the same and external turning tool and insert geometries are strikingly similar to that of boring tools. Accordingly, much of the information you find here can also be applied to the use of external inserted carbide turning tools. I hope you find this helpful.
> 
> 
> *The Force Is with you … like it or not*
> Anytime a cutter is used in a machining operation there are going to be cutting forces to contend with. Typically, cutting tool geometry is used to counter or alter these forces and boring is no exception. What is not readily evident is how intimately cutting forces relate to the use of a boring tool. Every part of the tool, from the lead angle of the bar to the rake of the insert to the nose radius of the cutter has some impact on cutting forces and we need to understand these forces in order to understand how the tool works and how our work methods impact on the results.
> 
> A boring bar is a single point cutting tool that is cutting to one side of a flexing bar deep in a hole where you cannot see. You can imagine that as the tip is pushed into the cut it will be also be pushed down and away as it encounters the rotating mass of the work … as a result, the bar deflects. Let’s look at this more closely.
> 
> Deflection is the movement of a structure or structural part as a result of stress, and the key components that stress our boring bar are three cutting forces: *Axial* or Feed forces, *Tangential* forces and *Radial* forces. These forces are ever-present and dynamic whenever the cutter is in contact with the work.
> ​
> View attachment 284097
> 
> 
> *Axial* forces run down the axis of the bar as we feed in and do not materially alter deflection. Therefore, anything that alters the other forces toward axial is a good thing.
> 
> *Tangential* forces (Ft) act to push the tip of the cutter down and can actually push the tip below centerline, especially when we have a long overhang. When this occurs the cutter can dig in and the clearance angle under the cutting edge is reduced, causing rubbing, vibration and chatter. This is most critical in small bores where the lower edge of the insert or bar is more likely to contact the work. In terms of magnitude, Tangential forces are the largest of the three.
> 
> *Radial* forces (Fr) tend to push the cutter out of the cut, toward the centerline of the bore. This results in variations in chip thickness as the tip moves toward and away from the centerline, causing inaccuracy, poor finishes, vibration and chatter. Radial forces tend to be the smallest in magnitude but are no less important.
> 
> _Just to make sure I am crystal clear about these forces, let’s look at them in Bubba terms. If you, Bubba, had a big stick and held onto one end and pushed the tip of the stick straight into a solid wall, the force you’re applying to push the stick are Axial forces. Now, if someone were to grab the far end of that stick and pushed it straight down (while you resisted), that someone is creating Tangential forces on that stick. Now say that same someone stopped pushing down and instead pushed the tip of the stick horizontally, directly away from him/her, again with you resisting. He/she is creating the equivalent of Radial forces.
> 
> What your boring bar is seeing is all three of these forces acting on it at the same time, all the time, and the magnitude of each force varies due to multiple factors in real time.
> 
> And don’t forget that Bubba is your boring bar holder. Think about the forces being encountered there as he tries to hold onto the end of that stick and keep the tip of it steady while these forces are trying to move the tip of the stick at the other end. Then think about what happens if he lets up on his grip or moves even a tiny little bit …
> 
> With that visual, let’s look at the stick._
> 
> In addition to these cutting forces, deflection is compounded by bar overhang, bar diameter and the material the bar is made from:
> 
> Overhang is the amount the bar is extended from the boring bar holder. It alters deflection exponentially to the 3rd power, such that its impact on deflection at 2” will be 8 times that at 1”. Overhang is dictated by the depth of our bore but it should be no longer than absolutely necessary.
> 
> Boring bar diameter influences deflection to the 4th power, such that deflection when using a ¼” diameter bar will be about 5-6 times greater than that of a 3/8” diameter bar. So, a bigger bar is better and more accurate.
> 
> The modulus of elasticity of the material the bar is made from also affects deflection. Steel can be extended in a 4:1 ratio (extension:diameter) beyond the holder, while carbide can go up to 8 - 10:1. For example, a 3/8” diameter steel bar would have a maximum overhang capability of 1.5”, while a carbide bar of the same diameter can extend 3.75”. Note that it’s the modulus of elasticity of the bar material that matters and the stiffer carbide bars will trump steel when reach is a concern.
> 
> This is why we are always advised to *keep overhang to a bare minimum, and use the biggest, stiffest boring bar we can fit in the hole and still clear chips* – to minimize deflection.
> 
> _So, again in Bubba terms, we have the three cutting forces trying to move the tip of the stick and we have Bubba hanging onto the end of the stick, a stick that we now know can move varying degrees under the influence of these forces depending on how fat, how long and how stiff it is. If you have ever wondered if your boring bar was possessed, the answer is no. It is simply dealing with this stuff! And we haven’t even introduced the human factor, the user, into this scenario yet. _
> 
> Okay, moving on …
> 
> Note that Tangential and Radial forces can be calculated mathematically and theoretically accommodated but it assumes the bar is held absolutely rigid, which is not possible. You must also have an NC lathe to input the data; most hobby guys don’t own a NC or CNC lathe. That is not to say you can’t do anything.
> 
> *Tangential forces* can be partially accommodated by setting the tip of the cutter *above* center height by 0.005-0.010” and then assessing the effect. What you are doing with this setting is anticipating and accommodating the downward movement of the bar brought about by tangential forces because it *WILL* occur. However, because Tangential forces are affected by a number of factors - the nose radius of the cutter, depth of cut, feed, speed, overhang, boring bar material and the material being cut, the lead angle of the bar and of the insert - it is impossible to tell you that any single setting will work for you. You need to set it to see if the cut is smooth and free with the depth of cut you dial in. The deeper the hole and/or the smaller the bore, the higher we need to set the cutter tip above center height. Get it right and the bar will hiss sweetly as it cuts.
> 
> *Radial forces* can be accommodated by assessing the change in inside diameter (ID) a given depth of cut (DOC) produces. For example, if you dial in a 0.005” DOC and find it produces only 0.0095” change in ID then the impact of Radial forces on your setup is that 0.0005”; adjust your DOC to get the change in ID you need. You may need to dial in a 0.0055” DOC to get a 0.010” change in ID. Doing this to determine a setting for finish cuts is very useful in planning your approach to final size. I know of no other practical way to directly offset the effect of Radial forces but this actually does work rather well.
> 
> These adjustments are only a small part of the picture. There are many other things that impact on cutting forces that we will cover later. For now, let’s look at setting up for boring.
> 
> *Tolerances*
> As tolerances get tighter the cost in time, materials and equipment goes up. The usual advice is to always ask yourself how tight your tolerances really need to be; why work to thousandths if hundredths will do. This is good advice, but until you are comfortable with boring I suggest you try to work to the tightest tolerances you can for every bore you make so that when you need to bore a truly precise hole it isn’t something you haven’t done before. Practice makes a difference. And besides, it doesn’t take much more effort to bore accurately than it does to bore casually.
> 
> *You can only cut as accurately as you can measure*
> Being able to measure accurately is a cornerstone of machining but it is especially true when boring because boring is a planned approach to a precision result. In order to do that you must be able to measure the bore accurately. There are many devices to measure a bore – bore gauges, internal micrometers, inside micrometers, telescoping and small hole gauges and so on. Telling you how to choose and use all of these tools is beyond the scope of this discussion but for most hobby guys it is my feeling that the “feel tools” – *telescoping gauges *on the left and *small hole gauges* on the right– are adequate for most work in a hobby shop. They are capable of reading to tenths if used with care, they’re relatively cheap and are easy to use with a work piece mounted in the lathe.
> 
> View attachment 284098
> View attachment 284099
> 
> 
> 
> For less precise work I prefer an inside micrometer (shown below) that reads in thousandths; they are fast to use and very accurate.
> 
> View attachment 284100
> 
> 
> Regardless of which tool you use to measure bores it is a good idea to have a standard to check your technique and your tool’s accuracy. The standard I prefer to use is a *ring gauge*. These rings are precision ground, hardened and are often calibrated in tenths; they will be stamped with their ID.
> 
> View attachment 284101
> 
> 
> If you can read the bore of a ring gage consistently to tenths with a telescoping gage then you can read any bore to tenths with those gages. The specific size of the ring gage is not important; my mid-sized one is a 0.7514”XX. Buy a smaller one for checking your small bore gages, too.
> 
> Another important tool is a micrometer stand or padded vise. It is very difficult to hold a micrometer in one hand, your gage in the other and then turn the knob without something moving. A stand makes this simple. These are available on eBay for very reasonable cost. If you have ever read a telescoping gauge three times and got three different readings then you might want to buy a stand!
> 
> View attachment 284102
> 
> _Use your ring gage to find out how your mic likes to read. Lock a telescoping gage in your ring gauge and bring it to the mic and read it. Knowing what the reading should be you can then tell how your mic should be handled. For most mics, you need to turn the spindle of the mic until it just touches your gage, and then take your reading. Just touch; cranking in on a ratchet can move the telescoping gage. Be sure the contact surfaces of the telescoping gage is in the center of the micrometer’s anvil/spindle and the handle of the gage is perpendicular to the spindle of the mic as shown in the picture. This orientation assures you are reading the gage at the widest point._
> 
> *Setting up the Lathe*
> Confirm that your lathe is level and that the headstock is aligned with the ways - we don’t want tapering bores unless it’s on purpose. Also check the gibs on your carriage to be sure there is no play. Boring creates a lot of stress on the lathe and any play in the carriage gibs, or even a worn leadscrew or nut, can lead to chatter in the bore. If you lock your carriage when boring then be sure to set your cutter center height with the lock engaged. Also be sure there are no burrs on your carriage top or compound that can allow a tool post to spin under load.
> 
> A depth stop is useful when boring to precise depths or when cutting a flat bottom. They are simple to make for most lathes. On my Sherline lathe I use either a stop screw or a micrometer head, depending on the situation. I typically bring the stop into contact with the carriage after I feed in to final depth (before I start boring); then I zero the hand wheel. While boring I use the stop to tell me when I am close to the bottom and use the zero on the wheel to stop precisely. The stop is a reference, not a solid stop. A DRO or even a piece of masking tape on the ways will serve the same purpose.
> 
> View attachment 284103
> View attachment 284104
> 
> 
> On my larger Emco lathe, I have a very robust and solid carriage stop. The stop is locked at the final depth of the bore and I put a mark with a Sharpie on the boring bar about ¼” before I hit the stop. I use power feed to get to that mark, disengage the feed and manually move the carriage into contact with the stop. Then I move the cutter away from the wall and retract the bar before making the next cut. My lathe has a clutch at the end of the feed shaft so I could just use power feed all the way and bang into the stop and then turn the power feed off but that seems too much like an assault by a newbie so I don’t use it that way.
> 
> *Your boring bar holder is VERY important. *
> Most of us use a Quick Change Tool Post on our lathes but the boring bar holder that comes with these posts typically locks directly down on the boring bar with 3 or 4 set screws. This is the least secure way to hold a boring bar. It is also guaranteed to mar your expensive carbide bars and can even break them. These holders can be a source of chatter – I suggest you buy or make your own.
> 
> View attachment 284105
> View attachment 284106
> 
> 
> The holder above was made for my Sherline lathe. It completely encases the boring bar in a finely finished bore and holds it very rigidly. 6061 T-6 Aluminum is fine for bars up to 3/8” diameter; for larger bars, use steel.
> 
> *The length of a holder should be at least four times the diameter of the bar.* This holder is just long enough to fit a 3/8” diameter carbide bar at full extension (4” hanging out); for a 6” long bar this amounts to just over 2” that can be held so this one is 2.125” long. The dovetail is made first. The 10-32 locking screw holes are drilled for a roll tap, and then the hole in the center of the 1-3/8” wide x 1” thick body is drilled, bored and reamed to a fine finish before the compression slot is cut. Then the upper screw holes are drilled to clearance diameter and the bottom half is tapped. In use, just snugging the screws is enough to hold the bar solidly. A QCTP using this holder can handle a 3/8” carbide bar at full extension making deep cuts but I would not trust the typical tool post found on most mini-lathes to handle a larger bar unless you brace it. Mini-lathe guys, you want to make this holder.
> 
> View attachment 284107
> 
> _Sleeves can be used to hold smaller bars (1/4” and 3/16” bars shown). These sleeves are made of mild steel and have a compression slot on one side and a shallow expansion slot on the other. Length matches the holder and the hole to receive the bar is reamed carefully. These sleeves fit the shop-made holder shown above. A flat is machined on top of the sleeve to also allow use in the standard QCTP holder._
> 
> If you’re using a larger lathe and have a solid Aloris-style quick change tool post then a standard tool holder that surrounds and clamps the bar will work fine. An example of this type of holder would be the *Aloris 4D *shown below; the AXA version works fine for bars up to ¾”.
> 
> View attachment 284108
> 
> 
> Unlike import boring bar holders that use a cotter arrangement to put pressure on the bar in just a few places or that uses set screws that bear directly on the bar itself, the 4D uses screws that compress the entire length of the tool holder to firmly and evenly grab the bar. It also allows you to fully compress adapter sleeves for smaller bars and hold them very rigidly. In my opinion, this is a superior tool holding design.
> 
> Regardless of the tool holder you use, keep in mind that the holder should be at least 3 - 4 X the diameter of the bar to assure rigidity based on the Lever Rule.  The Aloris AXA-4D is intended to hold a ¾” OD bar and is 3” long; this length is not a coincidence. This recommendation comes from Dr. Uwe Heinrich of Micro 100. See his article that I am attaching: Cutting Tool Engineering, December 2005, Volume 57, Number 12, “Stable Environment”.
> 
> *Part two
> 
> Boring Bars*
> There are two basic kinds of commercial boring bars – solid bars and inserted tip bars - and *many* other kinds of commercial bars, shop-made bars or between-centers bars. We’ll only look at the commercial bars commonly found in the typical hobby shop because I can’t possibly cover them all.
> 
> *Non-interchangeable tip/solid bars*
> 
> View attachment 284109
> 
> _The ubiquitous Chinese brazed carbide steel-shank boring bar set on the left and a high quality Borite solid M42 HSS Cobalt boring bar set. Both have 3/8” shanks and are the zero lead type of bar._
> 
> The sets shown above are typical of this type of bar. These are available as brazed carbide, solid carbide, HSS and cobalt HSS. As you can see, they include bar sizes from small to large that can bore shallow to deep, and will fit both a lathe bar holder or a boring head on the mill. Their tapered bars reduce resonance on this type of tool and their shank diameters appropriately increase as they lengthen. These bars typically have no axial or radial rake and have little to no nose radius from the factory.
> 
> View attachment 284110
> 
> 
> This diagram from Criterion depicts the different lead angles solid bars can have. The positive and zero lead bars can bore to a shoulder while the negative lead is for through holes only. Positive lead bars focus the cutting forces at the tip and shift radial forces toward axial. They also clean up the bottom of the bore better. Zero lead bars tend to alter radial forces toward axial also and cut with less chatter. Negative lead bars finish well and are stronger.
> 
> Most “sets” come with zero-lead bars. If you have a surface grinder, you can grind a 5 degree positive lead on the tips of these zero-lead bars and they will work better for lathe boring. This works especially well if you do a lot of closed bottom bores. You can also buy sets with a 5 degree lead angle already on them but they are not as commonly available.
> 
> These bars are typically mounted with zero radial rake (the top of the cutting tip is horizontal to the bed when viewed from the tip) and typically square to the bore (the center of the shank parallels the ways). However, since the clearance profiles under the edge are typically rounded, this allows some latitude in creating some positive radial rake by turning the tool in the holder so the cutting tip points up slightly; this lowers tangential cutting forces at the edge. You can also angle the tool along its axis (turn the tip toward the wall of the bore) a few degrees at the same time to further reduce radial cutting forces at the very tip; this reduces deflection from radial forces by decreasing the amount of side cutting edge engaged in the cut. There are limits to these maneuvers but used in small amounts the tool will cut with greater ease and accuracy. Give it a try.
> 
> Depth of cut and feed rates are not specified with these bars. When used on the mini-lathe I typically rough at around 0.010” to 0.015” and finish at 0.001 – 0.005” for the cobalt set and 0.020/0.005” for the Chinese set. This seems to work fine for me. I use speeds that are generally about 2/3 typical turning speeds for HSS because of the thinner profile of the tapered shanks but you will need to try it and adjust to produce the chip you want. In aluminum, brass, plastics and mild steels you can run at normal turning speeds or even faster with the shorter bars.
> 
> Personally, I rarely use this type of bar on my lathe anymore. They are mostly relegated for use in the boring head on my mill where they excel and I pretty much only use inserted carbide tooling for lathe boring. If you prefer this type of bar then I suggest spending some money on individual bars or a good set from a reputable maker (Borite, Bokum, Circle Machine/Widia, Criterion, Micro 100 are some good ones). There are many different configurations available from these makers and I can attest that they work exceedingly well … at a cost.
> 
> If you regularly bore small diameter holes then I suggest solid carbide bars from *Micro 100*. Their tiny boring bars are excellent and their higher modulus of elasticity allows them to bore quite deep for their diameters. For bores below about 3/16”, these are pretty much all I use. They also make bars for internal grooving, thread relief and threading and I can highly recommend all of them. They are very simple to sharpen by honing the flat top on a fine or extra-fine diamond stone and they will last for decades when cared for.
> 
> *Inserted tip bars, aka Indexable tip bars*
> In my opinion, an inserted tip boring bar is the most useful type of bar for most hobby shops, primarily because their constant diameters allow us to use only enough of the bar’s length to suit the bore depth. Moreover, inserts usually cut consistently until the insert wears. This is a major advantage because boring is a game of predictable accuracy; inserted bars, and especially those with solid carbide shanks, give you that … if you know how to use them.
> 
> The obvious caveat is that you need to choose the right bar and insert for the job and it can be really confusing when choosing one. You can go to most industrial supply houses to see checklists like the one below; they are intended to assist you in choosing a bar but lack the details you need to really make an informed choice.
> 
> I’m going to fill in the blanks so the items on the list make sense. I’ll also add a few things to the list that they don’t include but know that the best source of detailed information on a particular bar is the manufacturer’s product description; download their brochure that pertains to the bar/insert you are considering.
> 
> *Step 1: Determine hole size to be bored. Proper bar selection will have largest minimum bore dimension under hole size to be bored.
> 
> Step 2: Determine bore depth or how far boring bar will extend out of holder. Multiply bar diameter times 4. If bore depth is less, use a steel bar; if bore depth exceeds 4:1 ratio, use a solid carbide bar.
> 
> Step 3: Determine shank size needed.
> 
> Step 4: Determine if the machining application requires a boring bar with a 5° front lead angle or with a 0° lead angle. Also, if a left handed bar may be applicable.
> 
> Step 5: Select Boring Bar.
> 
> Step 6: Select insert geometry to fit boring bar.
> 
> Step 7: Select desired corner radius needed for particular machining application.
> 
> Step 8: Determine materials to be machined.
> 
> Step 9: Refer to carbide grade selection chart below.
> 
> Step 10: Select proper insert, corner radius and grade. *
> 
> 
> Let’s discuss this in more detail:
> 
> *Determine hole size to be bored. Proper bar selection will have largest minimum bore dimension under hole size to be bored.*
> 
> The manufacturer assumes that we would drill a starter hole that is within ~0.030” – 0.050” of the finished bore diameter, then finish bore the hole to size with a bar presumably bought for just that purpose. _Basically, they are asking you to choose the biggest bar that will fit in the starter hole and still be able to clear chips. _
> 
> Depending on the bore size, this one can catch you out if you aren’t careful. For example, say you need to bore a ½” precision bore. You would think that a 3/8” diameter boring bar would work, right? The minimum bore is around 0.421” for this type of bar so if you drilled a 27/64” hole the bar should fit. This theoretically leaves you with a whole 0.0395” of wall thickness to work with but since drills do not leave straight, clean holes we need to take a cleanup pass. If your insert has a 0.007”NR then we need to take an additional 0.005 – 0.010” depth of cut on top of that 0.007” (more on this later) for a chatter-free cleanup pass so, at minimum, we need to take a 0.012” deep pass. This leaves us with a wall thickness of 0.0155”. You now have two options. First, you can try taking a 0.00775” deep pass if you think you can dial it in or second, you can try taking two finish passes of 0.0038” and 0.0039” and hope you nail the bore. As we’ll see further along, the latter is probably the best option provided you can dial in a cut in tenths of an inch.
> 
> Personally, I would prefer to use a 5/16” bar and have much more room to approach the final bore. A smaller bar may allow me the option of using an insert with a smaller nose radius, say 0.002”. This gives me far more control over the cut by minimizing the effect of radial forces enabled by the smaller nose radius. This will make more sense later on but for now accept the point that bar selection requires us to consider not only the bar diameter but the insert nose radii the bar will accept because that determines how deep we can _accurately_ cut. Yup, there is more to it than just what will fit in the hole. Sometimes smaller is better if it allows you to go deep enough, gives you more room to work and clears chips easier.
> 
> Chip clearance is very important. The bar has to fit into the starter hole and have room to get the chips out. If the hole packs with chips they will get between the work and the cutter tip and impair your accuracy and finish – a lot. If they pack up enough they may even prevent you from reaching the bottom of the bore until you clear them. When the manufacturer lists a minimum bore size, what is unspoken is that this assumes you are using a depth of cut and feed rate that produces _coiled_ chips that clear readily, not long stringy chips that can pack up in a bore.
> 
> *How deep do you need to bore?* _This will determine the shank size of the bar and the material it is made from. The deeper the bore, the larger your bar’s diameter should be; and if the required bar extension is beyond a 4:1 depth to diameter ratio then you are looking at a carbide bar. _
> 
> So, you have to be asking yourself just how rigid this 4:1 (for a steel bar) recommendation really is. Can you exceed it, and if so then by how much? I tested my steel bars and I suspect your results will vary because we are using different bars and inserts but what I found is that I can get to almost 5:1 before I start seeing inconsistencies in how the carbide inserts cut. For me, that 4:1 is pretty close and I suggest you test your own bars to see how limited you really are.
> 
> Steel bars are fine for shallow bores but over the years I’ve found that if I opt for a steel bar to contain costs I will later buy a carbide bar of _the same diameter_ to go deeper and that steel bar will sit in a drawer. Consequently, I have learned to only buy carbide bars because I can own fewer bars and cover more range. In the long run, it’s cheaper.
> 
> There are some bars made of Tungsten that is halfway between steel and carbide, both in stiffness (extension capability is 6:1) and cost. They are meant to be more economical than carbide while giving you a bit more depth capacity than steel. These are not commonly found in the home shop. Some bars even have an internal damping feature; typically they are steel with a rubber mounted tungsten insert in the shank to reduce resonance. I have no experience with these but mention it here for completeness.
> 
> *What is the insert clearance angle?* Another factor that is related to bar diameter, especially for small bores, is the clearance or relief angle under the edge of the insert.
> 
> View attachment 284111
> 
> 
> An 11 degree insert works well in bores down to about 0.20” ID; this much relief angle is usually only found on inserts for smaller bars up to around 5/16”. This will give you more space as tangential forces push the cutter down. For bars above 5/16” diameter that have greater inherent stiffness this clearance thing becomes less of an issue. Inserts intended for larger bores will often have relief angles of 5-7°. Inserts for negative rake bars often have relief angles of zero; the bar itself beds the insert at a downward angle, thereby providing the needed edge relief.
> 
> The relief angle is usually tied to the nose radius. Most inserts with a 0.002-0.004” nose radius will automatically come with an 11° relief angle on it but you should check. As the nose radius gets bigger, the relief angle typically decreases.
> 
> *What kind of bar lead angle do you want?* There are two common types of bar geometry or lead angle on inserted tip boring bars: positive (typically 5-7°) and zero. Both can bore to a flat bottom, a shoulder or a through hole, and both also work on a small lathe. These also come in right and left hand but unless you plan on boring with the lathe in reverse, choose a right hand bar.
> 
> View attachment 284112
> 
> _Zero lead on the left, positive lead on the right. Note the positive lead insert is canted with a 5 degree tilt so that the point leads the way. The zero lead insert has the leading edge square with the front of the bar. Note that the tip of the insert just clears the side of the zero lead bar; it is very well supported so strength is enhanced._
> 
> Positive lead bars help alter force vectors from radial to axial based on the geometry alone. It holds a diamond-shaped insert at an angle that cleans up the bottom of the bore better because only the point cuts as you feed toward the centerline. These bars are better for blind bores. The inserts have only two usable tips but if used properly the inserts last for quite some time in a hobby shop.
> 
> The main advantage of a zero lead bar is that when used with a DOC (depth of cut) greater than the Nose Radius most of the cutting force is axial so it is less likely to chatter with heavier cuts, which really helps accuracy. This bar does not clean up the bottom of a bore well – it rubs across the leading edge – but it excels for through-cuts. The insert is supported very well so the inserts for zero-rake bars tend to last longer. Trust me; a good zero lead bar with a good operator behind it can bore really, really accurate holes.
> 
> So, which one to choose? I don’t know about you but the vast majority of the bores I make are closed bottom bores so I most often use a positive-lead bar because it cuts without rubbing; the geometry also reduces radial forces, a big plus when accuracy is critical. I also use a zero-lead bar, primarily for through holes or when I need to do a lot of roughing. For me, this is not an either/or; I own both types of bar geometry and use them for different scenarios but if I could only have one, I would choose the positive lead bar.
> 
> *Stuff to consider regarding insert rake angles.*
> I want to lead off by telling you about a positive _axial_ rake insert that reduces tangential forces at the tip. It also helps reduce the incidence of a BUE (built up edge) in softer materials.
> 
> View attachment 284116
> 
> _This illustration is from Circle Machine and clarifies the different rake angles._
> Circle Machine offers bars (the CCBI series) that uses _flat topped_ inserts that have a positive axial rake and zero radial rake (see illustration above). In my experience, they cut more accurately, feed easier and finish better than a zero axial rake insert; on a manual feed lathe you can definitely feel that they cut with less resistance and the chips literally fly out of the bore. From a geometry standpoint I believe positive axial rake acts like back rake on a turning tool; therefore, it should focus the cutting forces at the leading edge of the insert while also thinning the chip and accelerating chip flow. Now, if the bar itself also has positive lead geometry then only the very tip of the insert engages the material, not the entire leading edge.
> 
> This is an ingenious design. You have the positive-lead bar geometry creating a large axial component (lower radial forces) and the positive axial rake insert reducing tangential forces and boosting chip clearance … a LOT. These features make this bar design ideal for use on smaller lathes and/or smaller bores, which is probably why positive axial rake inserts are usually found only on smaller boring bars (3/16” to 5/16”).
> 
> This geometry is very accurate. The inserts for these bars come with nose radii down to 0.002”. What this allows the bar to do is take very small cuts without radial forces deflecting the bar much at all. A 0.003” depth of cut is already 1.5 times the nose radius and this insert will cut that depth, time after time. It will also take a 0.001” finishing depth of cut and is actually pretty accurate when doing so. This is a really useful capability when tolerances are tight or when you just have that smidgeon more to take off.
> 
> This seems like a lot of discussion about a little geometry thing that I would guess 99.9% of hobby guys are unaware of. And it really only applies to smaller bars, from 3/16” to 5/16” diameters. So what’s the deal, Mikey? Well, I just told you about the most accurate carbide boring bar and insert geometry that I am aware of. Given that a large proportion of bores that are done in a hobby shop will be within the reach of these bars, I thought it important to make you aware of them. A boring bar is not limited by bore diameter, but by bore depth. Just about any hole that is 2-3” deep or less can use these carbide shanked bars so keep them in mind when you need a precision hole.
> 
> Okay, moving on …
> 
> A zero axial rake insert is found on the *vast* majority of boring bars and cuts well, though it does so with greater tangential forces than a positive axial rake cutter. This can be offset by raising the bar above the lathe centerline so don’t avoid a bar because it has a zero axial rake insert. In addition, many inserts used on boring bars are called “positive rake” inserts and have their own unique upper surface topography. You will need to evaluate this on an individual basis because there are just too many of them to discuss here.
> 
> Negative rake tools (like bars that use Trigon inserts) bed the insert so the tip has both negative axial and radial rake. They cut with much higher tangential, radial and axial cutting forces – you will feel this in the higher resistance to feed when compared to a zero radial rake bar. The inserts tend to be much stronger since the tip is well supported; accordingly, they are most commonly used in production situations. They rough well and finish nicely but deflection can make holding tight tolerances a challenge, at least in my hands. I’ve have only owned three of these bars so I don’t have extensive experience with them but I personally avoid negative rake bars. I also avoid zero rake bars that have a tool orienting flat on top that forces the bar to have a negative approach angle (tip pointing down).
> 
> *When choosing inserts pay attention to the carbide grade of the inserts that are available for the bar you’re considering. *
> 
> There are many grades available, each with coatings and characteristics that may make one more suitable for a particular material than others. You are advised to consider inserts in light of the geometry and nose radius you prefer and then consider whether a particular coating or grade is preferable for the material you plan to bore. Typically, the maker will also tell you which ones are suited for general work.
> 
> Now, with that said …
> 
> Circle Machine lists 26 grades. Of these, I have CG5, CG6, CM1, and TN7 grades. I will be very honest with you and say that I cannot tell much of a difference between them. I tested them against each other for finish, wear resistance and consistency of cut. I used stainless steel, semi-hardened steel, mild steel, aluminum and brass for these tests. I even drilled a cross-hole across the bore to simulate interrupted cutting. While there may have been small differences that I could not measure or detect, I basically found little performance difference between these inserts.
> 
> I am absolutely sure a turning center doing production runs will show an advantage for one over the others but on a hobby lathe that bores all sorts of non-production or one-off projects in multiple materials at relatively low speeds (3000 RPM max) I see little to suggest one grade is much better than the others. If I find one of these grades with the nose radius I want at a fair price then I buy them with confidence. Your experience and pocketbook may vary.
> 
> If you work with a lot of aluminum then inserts with a TiC or Al2O3 coating are said to have anti-welding properties that may help to reduce the incidence of a built up edge (BUE). I have not confirmed this myself … yet.
> 
> With regard to insert geometry, they are referring to the shape of the insert – diamond, triangular, etc. You will find that most zero lead bars and negative rake bars use triangular inserts, while most positive lead bars use diamond shaped inserts. Personally, I tend not to choose bars based on insert geometry/shape because it tells me nothing. I choose based on the bar lead angle that I need, the nose radius available on the inserts I am interested in and whether or not I can find a flat topped insert for that bar. Others have their own opinions, I’m sure.
> 
> I should mention one very popular geometry, the CCGT AK inserts intended for use with aluminum. These inserts are ground to a sharp edge and are uncoated. They have a very positive rake geometry that improves chip flow and reduces the incidence of a BUE in aluminum. Although they are intended for aluminum they work with harder materials, too, although the edge won’t hold up as well. It puts an excellent finish on stainless steel and brass. If you bore with an SCLCR bar you probably already know about this insert. If not, give them a try.
> 
> *Chipbreakers*
> The vast majority of boring bars use inserts with chip breakers … but not all. You would think that a chip breaker is a critical feature for a boring bar insert, especially since most inserts have them. However, when boring, chip formation and chip clearance is more a function of DOC and feed.
> 
> Feed too slow and you get a nest of stringy tangled chips, regardless of the presence or absence of a chip breaker. Feed at the right rate and you get coiled chips that readily eject from the bore, chip breaker or no chip breaker. Take too light a depth of cut and you’ll get strings in most materials, chip breaker or no. The presence or absence of coolant or cutting oil also impacts on chip formation and clearance. Cutting oil, all by itself, can turn stringy chips into coiled ones that clear readily, chip breaker or no.
> 
> I’m sure there are good reasons to have chip breakers on boring inserts used in high speed machines. My problem is that I don’t own one of those machines. What I’ve found is that chip breaker inserts usually require a relatively heavy cut for the chip breaker to work properly, and that can be problematic for a small bar to handle in harder materials. This heavier cut also requires a faster feed rate so power feed would be recommended, which is fine in a through bore but may not be a good idea in a shallow blind bore.
> 
> This has led me to avoid chip breaker-type inserts for my bars if I can. I prefer flat topped inserts with no chip breaker, although I admit they are harder to find and therefore can be more expensive. In my experience, they cut with lower cutting forces and work well with much lighter depths of cut. They are also the most rigid of the available edge configurations and are ground to a sharp edge. Moreover, with the right feed, even with a light depth of cut, I get coiled chips that eject readily from the bore, coolant or no coolant.
> 
> All the Circle Machine carbide bars that do most of the work in my shop can use flat-topped inserts and I chose Circle Machine specifically because of their intelligent approach to bar and insert geometry. Of all the bars I have owned and used, well over 20 inserted carbide bars at this point, these bars are easily the most accurate and I believe the insert has a lot to do with that.
> 
> I am not saying to avoid bars that use inserts with chip breakers. I’m saying that if you have a chance to bore a hole with a flat top insert, try it and make up your own mind.
> 
> *The insert’s Nose Radius (NR) is the single most important feature to consider when choosing inserts. The impact of the nose radius on boring bar performance is significant, perhaps more so than any other feature in your boring set up aside from your bar holder.*
> 
> Inserts are available in a wide range of nose radii. In general, *the most useful NR is 0.007-0.008” for general work*, while a 0.002” – 0.004” NR works well for very small bores or tight tolerance work. I have tried but no longer use a 0.015” NR cutter due to the relatively heavy cuts it required; they also deflect too much to be useful _to me_ for precision boring.
> 
> *NR effect on cutting forces:
> The bigger the NR the greater the tangential forces will be* so a smaller NR is preferable for smaller bars used in smaller holes where running out of insert clearance angle is a concern. Smaller NR inserts do limit finishes, however, so increasing speeds and slowing feeds may be necessary.
> 
> *NR effect on radial forces depends on the depth of cut.* *If the DOC is less than the NR then radial forces increase with DOC until it exceeds the NR, at which point radial forces are determined by the lead angle of the bar.* For example, if you have an insert with a 0.007” NR, then any depth of cut less than 0.007” will be less consistent and has more potential for chatter. However, once the depth of cut exceeds the nose radius by even a few thou the insert is very stable and will tend to cut consistently.
> 
> This also implies that a zero lead bar will cut with lower radial forces than a positive lead bar when DOC>NR. In practice, however, I have not noticed a major difference in accuracy with either bar geometry provided the depth of cut exceeds the NR.
> 
> This depth of cut thing also explains why a finish cut that uses a DOC<NR often produces a variable change in ID; radial forces are higher so we must determine what the impact of those forces is in a trial and either accommodate it or, if it is consistent, use it.
> 
> Radial cutting force is a real thing, as is the deflection it causes. If you have tried to come in on size by taking tiny cuts with an insert then you’ve already discovered that it doesn’t work well. Sometimes it cuts, sometimes it doesn’t cut and then it suddenly cuts too deep! Radial forces are the reason why.
> 
> *NR greatly affects your cutting conditions* (feed, speed and depth of cut are collectively called your cutting conditions).
> Recommendations for setting your roughing depth of cut vary by the manufacturer. They will list the recommended DOC, feeds and speeds for the insert when used with all common materials on the box the inserts came in or in their literature. Note that the manufacturer will not usually give you different DOC settings for roughing and finishing; they only give you one recommended DOC.
> 
> Practically speaking, the best general recommendation I have seen comes from Criterion, a leader in the boring bar/boring head industry, who recommends *NR+ 0.005-0.010” for a roughing DOC*; this provides better support at the tip to reduce chatter and works with all the inserts I own.
> 
> Note that the recommendations for roughing cuts are not huge cuts, even from the manufacturer. Instead, they are usually just deep enough to bury the nose radius in the cut. This stabilizes the insert to minimize deflection, reduce chatter and improve accuracy. You can go deeper but then tangential forces rise; everything is a tradeoff.
> 
> For setting a finishing DOC, things are less clear but in no case should you use a DOC less than 1/3 NR. Again, the reason for this is that radial cutting forces become too large at very small DOC. *A general guideline is to use a DOC no less than about 1/3-1/2 NR to finish.*
> 
> Note that a “finish” cut is not so much about the actual finish, although that is important. A finish cut is about coming in on size after you rough out the bore so we’re really talking about accuracy. When we take cuts smaller than the NR, radial forces are very high and you will notice that what you dial in is not what you get, but what you get will tend to be consistent if you adhere to this 1/3-1/2NR thing. The reason is pretty simple. Within this range, the nose radius is just deep enough to bite but not enough of it is engaged to push the nose of the cutter out of the cut excessively. As more of the nose is engaged beyond about 1/2NR, radial forces rise. As less of the cutter is engaged below ~1/3NR, the nose tends to skip or skate and will not cut. That 1/3-1/2NR range is just about right. The reason that it matters is this will be the minimum depth of cut the cutter will reliably take. You have to confirm it with your insert but it will be close to this range.
> 
> If you’re wondering why the heck we shouldn’t rough with less than the NR but we can finish with a cut less than the NR, the reason is that our feed is usually reduced with a finish cut (see the next bullet below) so axial and tangential forces are proportionally reduced. I know, it’s confusing but try it and see. A cut with 1/2NR DOC taken at roughing feeds may chatter but the same cut at a slower finishing feed won’t. Yeah, weird.
> 
> You will find that the most accurate cuts will be larger than the NR, which is why having a bar that can accept inserts with small nose radii is really important when you need a precision bore. For example, a 1/2NR depth of cut with a 0.008”NR insert amounts to 0.004”, which is double the NR of a 0.002” NR insert. Clearly, if your bar can take both nose radii, switching to the smaller one when you’re close to your finishing passes is not a bad idea.
> 
> Regardless of which insert you use, you have to run trials with your inserts to determine how light a cut you can take and still produce consistent results. This is really important to do, and you must do it for every insert and bar you own.
> 
> Once you find reliable settings, write down the bar, insert designation, the size of the NR, the DOC that works for both roughing and finishing, speeds, feeds and type of coolant used. These settings may vary with the material being cut so note any differences if this is so. Keep this valuable information somewhere accessible. One day, you will be 0.006” away from final size and you will know how to cut it.
> 
> *Feed rates are also closely tied to the NR*. A general guideline is that a feed rate of about 50% NR, in inches per revolution (IPR), works for roughing and about 25% NR works for finishing. Much depends on the horsepower of the lathe, DOC, material being cut, and overhang. In no case do we want to exceed the NR when feeding.
> 
> Most folks tend to feed too slow when boring, thereby producing long stringy chips that plug up the bore instead of ejecting out of the bore as they should. Ideally, you are looking for a feed that produces chips that looks like a coiled spring; if you see that, you’ve got it right and the feed will be faster than you expect. If you calculate a feed rate and it produces stringy chips, just increase the feed rate and ignore the calculations.
> 
> Quite often, us hobby guys get hung up on precision this or precision that. Lathe work teaches us that we need to respond to the cut and this especially applies to boring. You can set a reasonable speed and depth of cut but it is also perfectly acceptable to use a manual feed to produce the chip shape you want; you are not tied to the feed your gearing mandates. Just an opinion.
> 
> *What about speeds?* I suggest using the recommendations from the manufacturer of your boring bar insert. They will have the SFM listings for all common materials and you can calculate your RPM, then adjust feed according to how your chip form turns out. Studies suggest the correct speed is more important in prolonging insert life than feed rates but in my hobby shop on a manual lathe this may be a moot point, especially in softer materials, because the proper speed is often well above what my lathe can produce. Hence, I try for the best speed and feed to get the chip shape I want and I often vary both to get the results I want.
> 
> I also find the following table from Parlec a useful guide:
> 
> View attachment 284115
> 
> 
> Okay, I hope that covers the list. There should be enough information here to help you to sort through the manufacturer’s listings and decide on what you need.
> 
> It should now be evident why paying attention to cutting forces matters. Just about everything we think about when choosing and using a boring bar is influenced by these forces. Try to keep in mind that sometimes smaller is better and this applies to both the bar diameter and the nose radius you choose. You are also going to find that if you need to do precision work then you need to invest in good tools.
> 
> *The Bars I Use*
> 
> These are the bars I currently use the most. They allow me to bore from about ¼” ID out to as large as my chucks can hold, and from shallow to about 6.25” deep. So far, this has been enough for me.
> 
> View attachment 284113
> 
> 
> From Left to Right:
> 
> 
> 3/8” Kennametal carbide zero lead bar with a hex shank, chip breaker insert with 0.008” NR. This is my through-hole-hogger and it will flat out remove metal but it does so with consistent accuracy. Inserts last forever on this bar. It has a hex shank and I suspect this is why I can feel everything when it cuts.
> 3/8” Kennametal steel positive lead bar with a hex shank and 0.008” NR chip breaker insert. Used mainly for shallower blind bores in stainless steel because I like the tactile feedback I get from this bar in that material.
> 3/8” AR Warner steel zero lead bar with a hex shank that uses a HSS insert, 0.008”NR. I tend to use this when I need a critically accurate cut in aluminum; the HSS inserts will usually cut what I dial in with that material. It works well with stainless steel, too. Inserts are easily sharpened and re-used; my current insert has lasted over 10 years at this writing and cuts like new. I once broke all the rules and bored a 30mm ID X 4” deep hole with this bar and got an air bearing fit in aluminum. Trust me; this is a good steel bar.
> 3/16” Circle Machine CCMI positive lead carbide bar that takes grooving and threading inserts. A specialty bar that allows me to cut a thread relief and then thread a ¼” or larger Class 3b hole almost 2” deep. Coupled with the two CCBI bars below, it gives me the same capabilities as the three QCMI bars below that.
> 
> View attachment 284114
> 
> 
> 
> 3/16” and ¼” Circle Machine CCBI positive lead carbide boring bars that take both 0.007”NR and 0.002” NR flat topped inserts. These 4” long bars are the most accurate bars in my shop and give the best tactile feel of all my bars. Both use positive axial rake inserts that literally shoot coiled chips out of the bore. Although they are available, I do not use chip breaker inserts on these bars. These bars will usually take a 0.001” DOC when I use a 0.002”NR insert in most materials … very useful.
> 3/8”, ½” and 5/8” Circle Machine QCMI positive lead carbide bars that use 0.007” NR flat topped or chip breaker boring inserts. Each one also accepts grooving and threading inserts so they can bore a hole, cut a groove or thread relief at the bottom and then cut a thread all the way down the bore with great accuracy. All bars use the same inserts. These bars cut silky-smooth and chatter-free and are the most accurate larger bars I’ve used, even when compared to other top tier boring bars. Each one will take a 0.004” DOC with a 0.007” NR insert in most materials and cuts consistently.
> For thread relief and threading requirements below what that 3/16” CCMI bar can do, I use Micro 100 solid carbide bars. If I need to thread deeper than the CCMI bar can go, my QCMI bars take over. So far, I have not needed more than this to thread a bore.
> 
> If I were going to set up my hobby shop today and had to choose a good combination of bars for general use, I would choose these three Circle Machine bars:
> 
> 
> ¼”:  *CCBI-250-4-5R*
> 3/8”: * QCMI-375-6-5R*
> ½”: * QCMI-500-8-5R*
> I know I said that I cannot tell you which boring bars to buy but if you’re stuck for a choice then you might want to take a look at these bars. If you do wish to buy them, wait for them to come up on ebay. Otherwise, their cost is ridiculous.
> 
> It should be clear that no single bar will do it all. Most of us accept that we will eventually need multiple bars. Some of this is repetitious, on purpose:
> 
> 
> Please don’t saddle yourself with a set of bars with a bar and insert geometry chosen by some unknown guy unless it is exactly what you need.
> Before buying the bar, make sure the inserts and nose radii you want to use are available to fit it. Pay very close attention to the nose radius you choose because when boring, this matters a LOT. Again, for general work, a NR of 0.007-0.008” is most useful. For smaller bars, the ability to use an insert with a 0.002” to 0.004” NR is really important, especially if accuracy is critical for you. I highly recommend you use good inserts and download the catalog of your bar maker to be very sure you know which inserts to use for your application. The bar is important but the insert is more important; pick the right one.
> Be sure the bar is long enough to be used at the full extension the bar material is capable of while also allowing enough length to be held in the tool holder. For example, a ½” _carbide_ bar can be extended about 5” beyond the holder; you also need about 3” to go inside the holder, so the bar must be at least 8” long.
> To save money in the long run, please try to buy carbide bars. Steel bars work, no doubt, but you can bet you will buy a carbide bar of the same diameter at some point and that steel bar will sit in a drawer. Why? Because you will someday be limited by the bore size you need but it will be deeper than that steel bar can reliably reach. If it is a precision bore that must have a consistent diameter all the way down the bore then carbide is your best bet. You’ll buy that carbide bar and that steel bar will sit, you’ll see.
> Furthermore, it isn’t just that carbide gives you more reach; carbide bars cover more range. For example, a ¾” steel bar can theoretically bore to a depth of 3” but requires a starter hole almost 1” ID, while a 3/8” carbide bar will fit in a hole less than ½” for a much more useful range of possible bore sizes and depths. It will easily go deeper than that ¾” steel bar will and will likely bore more accurately at that depth due to its greater inherent stiffness. Moreover, smaller bars typically allow you to use inserts with smaller nose radii, and this is a big deal when accuracy counts.
> 
> Bottom line: Carbide bars simply allow you to do more with fewer bars, do it more accurately and ultimately for less money. And you’ll have more drawer space!
> 
> Far and away, the cheapest place to buy boring tools is eBay. If you know what you want and are patient, you will eventually find what you need at a fraction of their retail prices.
> 
> 
> 
> *Putting it all together*
> Choosing a bar/insert combination is just half of the equation. The way we use the bar is the other half and it is arguably the more important half. Rather than just a tips and tricks approach, I have chosen to use a boring checklist that the newer guy may find more useful. I’ll throw tips in where they seem applicable. Cutting conditions may be specific to your insert; check the box the inserts came in or the maker’s catalog.
> 
> 
> Do all your calculations first – your anticipated initial roughing and finishing DOC and feed rates. If you already know the optimal depth of cuts your bars can take then you’re way ahead. Speeds and feeds are usually on the back of the box your inserts came in. Don’t get married to these calculations. You will need to adjust to the cut as you bore; this especially applies to feeds.
> For soft materials like aluminum it is likely that the required speeds will be faster than your lathe will go. Just run at max
> speed and alter feed to give you the chip shape you need.
> 
> 
> Know your final ID, tolerances and finish requirements if there are any. I always bore to the tightest tolerances I can. It takes me no longer to be careful than it would to be sloppy.
> Zero your mic and place it in a stand or padded vise. Lay out your hand tools. I always do some trial runs with my telescoping gauge and ring gauge to be sure my technique is up to snuff and that I’m able to perform consistently. Want to bore accurately? Learn to measure accurately!
> Mount your bar into the holder and set the extension to the bare minimum for the bore depth you need. If you have to start with a smaller boring bar to open up the starter hole, change to a larger bar as soon as the bore ID allows.
> Align the insert so the top of it is horizontal when viewed from the tip (this is called zero radial rake), and lock down the bar in the holder.
> 
> Put your tool holder on the tool post and set your cutting height about 0.005-0.010” above the lathe centerline to offset tangential forces. If your bar is ¼” or smaller, start at 0.010”; if larger, you can go a bit lower. Adjust this setting if the bar rubs.
> 
> 
> Set your lathe depth stop for the bottom of your bore if doing a blind bore. It is wise to mark your bar with a Sharpie so you know when you are approaching the bottom of the bore. Then you can shut off power feed before slamming into your carriage stop. If this is your first time boring, I suggest doing through bores until you get used to the process. Just don’t bore into your chuck!
> I would keep the boring bar square to the bore for most jobs (the bar parallels the ways). For shorter through holes you can angle the bar (tip toward the wall) to reduce radial forces further but the hole has to be big enough to allow it without the shank of the bar rubbing. Keep this under 15 degrees if you do it. This is most useful for solid bars with tapered shanks; I don’t feel it makes a significant difference with inserted bars.
> With the work piece faced and squarely locked into the chuck, drill your starter hole. Until you know how your bars like to cut it is wise to drill smaller and leave room to work. Skilled hobby guys who know their bar well can drill the starter hole closer to the finished ID, within about 0.030” to 0.050”, so that all that is needed is a finish pass or three. You must have enough room to take a cleanup pass and your finish pass – know how your bar likes to cut!
> Sometimes we need a small through hole at the bottom of the bore but the bore itself might be rather large, with a flat bottom. In that case, drill the small hole through, then drill a larger hole for a bigger boring bar but stop just short of the anticipated depth of the bore. As you bore you can clean up the bottom.
> 
> 
> Decide on whether or not to use coolant/cutting fluid. You may choose not to use coolant but don’t go without it and then shoot a bunch of coolant in the bore for a final pass – you may cause thermal cracks in your insert. Either use it from the start or don’t use it at all.
> Although it is common in industry you don’t need to flood the bore – just coat the walls and go. I use a syringe with a blunted needle and apply cutting fluid with the lathe running (before making a pass, obviously), from the bottom of the bore on out. Note also that cast iron, bronze, brass and plastics do not require coolant.
> 
> By the way, coolant also reduces the incidence of a built up edge (BUE), which is thought to be welding of the material being cut onto the tip of your insert; this totally stops the cutting action of the tip. This tends to occur with softer materials like aluminum and greatly increases cutting forces and may lead to breakage of the insert. You cannot usually just pick this deposit off. If I develop a BUE I make an entrance to the bore without coolant at a roughing feed rate and this sometimes knocks it off. If that fails, and it usually does, I index the insert to a fresh tip and boost feed rate. When you resume, it may take a pass or three before the insert cuts consistently.
> 
> 
> Start the lathe and bring the tip of the cutter into gentle contact with the wall of the starter hole, withdraw the bar and zero your cross slide handwheel. Remember that you are backing out the handwheel as you cut deeper. Set DOC to your calculated roughing value and make a pass to full depth at a feed rate that you calculated. The goals on this first pass are: 1) Find out how your cutter height setting is working out; if there is rubbing or chatter then make a slight adjustment (usually higher); 2) See if the DOC you set allows you to maintain a steady feed rate and produces the chips you want. Listen to the cut – it will hiss or sizzle when it’s right. If feeding manually, feel the cut – it should feel smooth and steady, with a slight positive resistance to the feed. Adjust your feed to get the chips you want.
> Make a pass all the way in, then stop the feed and move the cutter away from the wall of the bore. Back out, reset the handwheel to your next cut and repeat the pass.
> 
> Once you know that your roughing cut settings work well, measure the change in ID those cuts produce. For example, say your DOC is 0.010” deep and this produces an increase in ID of 0.019”. You can either write it down for reference or adjust the DOC to produce a number you prefer to work with. In either case, be sure the DOC remains consistent from pass to pass and that the change in ID is also consistent. This will become very important when we set up for a finishing pass.
> 
> Making a roughing pass at NR+0.005-0.010” works for most materials. I suggest restraint in taking really heavy cuts unless you can keep up with the feed rate. If you can’t, the finish usually gets mucked up and you get all sorts of irregularities in the bore that will have to be cleaned up. The advice commonly offered on the forums in this situation is to run the bar in and out several times to take off the irregularities caused by “spring” in the bar. If you do this be sure you have a lot of room because you won’t know where the bore ID will end up. You may also chip the insert by running it out of the bore. The need for this sort of thing can be avoided by resisting a cut you cannot keep up with. Boring is not a race; it’s about accuracy. Finding a roughing DOC that works well and making successive passes at that setting is actually the quickest and most accurate way to bore on a lathe.
> 
> As mentioned, carbide inserts do not like when you drag them out of a cut; to demonstrate this they will sometimes chip on you. It also wears the insert prematurely. I suggest you make a cut all the way in, then move the cutter away from the wall before you withdraw for the next pass. Try it and see – it’s *much* more accurate this way, and your expensive inserts will live a lot longer.
> 
> Inserts also do not like when you are tentative or if you stop on entrance to the bore; they can chip on you if you do this. Start your carriage moving at the correct feed rate and maintain it as the bar enters and proceeds down the bore.
> 
> 
> Once you know that your roughing passes are going well and that all settings are okay it is a good idea to find out what your finishing pass requirements will need to be. We want to deal with those pesky radial forces now, not when we are a few thou short of final size.
> First, clean and measure the bore. Zero your hand wheel, then dial in your calculated finishing DOC. Make a pass at the finishing feed rate and then measure the bore.  You are looking for a DOC that will give you a change in ID that you can work with. For example, say we are using an insert with a 0.002”NR; if you find you need to dial in a DOC of 0.0025” to produce an increase in bore ID by 0.004” then you can rough the bore until are 0.004” away from final size and dial in your 0.0025” DOC to make the final pass. OR, say we are using an insert with a 0.008”NR and dialing in a 0.004” DOC produced a change in ID of 0.0075”. Again, you would rough until you are 0.0075” away from final size and then dial in your finish cut. This actually does work rather well.
> 
> We know that a carbide insert does not cut accurately when we take too light a cut due to increases in radial forces. When making finish passes trying to split hairs to come in on size rarely works; it usually just makes things worse. Find out how your bar likes to cut with a reasonable finishing DOC, as above, and you’ll come in on size – I mean dead on - more often than not. If the finish is not as nice as you need then try increasing speed and decreasing feed a bit. If that fails then you need a larger nose radius.
> 
> 
> Once you know what your finishing DOC is, continue to rough out the bore until you are within about 0.030-0.050” of final size. You need to slow down and adjust your cuts until you reach the point where you can precisely dial in the finishing pass that you determined earlier. This is why you need to know how your roughing DOC correlates with the increase in ID … so you can get to that point. Measure carefully and often and adjust your roughing cuts to get you to the start of your finish pass as closely as you can. Then make your finish pass.
> When I have a critical bore to do, I usually adjust my roughing cuts until I am two finishing passes away from final size. Then I make a pass with my finishing DOC and feed rate and check it again. If I am on track then I make the final pass. If I am not on track I can make a small adjustment to come in on size. The reason for being so careful is laziness; I don’t like to bore a piece twice and the small amount of time it takes to make those two final passes is nothing compared to the time it takes to bore it all over again.
> 
> The other thing I do with a critical bore is to stop and let the part cool to ambient temperature before measuring the bore for a final pass. Metal expands when it heats up and boring can really heat things up, especially with heavy roughing cuts. Allowing the metal to cool down will give you a truer reading of the ID and you can then make a more accurate finish pass.
> 
> 
> Before withdrawing the cutter after your finish pass leave it at final depth and feed the bar in toward center to clean up the bottom of the bore. Be careful not to run the tip of the cutter much past center in a closed bottom bore – that can chip an insert, too. (Wow, lots of things can chip a cutter!)
> 
> 
> Your finished piece should be at the precise depth and ID you need with a finish suitable for your purposes. Just chamfer or ease the edges of the ID and OD and you’re ready to part off.
> 
> This looks like a lot but it really isn’t – it just looks that way in words. I do all of these things and more without even thinking about it when boring a hole. Do a few practice bores and you’ll see that it really is a simple process. Once you know how your bars and inserts like to cut and you can measure consistently and accurately, you will find boring accurately to be just another job and not the obstacle it once was.
> 
> *End Notes*
> 
> Experienced hobby guys know that I left a lot out of this piece but hey, I did also leave out any trace of math! I think this is the meat though, and it should get the new guy started.
> 
> For new guys: Practice! As I said earlier, boring is a planned approach to a precision result and boring accurately requires you to think and plan; it takes some time to learn how to do that. Once you learn how to work with your bars and discover how they like to cut the whole thing becomes sort of anticlimactic and you’ll wonder what the fuss was all about.
> 
> I suggest using 12L14 mild steel to practice with, not soft stuff. 12L14 cuts easily and will teach you when your speed and feed rates are right by giving you a good chip and a nice finish. This material is hard enough to give you reliable cuts that produce an accurate ID so you can learn to plan your cuts. It will also teach you the sound and feel that tells you when the cutter is working right. And it’s cheap!
> 
> Finally, let me re-emphasize that Bubba, your boring bar holder, is the foundation of your boring set up. You can have the finest boring bar in the world but if you put it into a holder that doesn’t keep it absolutely solid then that bar cannot do its job. In my opinion, *the holder is more important than the bar* and I strongly urge you to buy or make one that works on your tool post. You’ll see … a good holder will transform your boring experience.
> 
> It is my sincere hope that this helps you get started. What I’ve written here may not be right but it has all been confirmed to work for me in my shop. I hope it works for you as well.
> 
> 
> 
> Mikey
> 
> September (2005, 2009, 2011 and finally,) 2018!
> 
> 
> 
> *A brief case in point: An exchangeable tip live center for a Sherline lathe.*
> 
> View attachment 284117
> 
> 
> I wanted to illustrate how the information we’ve discussed was used to bore an accurate hole.
> 
> The live center above was made for my Sherline lathe and is a good example of a project we run into in a hobby shop. The OEM Sherline live center it replaced has 0.0008” TIR, which is unacceptable for close tolerance work. This one, patterned after a Royal live center, has zero TIR with the short tip and less than 0.0001” TIR with the extended tip so it works pretty well. The bearing housing and arbor are 1144 Stressproof steel and the tips are hardened and tempered O-1 steel. All work was done on a Sherline lathe.
> 
> I chose to install the Nachi double-row sealed angular contact bearing used for this project with a line-to-line press fit, which means the ID of the bearing bore must be the same dimension as the OD of the bearing so as not to impinge on internal bearing clearances once the bearing is installed. A fine finish is usually required for these fits. Recommended tolerances were +/- 3-5 microns, which I cannot even measure directly, so this was one of those projects where it took several hours just to get to the boring stage and you either hit the bore dead on or you get to start all over again.
> 
> The arbor that fit inside the bearing required the same fits so I used a sharp HSS tool to cut the arbor.
> 
> I used a 3/8” QCMI Circle Machine carbide bar with a 0.007” NR flat-topped grade CG5 insert to rough the bearing bore in the housing. The 0.007” NR insert roughs very accurately but I preferred not to risk taking fine cuts at these tolerance levels with it so I switched to a ¼” Circle Machine CCBI carbide bar with a 0.002” NR flat-topped CG5 insert to finish. I used the smaller bar because a 0.002” NR insert is not available for the 3/8” bar. From previous experience, I knew that the smaller insert usually cuts very accurately if I use a depth of cut that is two times the NR. This fully supports the nose radius and allows it to cut consistently and without chatter. While the smaller bar is more subject to deflection just based on the diameter of the bar, the bore depth was well within the extension limits of the smaller bar. I also expected that the planned finishing depth of cut would shift radial forces toward axial to further limit deflection so it was an acceptable option.
> 
> When I was a bit more than one roughing pass away from the anticipated starting point for the finishing passes I cooled the part to ambient temperature and measured it, then took my final roughing pass. I then switched to the 0.002” NR insert that allowed me to take two 0.004” deep finishing passes, measuring once more between them. The finish was excellent despite the tiny nose radius and the bore came in exactly on size.
> 
> 
> 
> Mikey
> 
> December 2018
> 
> 
> 
> 
> ​


Thank you for the primer.


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## WobblyHand

Don't know if this is the right place to ask, but, I was boring some 1045 with a QCBI 3/8" bar.  Used a new insert with NR = 0.002". Made a boring bar holder of 7075 for my mini-lathe.  Was taking 0.010" off the radius.  Chips were nice.  Finish is satin and a little rough looking.  Is this normal for 1045?  Lighter cuts 0.002" seemed to have the same finish.


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## WobblyHand

Not a Circle Machine bar, but is this a good deal for a small carbide boring bar?  eBay 6mm solid carbide bar  Seems to use CCMT06 style inserts.


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## rwm

That shaft is metric (obviously) so you may need to make a sleeve. All my holders are Imperial.
Robert


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## WobblyHand

rwm said:


> That shaft is metric (obviously) so you may need to make a sleeve. All my holders are Imperial.
> Robert


Understood.  My shop made boring bar holder is for a 0.500" bar.  I made a sleeve for my 0.375" bar out of 12L14.  Need the right drills and a 6mm reamer, (for the 6mm bar) but sleeves are really easy to make.  6mm reamers are pretty inexpensive.


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## soniamann

This is very useful information, thank you for sharing it.


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## Stuartd

As a new model machinist this is a fantastic article to help. so I need more than 1 boring bar


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## VicHobbyGuy

Excellent article, THANKS!
It will help me to pick among the bars I can afford. One of those recommended Circle bars retails for more than I paid for my lathe!


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## mikey

VicHobbyGuy said:


> One of those recommended Circle bars retails for more than I paid for my lathe!


Yup, boring can get expensive. Wait for one to come up on ebay ...


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