# Boring bar sizing



## wildo (Mar 10, 2017)

I would like to bore a very small hole. I want to press fit a .1875 ball bearing (±2.5 micron) into brass as an interference fit. Engineer's Edge shows that I would want a max of .0014" and a min of .0002" for an interference fit, though it doesn't state the material. I would think that it would matter if it was steel vs brass vs aluminum, etc. So I guess I'd want to bore a hole to .1865" (Honestly that doesn't seem like much of an interference to me, but what do I know.)

I found this boring bar on ebay that states:

 Extended Reach Micro Boring Bar Uncoated 0.0600in Diameter 1/8in Shank 0.250in LOC 1 1/2in OAL Series MBE (MBE-0600.250)

Can you guys help me understand what some of those terms mean? Can I assume that .060" diameter means that the cutting edge is probably half that- .030" in order to cut that radius? The shank is .125" so how can you cut a hole smaller than the shank? Wouldn't the bottom of the shank interfere with the hole as you cut? And finally- are these boring bars meant to plunge in, or does one drill a small hole first?

Thanks!

[EDIT] - since this is a manual bench lathe and not a $50000 super rigid CNC lathe- probably a boring bar as close to nominal size as possible would be better in order to not instantly break it, yes? If I'm understanding the diameter measurement correctly, then perhaps this .1800" diameter boring bar would be a better option. Really, the question remains on if the diameter spec means what I think it mean.


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## RandyM (Mar 10, 2017)

Personally, I'd think you'd be better off reaming the hole rather than boring it. That is a really small hole to bore accurately. Are you doing this on a lathe or milling machine?


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## wildo (Mar 10, 2017)

On a lathe- and it will be a blind hole most likely. The hole depth will be just slightly more than the bore diameter- around .190" or so.


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## RandyM (Mar 10, 2017)

The .180 diameter bar is going to be way to big for you to use. The ad states it is .180 in diameter, you still need to allow for the protrusion of the cutting edge. Is there any way you can just drill it oversize and glue the ball in, or even just to size?


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## wildo (Mar 10, 2017)

Well that's what I'm trying to learn. If I need to bore a hole .186, then it's unclear to me why a .180 bar wouldn't work. A #14 drill bit is .182" so I would think you could drill the #14 and then use that bar to bore to .186. Is my thinking wrong? I don't know how the terminology works for boring bars, so that's what I'm asking about.

I need the ball bearing perfectly on center. Drilling oversize and gluing will not work for my application. Actually, I'd prefer an interference fit and no glue.


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## mikey (Mar 10, 2017)

wildo said:


> Can you guys help me understand what some of those terms mean?



The shank is 1/8" in diameter; this is the part that must fit into your boring bar holder. The "0.0600" part should be the size of the cutting tip. You will need to drill a hole larger than this to begin boring. I should think a #45 or #46 drill should be large enough. Its hard to tell from the pic but this looks like a positive lead bar, which means the end of the bar is angled so that the tip leads the way; this allows you to cut to a flat bottom without rubbing, among other things. 

This tool has a neck 0.250" long so that is your limit for the deepest bore you can make. The overall length of the tool is 1-1/2". 

You will need a sleeve to use this in your boring bar holder. It is solid carbide so it should be sharpenable with a diamond stone. 

Another brand to look at is Micro 100 - their bars are top of the line and is the brand I prefer for these tiny bars.


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## wildo (Mar 10, 2017)

Mikey- that is exactly the info I was looking for! Very helpful- I can now find the appropriate sized bar with this info. Much appreciated!


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## 12bolts (Mar 10, 2017)

Yeah the part of the bar on that ebay listing that is round is probably only 040"-050" in dia. Want to be careful using that tool. A reamer would be my choice

Cheers Phil


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## wildo (Mar 10, 2017)

A 6" vise is overkill for a RF45 style mill. A 4" is more appropiate for that size mill & IMO 5" max. I have a 5" GMT vise on my PM45 & it's slightly too big. Not enough Y axis travel to make use of the 5" full capacity. Better to save your money rather than getting something too big & most importantly the weight. I take my vise of the table quite often, a 6" is still light enough for me to be carried by hand but I'm glad I have a 5". I also have a 4" vise as well. I prefer the 5" though.

But those GMT 6" Premium vises are pretty nice. I'd love to have one but don't need one on my current mill. But if you plan on upgrading to a full size knee mill in the future than the 6" will be perfect.


Here's what the 5" looks like on my mill.




I couldn't even complete this cut without my bellows & DRO scale getting in the way. Not enough Y travel & the 5" vise is not even maxed out.




Here's what a 6" vise looks like on another PM45 (gt40's)
View attachment 253544


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## wildo (Mar 10, 2017)

Oh- I guess I have misunderstood the shank diameter a bit. In my illustrations, the circle would represent the necked-down part of the shank, in which the necked down portion is less than the cutting diameter. The shank would be the area that fits into the tool holder. Ok, pretty sure I got this. The main point is that with the cutting edge on center of the actual shank, then the necked portion can be nearly as large as the drilled hole, not half the size as I improperly thought in the first illustration.


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## RandyM (Mar 10, 2017)

Maybe this will help.

Boring Bar Basics


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## mikey (Mar 10, 2017)

It isn't just about fitting the tip in the hole - you gotta' be able to get the chips out. Initially, it can be a tight squeeze but as the hole opens up the chips eject easier.

No, the tip is not on the centerline of the work. It is raised about 0.005 - 0.010 above the centerline to offset tangential forces.

Normally, the flat on top of the tool is horizontal; this is called zero radial rake and allows for as much clearance as the relief angles under the tool provide. This relief under the cutting edge is important because as tangential cutting forces are generated the tool is pushed down and may contact the work. This is why you cannot use too large a tool in a tiny bore; you need to account for cutting forces and how they will affect the tool. This is also why carbide makes the best material for boring bars. It's modulus of elasticity is the stiffest of the common boring bar materials and will be least affected by tangential forces.


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## jbolt (Mar 10, 2017)

.1875" is not that small of a hole. A .0005" to .001" interference fit is about right. Much over that the bearing may get tight depending on the wall thickness around it.

I would use* this* boring bar. You need a little clearance for chips. Pre-drill with a spot drill then a #14 or #15 drill and then bore to final dimension. If I were doing a lot of these I would put the compound at Zero (parallel to the x-axis), keep the Z axis snug, lock the x-axis, put a 0-4-0 TDI against to tool post and use the compound to repeatably set the final bore depth. 

I would not use a reamer unless your tail stock/drill chuck is dead nuts concentric to the spindle. Also most reamers have too large a chamfer for small bearings to seat fully to the shoulder.

In a pinch you could use a one or two flute 1/8 in end mill for a boring bar.


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## wildo (Mar 10, 2017)

Just so much to learn! Honestly, it's all a bit overwhelming. You guys are really a wealth of info. Much appreciated guys!


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## willthedancer (Mar 10, 2017)

I would just grab a 1/4 " tool bit and grind a bar out of it. For that depth it will be easy. Just FYI, Bronze tnds to spring back a bit when cutting or pushing a ball into it. You don't need a lot of interference.


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## Wreck™Wreck (Mar 10, 2017)

Boring bars are the easiest tool to size.
If it will not fit into the hole it is to big (Minimum bore diameter) if it will not cut to depth it is to short (Depth/Length of cut)  if it still does not perform and both of these parameters are met it will then take a bit of work.

A good place to start with a carbide tool at that diameter would be 200 SFM in aluminum or free machining brass, about 4000 RPM's for a bore that shallow.
How many parts is the big question when buying such tools.

As a sidenote, you will find that measuring such a feature will cost more then the tools used making it. You do have a dial bore gauge for that diameter do you not?


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## wildo (Mar 10, 2017)

Wreck™Wreck said:


> A good place to start with a carbide tool at that diameter would be 200 SFM in aluminum or free machining brass, about 4000 RPM's for a bore that shallow.



4000 rpm!? Is that correct??


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## wildo (Mar 10, 2017)

A 6" vise is overkill for a RF45 style mill. A 4" is more appropiate for that size mill & IMO 5" max. I have a 5" GMT vise on my PM45 & it's slightly too big. Not enough Y axis travel to make use of the 5" full capacity. Better to save your money rather than getting something too big & most importantly the weight. I take my vise of the table quite often, a 6" is still light enough for me to be carried by hand but I'm glad I have a 5". I also have a 4" vise as well. I prefer the 5" though.

But those GMT 6" Premium vises are pretty nice. I'd love to have one but don't need one on my current mill. But if you plan on upgrading to a full size knee mill in the future than the 6" will be perfect.


Here's what the 5" looks like on my mill.




I couldn't even complete this cut without my bellows & DRO scale getting in the way. Not enough Y travel & the 5" vise is not even maxed out.




Here's what a 6" vise looks like on another PM45 (gt40's)
View attachment 253544


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## Wreck™Wreck (Mar 10, 2017)

wildo said:


> 4000 rpm!? Is that correct??


Yes, that is the low side however, 200 SFM is low for such materials with carbide tooling.
Read this: http://www.bravobronze.com/info/blog/details/use-our-c360-free-machining-brass-machining-guide
300-1000 SFM recommended for 360 brass, I calculate by hand but a handy cutting speed calculator may be found here.
http://www.carbidedepot.com/formulas-turning.htm

I do not use such speeds on lathes with jawed chucks because the jaws turn the coolant into mist.

Currently running 3/8" steel rounds, face turn and groove on both ends, 1500 RPM's in an 8" 3 Jaw chuck. A collet chuck does not fling coolant everywhere

500 parts 33 3/4" long. I could turn them at 3000 RPM's but it would mist the entire shop in coolant from the chuck/fan. Also a .5" deep 10-32 thread in one end, estimated lathe time is 91.4 hours.


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## Wreck™Wreck (Mar 10, 2017)

A 6" vise is overkill for a RF45 style mill. A 4" is more appropiate for that size mill & IMO 5" max. I have a 5" GMT vise on my PM45 & it's slightly too big. Not enough Y axis travel to make use of the 5" full capacity. Better to save your money rather than getting something too big & most importantly the weight. I take my vise of the table quite often, a 6" is still light enough for me to be carried by hand but I'm glad I have a 5". I also have a 4" vise as well. I prefer the 5" though.

But those GMT 6" Premium vises are pretty nice. I'd love to have one but don't need one on my current mill. But if you plan on upgrading to a full size knee mill in the future than the 6" will be perfect.


Here's what the 5" looks like on my mill.




I couldn't even complete this cut without my bellows & DRO scale getting in the way. Not enough Y travel & the 5" vise is not even maxed out.




Here's what a 6" vise looks like on another PM45 (gt40's)
View attachment 253544


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## mikey (Mar 10, 2017)

Wreck™Wreck said:


> Boring bars are the easiest tool to size.
> *If it will not fit into the hole it is to big* (Minimum bore diameter)* if it will not cut to depth it is to short *(Depth/Length of cut) _* if it still does not perform and both of these parameters are met it will then take a bit of work.*_



Great advice, Wreck - made me smile!!


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## jbolt (Mar 10, 2017)

I would make go and no-go gauge pins to do the measuring.


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## Wreck™Wreck (Mar 11, 2017)

This would require making  .1873  No Go and .1860  Go pins, fussy work with a lathe.
This also does not tell you the bore size just that it is within the range.


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## AGCB97 (Mar 11, 2017)

RandyM said:


> Maybe this will help.
> 
> Boring Bar Basics



In the last table of this document, what is a' heavy metal shank'?


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## mikey (Mar 11, 2017)

AGCB97 said:


> In the last table of this document, what is a' heavy metal shank'?



Haven't read the link but a heavy metal boring bar is usually made of tungsten. They are between steel and carbide in modulus of elasticity (stiffness) and cost. They're intended to give you a bit more overhang capability at a lower cost than a solid carbide bar.


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## jbolt (Mar 11, 2017)

Wreck™Wreck said:


> This would require making  .1873  No Go and .1860  Go pins, fussy work with a lathe.
> This also does not tell you the bore size just that it is within the range.



He does not need to measure the hole. He only needs to be within the fit tolerance for the bearing. 

I don't find making pin gauges that much trouble. For those who do not machine for a living its not always practical to spend that kind of money on bore gauge with a limited measuring range for a few parts.


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## Wreck™Wreck (Mar 11, 2017)

jbolt said:


> He does not need to measure the hole. He only needs to be within the fit tolerance for the bearing.


I realize this, I simply like having a number so that there is a basis for the next move if any. - .0014 is a considerably tight bore for an annular bearing of that size.


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## JPigg55 (Mar 11, 2017)

mikey said:


> The shank is 1/8" in diameter; this is the part that must fit into your boring bar holder. The "0.0600" part should be the size of the cutting tip. You will need to drill a hole larger than this to begin boring. I should think a #45 or #46 drill should be large enough. Its hard to tell from the pic but this looks like a positive lead bar, which means the end of the bar is angled so that the tip leads the way; this allows you to cut to a flat bottom without rubbing, among other things.
> 
> This tool has a neck 0.250" long so that is your limit for the deepest bore you can make. The overall length of the tool is 1-1/2".
> 
> ...



Hey Mikey, sent you a PM. How goes the "PROJECT". Any new updates you're willing to share ???


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## cmantunes (Mar 12, 2017)

RandyM said:


> Personally, I'd think you'd be better off reaming the hole rather than boring it.



What Randy suggests is what I would do. With the tailstock properly aligned, I'd drill a 11/16 hole (3/16 - 1/64) and then ream the hole with a 0.1865 (3/16 - 0.001) reamer. Done!


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## mikey (Mar 12, 2017)

Might be an interesting exercise to ream a hole only 0.095" deep (I assume just less than half of the bearing ball will protrude) with a 3/16" undersize reamer. Not sure the cutting edges would even engage at such a shallow depth. I think the only way to make this hole is to bore it. That isn't the challenge; the challenge is to measure it accurately with the equipment a hobby guy has available. I think this is going to come down to a go/no-go gauge.


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## francist (Mar 12, 2017)

I'm wondering if there would be enough material in the design to allow drilling the hole greater than the finished depth so as to facilitate use of a reamer, but then insert a short plug in the hole to keep the ball from being pressed in too deeply. I believe we're talking about making a spinning top, yes? There should be ample material to support an over-depth hole.

-frank


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## cmantunes (Mar 12, 2017)

mikey said:


> Might be an interesting exercise to ream a hole only 0.095" deep



With a flat-bottom reamer. You can buy one or grind the bottom of a typical chamfered one.


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## mikey (Mar 12, 2017)

I guess I don't know much about reamers. I've never seen or heard of a flat-bottomed reamer. I was under the impression that all reamers required a chamfer to enter a hole that is smaller than the reamer so grinding it off might be interesting.


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## wildo (Mar 12, 2017)

francist said:


> I'm wondering if there would be enough material in the design to allow drilling the hole greater than the finished depth so as to facilitate use of a reamer, but then insert a short plug in the hole to keep the ball from being pressed in too deeply. I believe we're talking about making a spinning top, yes? There should be ample material to support an over-depth hole.
> 
> -frank



Yes, we're talking about top making. Sure, anything is possible, and with enough skill that would certainly be an option. However, to get those insanely long spin times (+15 mins or so) you have to have the utmost in balance. To me the idea of a plug like that sounds a lot like an operation that could introduce some balance issues. I'd prefer to just get good enough at boring to be able to do it as a bore.

I will note, however, that one could drill a hole deeper than needed, ream it to size, press the ball to the bottom, and then turn off the extra material exposing the bearing. I found this video last night that did just that.






That top spins for around 8-10 mins, as seen here:


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## cmantunes (Mar 12, 2017)

mikey said:


> I've never seen or heard of a flat-bottomed reamer.



They aren't the most common thing in the world but they are out there. For example: http://bondeproducts.com/index_files/FBRHLSET.htm


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## mikey (Mar 12, 2017)

wildo said:


> I will note, however, that one could drill a hole deeper than needed, ream it to size, press the ball to the bottom, and then turn off the extra material exposing the bearing. I found this video last night that did just that.



There's an idea, and not a bore gauge in sight, too.


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## wildo (Mar 12, 2017)

mikey said:


> There's an idea, and not a bore gauge in sight, too.



Ring gauge, boring bar, and boring bar holder purchased. I'm all in now!


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## cmantunes (Mar 12, 2017)

wildo said:


> I found this video last night that did just that.



Wow, insanely beautiful! Nice fine.


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## mikey (Mar 12, 2017)

cmantunes said:


> They aren't the most common thing in the world but they are out there. For example: http://bondeproducts.com/index_files/FBRHLSET.htm



Hmm, this is interesting. A reamer is guided into and down a hole by the chamfer on the front edge; otherwise, it will tend to drift and cut oversize. These reamers lack that chamfer and looks like they would cut on contact. It would be interesting to see how accurate a hole they cut and if they would produce an interference fit as required here, assuming you could special order one to the right specs. 

I've reamed hundreds of holes over the years and have never seen a reamer like this, cmantunes. I will admit that they exist; I just don't see the logic.


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## Randall Marx (Mar 13, 2017)

I wonder about drilling deep enough to use a reamer, then ream to the right depth. The step in the hole would stop the bearing from getting pressed too deep. I don't know and have not tried it, but it might be a workable idea.


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