# Thread cutting dilema?



## porthos (Aug 17, 2018)

cutting my first thread on a south bend heavy 10. before this i cheated and used dies. the thread to be cut is 1.00-14. material is TGP something (i forgot to mark what it is; bought it about 3 years ago). turned the TGP down from 1.250 to 1.00. using a brand new  factory insert from A.R. Warner. watched Tubalcain video. adjusted lathe properly. started the cut, did everything right; and from the beginning the cut was very rough and finished rough; and, the nut won't go on. Oh, by the way, this first piece is a "practice piece". so, thinking that it is the steel, i went to a piece of 12L14. same results. the od of the round was down to .985. think, think think. tubalcain's video states to set the compound at 29.5 degrees, which i did. AND, to feed with the compound which i did.  BUT, SouthBend book "HOW TO RUN A LATHE" says to feed with the cross slide. i did this first with the leaded steel; perfect threads. next did it on the excess length of the TGP. perfect threads. what is happening here????. its much eaiser to feed the cutter by using the compound and not the crossfeed dial.  oh, after reviewing my post; i didn't mention that the thread cutter is also AR Warner thread cutting insert


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## Dave Paine (Aug 17, 2018)

If the plunge cut with the carriage created a good thread, but the compound did not create a good thread, it feels like the compound was not set to the correct angle.   I have made this mistake myself.

Many threads on the forum about threading issues, one example.

Threading help on HM


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## SCLead (Aug 17, 2018)

Your thread cutting tool is absolutely square to the work, and on center (height-wise)? 

The purpose for 29.5* compound is because you then only cut on one face of the tool. Tough to explain clearly, but if you look at the compound from directly above, the angle of movement should be DIRECTLY in line with the left/headstock cutting edge of the tool (ok, not directly. 0.5* angled "into" the work). The purpose for this, instead of a solid 30* cut, is so the right/tailstock side of your tool doesn't contact the thread _at all_, and in fact has a wider clearance nearer the toolpost than at the tip of the tool. When you feed with the compound, you're cutting the full thread profile (up to that depth) with the left side of the tool, which theoretically reduces chatter as opposed to cutting the full thread profile on both sides of the tool simultaneously, where chips will be constantly crashing into each other on top of your tool. The SB manual likely "predates" this concept, which is to say feeding with the cross feed worked perfectly well for many people, for many years. If you find it works better for you, there's no harm in using this method.

So if you're getting better threads cutting with the cross slide, I would first look at your setup. Tool is perfectly square to work. Tool is correct height - on center is the "one size fits all" answer. Everything is tight - tool is tight in post, post is tight on machine, compound angle is tight, cross slide gibs are tight. You shouldn't see a ton of springing with 1" OD, but are you supporting the tail end of the work with a center or anything? What's your cutting speed? You're not trying to take hog-depth cuts, right? Double check the angle on your compound, you're not trying to take 29.5* or 30.5* cuts, are you? I know I've mixed up a couple compound angles in the past.


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## MarkM (Aug 17, 2018)

If it s an asian lathe the angle setting is different then an american machine.  The 0 degrees has your compound in line with your shaft.  So if so it may be set closer to sixty then thirty.  An american machine 0 degrees has your compound at 90 degrees to your shaft.  Whatever you set your angle tomyour shaft being threaded. Also it s not critical to be exactly 29.5 degrees. Just less than 30 and more than 29.  Some may argue but in really doesn t matter.


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## brino (Aug 17, 2018)

How far from the chuck are you working? was there centre support?
Any chatter? These things can all contribute to bad turning finish.

When you say "rough" do you mean the surface finish?
Or do you mean the threads looked "wonky" with a different angle on each side?

If the later, double check the compound angle.....some lathes call zero degrees at parallel to the lathe centre axis, while some call zero degrees perpendicular to the lathe centre axis.

Here's an example of setting to the wrong angle:
https://www.hobby-machinist.com/threads/threading-help.49488/post-416851
Notice the slopes on the leading and trailing edges are not the same; one is steeper than the other.
Also one side is "stepped" and not one flat surface.

Here's an example of "double tracking" where the half-nut was engaged on the wrong number:
https://www.hobby-machinist.com/threads/threading-something-went-wrong-here.33772/post-284757

Some photos of your set-up and results would help to debug it.

-brino


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## Technical Ted (Aug 18, 2018)

You've been given a lot of good advice. Here's my take: if it cuts fine feeding straight in with the cross slide, but not when feeding in with the compound, the first thing I would check is to verify the compound is set at the proper angle.

I have two SB lathes (see pic of my 13"). I have both set at the proper 29 degree angle for threading and leave them there most of the time. But notice that they are set at 61 degrees on the graduations to achieve this 29 degrees of actual movement needed.

Ted


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## porthos (Aug 18, 2018)

everything that i did so far is done properly. however, i like Techinal Teds  remark about 61 degrees on the South bend. i'll try that this afternoon and get back to the site with results. thanks for all the help so far


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## porthos (Aug 18, 2018)

i'm back!! set the compound at 61 degrees as per TechnicalTeds instructions. PERFECT threads... a little confused as to why everything that I have read states to use 29.5 degrees. i know that 61 degrees is the same; but, from a different direction. its not quite the same; it is at a slightly different angle on the compound. i'll bet there have been a lot of bad threads cut because of the miss information out there and not telling a novice about this issue with different types of lathes.  THANKS  TED


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## Technical Ted (Aug 18, 2018)

You will find often that the markings on the compound don't match up with the actual angle you want to turn or feed.... you need to do the math at times. 

It can be a little confusing. When you set the compound per the markings to 90 degrees it is set perpendicular to the turning axis of the lathe and feeding in that direction will face off a part at 90 degrees the way it should be faced. When you are threading, if you look at a diagram of a 60 degree sharp V thread profile it shows the thread at being 30 degrees on both sides for an overall angle of 60 degrees.

But look closely and you'll notice the 30 degree is actually off from the 90 degree setting on the lathe. So, 90 degrees minus 30 degrees is 60 degrees. Back it up 1 degree so you're on 29 and that equals the compound being set at 61 degrees per the compounds markings. 

But, thread wise, you will be feeding in a 29 degrees. 

So, in the case of threading, the compound's 90 degree setting (per the marks) is actually 0 degrees (feeding straight in). You need to shift the compound 29 degrees off from this 90 degree position to get the correct angle for threading. 

Hope I'm explaining that well enough.

Great job glad you got things working.
Ted


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## T Bredehoft (Aug 18, 2018)

The logic says "set the angle at 29.5 degrees," Not "set the dial on 29 1/2 degrees. " Many of us don't realize the difference.  I know I had trouble with that on a new  (to me) lathe.


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## brino (Aug 18, 2018)

porthos said:


> i'm back!! set the compound at 61 degrees as per TechnicalTeds instructions. PERFECT threads.



I believe that's the same thing multiple people have said in this thread.....however it is a difficult thing to explain with words.
I am glad that @Technical Ted 's explanation made sense to you.

Did you notice this post by @Dave Paine: https://www.hobby-machinist.com/threads/threading-help.71189/post-596785
(it was in the thread that he pointed you to above.)
He shows a no-nonsense way to ensure you have the cross-slide set properly; independent of the angle markings on the lathe.



porthos said:


> a little confused as to why everything that I have read states to use 29.5 degrees.



It depends how your lathe is marked; which end is 0 degrees and which end is 90 degrees. (90-29.5 =60.5)

The bottom line is that we got it figured out.

-brino


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## Technical Ted (Aug 18, 2018)

Yep, I wasn't the first or only one. I got a sense that the point wasn't hitting home. Sometimes we need to hear things a few different ways before it clicks and the light bulb goes off. Pictures and drawings help too especially with those that are more of the visual leaning type.

Sometimes it is hard to explain things from a keyboard on these forums and at times I think I go overboard when trying to make a point and get it across the way it is intended. But, then again, sometimes that's what it takes.

Agreed, bottom line he got it working and learned! This forum is a team effort! 

Ted


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## MarkM (Aug 19, 2018)

It s also interesting that in north america we use the 29.5 or so setting and in asia and many other areas around the world they just plunge straight in with the compound 90 degrees to the work.


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## brino (Aug 19, 2018)

MarkM said:


> It s also interesting that in north america we use the 29.5 or so setting and in asia and many other areas around the world they just plunge straight in with the compound 90 degrees to the work.



I believe that is more a function of how stiff the lathe is.
I have found that when plunging straight in with the cross-slide (and cutting on both faces of the tool) any slack in the dovetails causes chatter presumably due to higher cutting forces.
I use the compound set at 30 degrees to cut on only one tool face, it seems to chatter less, on my old lathe with some wear.

Some folks get rid of the compound to get rid of one more set of dovetails and provide a stiffer tool, then they have to plunge straight in.

-brino


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## savarin (Aug 19, 2018)

I plunge straight in usually but I did use the 29.5' when I cut the 39 x 4mm thread for the lathe back plate test bar thread


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## Boswell (Aug 19, 2018)

I use the 90-deg straight in plunge approach. Found it much less likely that I would make a error and screw it all up  and also have not had any problems with that approach. However I don;t do that much single point thread cutting.  If your cutting inside threads it may be the only way you can do it due to clearances.


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## C-Bag (Aug 20, 2018)

I just wanted to thank the OP and everybody for this thread. Once again I was able to find exactly what I was going through and the fix. I had successfully avoided single point threading for all this time and when I finally got backed in a corner and HAD to do it I was getting the same results. Who knows when I'll next have to single point something so that was another reason I wanted to reply so I could look through the history and find it again


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## Janderso (Aug 21, 2018)

On my old South Bend, (new screw and nut on the compound and cross slide) I feed at 29.5 with the compound. But, I find when I take out the play moving into the piece, I can see the load push the compound back to the opposite side of the free play in the screw. Am I explaining it right?
I thought you guys were going to suggest HSS cutting tools to improve finish. Threading slow with carbide is tricky no?


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## C-Bag (Aug 21, 2018)

Janderso said:


> Threading slow with carbide is tricky no?



Is this true? My 9x20 cannot go slow enough for the super short threads I was cutting so I made a hand crank and it totally does the trick. But my cutter is carbide and freshly sharpened correctly(except for the tip, just a little off as I'm not sure of the exact spec). But I was using some gummy steel and I'm sure that was the biggest reason the threads were a little rough. But I'm still so new to this I could be committing all kinds of faux paus.


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## Technical Ted (Aug 21, 2018)

As with most things, it depends. But, generally, HSS can be sharpened to a very sharp edge and it will hold up. Carbide, on the other hand, if sharpened to a very sharp edge has a tendency to chip and break down and the sharp edge will  not last. A lot of times when I want a carbide edge to hold up I will lightly hone the edge at a 45 degree angle. Running carbide with edges like this performs best with faster surface speeds.

I get my best threading results on my slower running lathes with a good quality HSS tool bit, sharpened with a very sharp edge (with light honing) using a good quality cutting oil.

YMMV,
Ted


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## Creativechipper (Aug 21, 2018)

Great thread going here, I need this info as I have not cut any threads yet.
 Recommended minimum tooling/measuring for this single point threading? 
 Currently looking at some screw pitch gauges, but not sure whats needed.


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## Technical Ted (Aug 21, 2018)

My standard recommendation for anyone learning how to run a lathe is to download and study South Bend's "How to Run a Lathe". There are other books and resources available as well, but this one is my favorite.

Just Google it and you'll find many sources and different versions to download. Or, if you're so inclined, you can order a paper copy from Amazon.

There are also a lot of threads on this site about this very topic. This subject has been discussed several times and there is a lot of good information already posted which will help. 

Ted


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## Tozguy (Aug 21, 2018)

Just a comment on the 29.5 degree thing when feeding with the compound.....
The protractor scale on most compounds is not absolutely accurate. However the actual angle of the compound for feeding must NEVER be over 30 deg. So to hedge on inaccuracy of the scale we set with a safety margin of 0.5 deg. or 29.5.
In fact any setting of something less than 30 degrees would work but 30.1 deg would not.


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## mikey (Aug 21, 2018)

Creativechipper said:


> Great thread going here, I need this info as I have not cut any threads yet.
> Recommended minimum tooling/measuring for this single point threading?
> Currently looking at some screw pitch gauges, but not sure whats needed.



Depends on how accurate you want your threads to be. At minimum, you need a thread pitch gauge to make sure you've identified your thread pitch correctly. It is also used to make sure your first scratch pass conforms to the thread you're cutting so that you know your gearing is correct. You also need a fishtail or centering gauge to correctly orient the threading tool to the work piece; a Starrett C391 is common but many others make them. With these tools you can cut threads to fit an existing female piece or thread internally by cutting the thread and checking for fit.

If you want to cut threads to some spec then you will need a more accurate way to read pitch diameters. The cheapest way is to buy a set of pitch diameter thread wires and use your micrometer; read up on the 3-wire technique. Thread micrometers that read pitch diameter directly are also popular and nowadays are pretty cheap. There may come a time when you need to cut a Class 3 thread and you need to be able to read pitch diameters accurately; that is when you need these tools. 

Lastly, you need good thread cutting tools. These are available in carbide, HSS or you can grind your own tools. The cleanest threads I can cut are done with a shop-ground HSS tool so that is what I prefer. 

Threading is not hard to do but you sort of need to know what you're doing. Read, ask, try and try again. It will come.


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## RWanke (Aug 21, 2018)

And don't let anything distract you're attention lest you end up with a nice crash like this. 3.5"- 4TPI on a shaft worth probably $800-$1000. Saved the shaft after ordering a new threading tool (probably $250). What's really bad is the boss and his son and the company owner and a customer where standing in the middle of the shop when I crashed it.    Still not sure how I got my timing/rhythm whatever off so bad, but it only takes a heartbeat.


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## brino (Aug 21, 2018)

Technical Ted said:


> Just Google it and you'll find many sources and different versions to download. Or, if you're so inclined, you can order a paper copy from Amazon.



...or choose a PDF copy from one of several hosted right here:
https://www.hobby-machinist.com/res...a-lathe-for-beginners-1914-15th-edition.3217/
https://www.hobby-machinist.com/resources/sb-how-to-run-a-lathe-1966-27th-edition-56-pdf.2909/
https://www.hobby-machinist.com/resources/sb-how-to-run-a-lathe-1914-3rd-edition-pdf.2890/

-brino


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## brino (Aug 21, 2018)

RWanke said:


> And don't let anything distract you're attention lest you end up with a nice crash like this.........................Still not sure how I got my timing/rhythm whatever off so bad, but it only takes a heartbeat.



Ouch! Kinda hard to hide something like that when it goes bad.
Glad you saved the shaft.

-brino


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## coffmajt (Aug 21, 2018)

My SB9A compound degree markings are very small and difficult to read, so I made a 29.5 degree jig that lets me reset to that angle for threading whenever I have moved the compound to some other angle such as cutting an R8 taper on a piece. A one time gauge is much easier for me to reset to == Jack


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## C-Bag (Aug 21, 2018)

I read the SB book. I watched videos. I read forums. But there seem to be exceptions and variables depending on the thread and the machine and who's doing it. But nobody explained that in order to get the correct angle I can't go by the markings on the machine. 

To me the one tip I got early on was you're just going to have to do it. In this case I had so much other stuff like a proper grinder and gage to hold the tool at the correct angle, didn't understand what I was looking at on the gear chart to set up the feed, the cluster that is the gear train on the 9x20, my machine just doesn't go slow enough so I had to make a hand crank, the real way to use the fishtail and on and on. There was a ton of side trips and detours so by the time I was actually ready to cut the threads it was kind of anticlimactic. But finally getting how to not go by the deg on the compound, but using my Starrett gage I was able to get the correct angle. There is still a lot I need to learn but after three tries I got it done. I have another project, a graduated carriage stop, that needs to be threaded. Hopefully I'll be able to get to that before all this fades. I feel like it will stick though because each try taught me a lesson and those stick with me more.


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## Creativechipper (Aug 21, 2018)

Excellent info again!!

 I am still gathering the devices specific to threading. Any suggestions on thread gauges and fishtails and thread mics? Are the gauges and fishtail ok to get new cheap ones or does the quality vary enough to warrant a name brand set? Are the Mics for threading good bad, where to start with equipment?

 Thanks


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## RWanke (Aug 21, 2018)

brino said:


> Ouch! Kinda hard to hide something like that when it goes bad.
> Glad you saved the shaft.
> 
> -brino




LOL. Wasn't any hiding it, especially when they watched me just shut the lathe off and walk out of the shop to calm my rattled nerves. The bosses son actually came over after I walked back in and apologized because he had just been standing over my shoulder a moment before I did it and I stopped and politely asked him to not distract me as I needed to really concentrate. I told him it was definitely my screw up because he had left me alone and I just brain farted. Stuff does happen. He wasn't mad.


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## RWanke (Aug 21, 2018)

coffmajt said:


> My SB9A compound degree markings are very small and difficult to read, so I made a 29.5 degree jig that lets me reset to that angle for threading whenever I have moved the compound to some other angle such as cutting an R8 taper on a piece. A one time gauge is much easier for me to reset to == Jack



And isn't it great the way South Bend put the index mark on the back side of the cross slide. I run my carriage all the way to the end of the lathe where I can stand at the end and get a light shining on it to see it to get it right.


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## Creativechipper (Aug 21, 2018)

At least they understood and didn't give you too much grief.


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## Technical Ted (Aug 21, 2018)

Creativechipper said:


> Excellent info again!!
> 
> I am still gathering the devices specific to threading. Any suggestions on thread gauges and fishtails and thread mics? Are the gauges and fishtail ok to get new cheap ones or does the quality vary enough to warrant a name brand set? Are the Mics for threading good bad, where to start with equipment?
> 
> Thanks



Probably boils down to what exactly you will be doing. I guess my generic recommendation would be: if you want to save a little money you could either buy mid level quality new or top quality used. I wouldn't go out and buy the cheapest set you can find. You want a quality set, but you don't have to buy top of the line for most home use.

I do a fair amount of threading and don't own/use thread mikes. We did have them at work and they are nice if you have to hold a tolerance from a drawing. Usually I thread to fit a mating part. I do have a set of thread wires. They take a knack to use but they do a good job and are accurate. Just make sure to clean out your chip pan before using them. They turn invisible when you drop one. I just got a cheap set of these off eBay. All they are is a set of ground wires.

Ted


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## savarin (Aug 21, 2018)

I guess it all depends upon how much you wish to spend. I am a notorious cheapskate so for things like that I only buy the cheap stuff.
Even my micrometers are cheap Chinese ($10 each) but when measured against my mates top of the range items we cant measure any discernible difference.
Does the work you will be doing depend upon sub micron accuracy? Unless working for NASA then probably not.
Are you working in the optical fields then probably yes.
If you want an excellent fishtail gauge then check this one out


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## mikey (Aug 21, 2018)

Creativechipper said:


> Excellent info again!!
> 
> I am still gathering the devices specific to threading. Any suggestions on thread gauges and fishtails and thread mics? Are the gauges and fishtail ok to get new cheap ones or does the quality vary enough to warrant a name brand set? Are the Mics for threading good bad, where to start with equipment?
> 
> Thanks



All the thread pitch gauges from the major makers are fine. I prefer Starrett but have B&S, too, and it works fine. I mostly use the Starrett 472; it goes from 4 to 84 tpi. I also use a Starrett metric gauge, too. Check ebay or Amazon.

Fishtails are also pretty accurate. I use a Starrett 391 but also have one from Mitutoyo and several from PEC. All are about the same. Check Amazon for a Mit - its accurate.

Thread mics do vary in quality but not enough to warrant spending major bucks on a high quality set. I own a 0-1 and 1-2 set from Tesa - you don't even want to know what that retails for. If I were you, I would buy an import like this one: https://www.ebay.com/itm/0-1-SCREW-...895275?hash=item519992f22b:g:nDAAAMXQrvpREJLy

You couldn't even buy a single anvil from Tesa for what that Shar's set cost and I doubt you will see a difference in outcome. 

You should be clear that you do not need to use thread mics for a rough and ready thread; cutting to fit will work fine for that. But when you need to cut a thread to tight tolerances or to fit an existing part and you need it to fit right the first time, that is when you pull it out. You can also use the three wire technique. I no longer do this because I absolutely hate those things, or maybe they hate me. Either way, I'm constantly dropping or bending them so I went to thread mics.

Bear in mind that when cutting threads to tighter tolerances you need to turn the blank to the right size for that class, then cut the thread to a pitch diameter range. That might be beyond the scope of this thread. If you need more details, start a thread of your own and we can go at it there.


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## Technical Ted (Aug 22, 2018)

If you find you have trouble holding the fish square/level with your stock, you can pick up or make one of these (or make something like in the video savarin posted above which is a lot more work). If I remember, mrpete222 on YouTube shows how to make a holder similar to the one in this picture, which is a Starrett brand.

Ted


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## C-Bag (Aug 22, 2018)

mikey said:


> Bear in mind that when cutting threads to tighter tolerances you need to turn the blank to the right size for that class, then cut the thread to a pitch diameter range. That might be beyond the scope of this thread. If you need more details, start a thread of your own and we can go at it there.



With all due respect Mike, personally I think this is part of the scope of thread on threading. Several of you who understand threading are making things much clearer than I've been able to get on my own. I relate this to when I was trying to understand computers and was learning from guys who had been at it forever. They couldn't drop down enough because they didn't understand I had picked it up piecemeal and there were certain pieces they took for granted, but I didn't get. Putting everything in one place would help and probably it is somewhere and I've not run across it. If it is I apologize. 

So where do you find the "blank size"? A lot of what keeps me from finding info is I don't even know what it's called.


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## MrWhoopee (Aug 22, 2018)

Creativechipper said:


> Excellent info again!!
> 
> I am still gathering the devices specific to threading. Any suggestions on thread gauges and fishtails and thread mics? Are the gauges and fishtail ok to get new cheap ones or does the quality vary enough to warrant a name brand set? Are the Mics for threading good bad, where to start with equipment?
> 
> Thanks



Any will do, these are not high precision tools. As for the thread gage, you can get by using a scale and counting the number of threads in an inch. If the thread you're cutting is not that long, set up on a test piece. It's good practice for refining your technique.


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## Creativechipper (Aug 22, 2018)

"A lot of what keeps me from finding info is I don't even know what it's called."

 I here that C-Bag, I have to google a lot of new tool names, terms etc. Keeps it fun always learning new stuff..lol


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## GrayTech (Aug 22, 2018)

There are thread charts that list the outer and inner diameters for internal and external threads in the range of classes from loose fitting to very tight tolerances. 

Sent from my H3123 using Tapatalk


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## P. Waller (Aug 22, 2018)

Screw thread micrometers will only measure a limited number of thread leads per anvil set and will require a standard for setting them.
Wires and plain micrometers will measure any lead also Whitworth and Acme/Metric Trap threads and gears and splines. They are far more versatile unless you are in a production setting.


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## P. Waller (Aug 22, 2018)

GrayTech said:


> There are thread charts that list the outer and inner diameters for internal and external threads in the range of classes from loose fitting to very tight tolerances.
> 
> Sent from my H3123 using Tapatalk


The IMPORTANT dimension is the Pitch Diameter, you will notice that it always has the smallest variance between Min. and Max.
All straight thread gauges are marked with the PD
For example https://www.threadcheck.com/thread-ring-gages-unified-inch/


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## mikey (Aug 22, 2018)

C-Bag said:


> So where do you find the "blank size"? A lot of what keeps me from finding info is I don't even know what it's called.



Oh, trust me, I remember the beginnings and I totally understand what you're saying.

Okay, lets make things as simple as we can. The first thing to do is to sort out the nomenclature of threads. This is a good but brief summary that will help in our discussion: http://portal.ku.edu.tr/~cbasdogan/Courses/MDesign/course_notes/fasteners.pdf

I won't go through the different types of threads (UNF, UNC, etc) but I want to focus you in on the fit classes, 1-3. Loose is class 1, normal is class 2 - this is the typical hardware store fit, and class 3 is a fine, high tolerance fit. You will rarely go for a class 1, usually go for a class 2 and sometimes go for a class 3. Note that this class stuff applies to both internal and external threads. As a hobby guy, you can use whatever you wish and this is especially true when both internal and external threads are made by you; you can cut one and make the other one to fit.

The problem comes when you have to cut a thread to fit an existing part that you do not have in front of you or that is difficult to reach, or that you wish to have a specific fit for some reason. Then you need to cut the thread to some standard that will guarantee a fit. That is when you really need thread wires or a thread mic, and you need to know what those standards are. Fortunately, this is not difficult if you have a good chart. I am attaching the best chart I've ever seen for this kind of thing.

Let's use a common thread as an example, say 1/4-20. Here is an excerpt from the chart:




Find 1/4-20 in the Thread Designation column and decide what class thread you want. Say we want a class 3 external male thread, or class 3A. Now look under Major Diameter and you'll see two figures, 0.25 and 0.2419. That is the size range your blank must be to cut a class 3 external thread so turn the work piece down until it falls somewhere inside this range, then chamfer the nose of the work piece at about 30 degrees for a length of about 1 to 1-1/2 threads. Similarly, if you wanted a class 2 fit, note the size range and cut the work to that size.

It is wise to also cut a thread relief at the end of your thread. This allows your tool to run off into that space and leave a clean thread end. There are specs for this sort of thing but I usually just make it about two to three threads wide and about 0.003" - 0.005" deeper than the minor diameter. This will not significantly impact on strength. *EDIT*: use your parting tool to cut the thread relief. I use a P1-N blade that is only 0.040" wide; this allows me to cut as many side-by-side slices to get the thread relief as wide as I need it to be.

Now you have a blank that is the right size, is chamfered properly and has a thread relief. To be sure your gearing is correct, make a single pass at about a 0.001" depth of cut and use your thread pitch gauge to make sure the pattern matches the gauge. If it does then your gearing is okay and you can proceed to cut the thread. What should be entering your mind now is ... how deep do I need to go to cut this thread?

There are formulas for this kind of thing. The simplest one I know is from Darrel Holland of API:  *0.750/TPI*. In our example, 0.750/20 = 0.0375". So, you would need to feed in 0.0375" when feeding in with the cross slide only. If you prefer to use the compound set at 29.5 degrees then multiply the above number by 1.143 and use that depth. Keep in mind that these are estimates; you need to stop your cuts about 0.005" or more before you hit the calculated depth and check the fit with your wires or thread mic.

*EDIT*: I suppose I should tell you how to stage your cuts. When you screw cut a thread, the deeper you are, the greater the cutting forces become. This is because more and more of the flank of the cutting tool is in contact with the work piece. This is especially true when feeding straight in with the cross slide so you don't want to try and cut the entire thread in a few passes; you should stage your cuts so that the deeper you go, the shallower you set your depth of cut. Personally, on threads below 3/8", I make the first pass a scratch pass of about 0.001" deep and check to be sure my gearing is okay. If it is, then I make a 0.009" deep pass to debulk the thread. Then I reduce the depth of cut to 0.005" deep for the next two passes, then 0.004 X two passes, then 0.002" deep X two passes and then 0.001" until I am at final depth. I've tried all kinds of patterns but on smaller threads under 3/8" I have found that going lighter gives me less deflection so my thread form is better and definitely more accurate. Lots of guys use deeper cuts and that's fine if it works for you. I prefer to go slower so I hit the size I want dead on. For me, the smaller the thread, the lighter I make my passes. When cutting threads greater than 3/8", I will make several passes at 0.009 - 0.010 to debulk, then step down as above. It takes longer to thread like this but I'm usually in no hurry; I would much rather go slow than have to make the piece all over again.

Note that the next two chart columns fall under Pitch Diameter and again, this is a range. The idea is to check the pitch diameter of your thread until it falls inside this range and that will give you the fit for the class of thread you're cutting. If using wires, you have to do some calculations. If using a thread mic then the reading is direct. Cut until you're inside the desired range and you're done.

So, what about estimating depth of cut for metric thread? They use pitch, not tpi. Just use the Newmann Tools chart below and plug the conversion into the Holland formula and it works fine.

See, not hard, right? If you use a really good HSS tool then hitting these numbers is much easier. Carbide inserts also work but you will find taking micro cuts to be less precise. On large threads this isn't usually a problem but when cutting smaller threads, it can be.

Cutting internal threads is much more difficult to do, especially when cutting to a class 3 fit. The best option is to make a plug gauge to specs then cut the internal thread to fit. I use solid carbide threading tools for small holes and a solid carbide inserted bar for larger holes. The reason for using carbide shanked tools is accuracy. When you're threading in a relatively deep hole, tangential and radial cutting forces are very high and you need to reduce deflection as much as possible; carbide will do that for you. For small holes, 3/16" and under, I use Micro 100 solid carbide internal threading bars; they are extremely accurate and stay sharp for a long time. Plus, they are easily sharpened with a diamond stone when needed (just hit the flat on top).

When you cut threads with a good HSS tool, it will usually leave a good finish and just require a quick pass with a steel or brass brush. Carbide tools sometimes need a quick pass with a fine file to clean up.

Hope that makes it easier for you guys. Thread cutting is not difficult; you just need to practice.


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## mikey (Aug 23, 2018)

Made a few edits to my post above.


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## C-Bag (Aug 23, 2018)

Bullseye Mike, especially the first link. For some reason I can't open the last two attachments.


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## Creativechipper (Aug 23, 2018)

Amazon to the rescue thread gauge set and fishtails for 9.99 shipped. https://www.amazon.com/gp/product/B07DC2RKX7/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
Now that will go nice with my Blue Point tap and die set, gotta skimp somewhere..lol


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## C-Bag (Aug 23, 2018)

Yeah, I wish they still had the Starrett no. 392 fishtail holder. I think they quit making them. That would be so handy I think I'll have to make one. Use a fishtail once and you'll wish you had one too.....


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## MrWhoopee (Aug 23, 2018)

mikey said:


> *EDIT*: I suppose I should tell you how to stage your cuts. When you screw cut a thread, the deeper you are, the greater the cutting forces become. This is because more and more of the flank of the cutting tool is in contact with the work piece. This is especially true when feeding straight in with the cross slide so you don't want to try and cut the entire thread in a few passes; you should stage your cuts so that the deeper you go, the shallower you set your depth of cut.



I think Cat Stevens summed it up best (or Sheryl Crow for you youngsters):

"The first cut is the deepest,
baby I know,
the first cut is the deepest"


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## Creativechipper (Aug 23, 2018)

The younger generation may also know Cat Stevens as, Yusuf


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## mikey (Aug 23, 2018)

C-Bag said:


> Bullseye Mike, especially the first link. For some reason I can't open the last two attachments.



Not sure why - they're just pdf files. Maybe try right clicking and choosing "save file as" and save it on your desktop, then try opening it there. If that fails, PM me.


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## mikey (Aug 23, 2018)

C-Bag said:


> Yeah, I wish they still had the Starrett no. 392 fishtail holder. I think they quit making them. That would be so handy I think I'll have to make one. Use a fishtail once and you'll wish you had one too.....



I know this one to be of high quality and the price is good: https://www.amazon.com/Mitutoyo-950...3687&sr=8-1&keywords=mitutoyo+60+degree+gauge


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## mikey (Aug 23, 2018)

I was searching my computer for a pic of a screw with the features I mentioned  -  chamfer, thread relief. I couldn't find the one I was looking for but I found this one and I hope it sort of gives you an idea of what I meant. This is an 8mm Class 3 thread in 1144 Stressproof steel. It has a 30 degree chamfer at the end X 1-1/2 threads (most of it is on the other side of the screw - sorry) and a thread relief. It was cut with a Rex AAA cobalt HSS threading tool with 15 degree relief angles and the thread is as it comes off the lathe, no filing or anything. The thread relief is about 2-3 threads wide and ends in a clean shoulder; made with a P1-N parting tool. For those of you with calibrated eyeballs, the depth of the thread relief is 0.003" deeper than the minor diameter.

Hope this makes things clearer.


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## C-Bag (Aug 23, 2018)

mikey said:


> I know this one to be of high quality and the price is good: https://www.amazon.com/Mitutoyo-950...3687&sr=8-1&keywords=mitutoyo+60+degree+gauge



I think there was a miscommunication there. I wasn't looking for the fishtail(got one) but the attachment like this:
https://www.amazon.com/Starrett-392...=2025&creative=165953&creativeASIN=B000JERN5C

Outta stock and possibly outta production. Personally the one thing I don't like is trying to hold the fishtail in alignment while trying to weasel everything else. Was a really good idea and might have even seen one in the old machinist boxes I've looked through but didn't know what it was.


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## C-Bag (Aug 23, 2018)

mikey said:


> I was searching my computer for a pic of a screw with the features I mentioned  -  chamfer, thread relief. I couldn't find the one I was looking for but I found this one and I hope it sort of gives you an idea of what I meant. This is an 8mm Class 3 thread in 1144 Stressproof steel. It has a 30 degree chamfer at the end X 1-1/2 threads (most of it is on the other side of the screw - sorry) and a thread relief. It was cut with a Rex AAA cobalt HSS threading tool with 15 degree relief angles and the thread is as it comes off the lathe, no filing or anything. The thread relief is about 2-3 threads wide and ends in a clean shoulder; made with a P1-N parting tool. For those of you with calibrated eyeballs, the depth of the thread relief is 0.003" deeper than the minor diameter.
> 
> Hope this makes things clearer.
> 
> View attachment 274376


So the other factor amongst the many is the type of steel you are trying to thread it would seem. What's the best steels for single point threading?


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## mikey (Aug 23, 2018)

C-Bag said:


> So the other factor amongst the many is the type of steel you are trying to thread it would seem. What's the best steels for single point threading?



Doesn't really matter. If you can turn it, you can thread it.


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## mikey (Aug 23, 2018)

C-Bag said:


> I think there was a miscommunication there. I wasn't looking for the fishtail(got one) but the attachment like this:
> https://www.amazon.com/Starrett-392...=2025&creative=165953&creativeASIN=B000JERN5C
> 
> Outta stock and possibly outta production. Personally the one thing I don't like is trying to hold the fishtail in alignment while trying to weasel everything else. Was a really good idea and might have even seen one in the old machinist boxes I've looked through but didn't know what it was.



Sorry, my mistake. I grind my HSS threading tools very accurately so I don't bother to align it with a fishtail. I use an accurately turned and faced piece of steel and butt it up against the chuck face on one end and the side of the tool on the other end. This gets the tool aligned perpendicular to the work piece. This works for me. I use the fishtail to check my 60 degree angle when grinding. I also use it to align my tool when internal threading.


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## Z2V (Aug 24, 2018)

That’s a lot of good information there Mikey, thanks


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## whitmore (Aug 25, 2018)

MarkM said:


> It s also interesting that in north america we use the 29.5 or so setting and in asia and many other areas around the world they just plunge straight in with the compound 90 degrees to the work.



I think the disparity is related to the tool used; if your tool is HSS (and that's the old style), the easiest sharpening
is to make it a left-cutting or right-cutting tool, only ONE of the two edges of the tool is at the optimum height for cutting.
With a carbide insert, a one-edged insert might have a chipbreaker on the left edge but not the right.   A symmetric
two-edge insert will cut left and right flanks equally (but it's problematic to include a chip breaker).

For fine vee threads, with HSS tool, and for the relatively heavy chip loads that a single-point tool
does most efficiently, the 29.5 degree setting is still a good choice.     If  you need special accuracy,
the sideways-load-flex issue might point you to 90 degree plunge cutting (but don't expect the chip
to come out as a nice curl).


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## mikey (Aug 25, 2018)

Not disagreeing at all with the above but my simple understanding about the crossfeed vs compound feed thing is related to tool loads, cutting forces and heat distribution. 

When feeding straight in with the cross slide, both flanks of the tool are engaged in the work and as you go deeper into the thread, cutting forces climb with flank engagement. This generates heat, which affects tool wear and tool life. This is not a big deal in a hobby shop and many of us do feed in with the cross slide using both HSS and carbide inserts. However, this works best for fine threads (maybe 20 tpi or greater) because the degree to which both flanks are engaged is limited. 

Feeding with the compound causes most of the cutting to be done by the leading edge of the tool, be it left or right handed. This keeps cutting forces and heat generation lower while also allowing for better chip clearance. Most (not all) threading inserts and almost all HSS tools (except custom ground tools) have a flat top rake so chip clearance is very much improved when only one flank is engaged. One way to improve this on a HSS tool is to grind 5 degrees of side rake; if you only feed with the compound then this is a very good option. Of course, you will need to grind LH and RH tools this way. 

Many of us use both methods in combination. We use the compound to do most of the thread cutting and then use the cross slide to feed in the last few thou to clean up both flanks of the thread. This actually works rather well, especially with a sharp HSS tool. 

In terms of which is better, both work. For most of us with manual lathes that thread at low speeds I suspect heat and tool life will not be greatly affected either way. I use the cross slide to thread on my Sherline lathe and the compound to thread on my Emco lathe and I don't see a whole lot of difference in thread form. What does make a difference is the tool. I find my HSS thread tools produce much cleaner and more accurate threads than does my Carmex carbide threading inserts. Nothing against carbide or Carmex; its just that I have some really good HSS tools, that's all.


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## Technical Ted (Aug 25, 2018)

Creativechipper said:


> Amazon to the rescue thread gauge set and fishtails for 9.99 shipped. https://www.amazon.com/gp/product/B07DC2RKX7/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
> Now that will go nice with my Blue Point tap and die set, gotta skimp somewhere..lol



I hope you realize the one thread gauge is for Whitworth threads, not 60 degree threads like we use here in the USA... The other is for metric. 

55 degree threads aren't something that is typically used in the USA.

Ted


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## Creativechipper (Aug 25, 2018)

I thought whitworth was a brand name whoops, thanks for letting me know


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## Technical Ted (Aug 25, 2018)

Creativechipper said:


> I thought whitworth was a brand name whoops, thanks for letting me know



Live and learn... just have fun while you're doing it! Here's some info on the whitworth thread:

https://en.wikipedia.org/wiki/British_Standard_Whitworth

Ted


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## Creativechipper (Aug 25, 2018)

So looking to thread with 1/4-1/2" tap and dies what tooling measuring tools do I need to be able to cut and measure?  
 It looks like the screw pitch gauge set for whitworth is useless for my set up.


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## savarin (Aug 25, 2018)

A good article of threads
https://en.wikipedia.org/wiki/Screw_thread


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## BaronJ (Aug 26, 2018)

Hi Guys,

I must admit to laziness here and say that I do use carbide threading tips.  I also run the lathe in reverse, cutting away from the chuck, and yes I do just plunge straight in.


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## Technical Ted (Aug 26, 2018)

Creativechipper said:


> So looking to thread with 1/4-1/2" tap and dies what tooling measuring tools do I need to be able to cut and measure?
> It looks like the screw pitch gauge set for whitworth is useless for my set up.



You could still use the thread gauge to help determine 60 degree threads... the threads per inch are still the same, just the profile won't fit 100% because it's 55 degree instead of 60, but it would be close. Usually, you're not trying to determine the profile of the thread when using a thread gauge, just the threads per inch and it should work for that. 

The only tools I would say you really "need" would be a set of mics to turn/measure the major diameter, a fish to align your threading tool (or some other means) and a nut to use as a gauge for sizing the pitch diameter of the threads. If you really want to measure the pitch diameter (which I don't 99.9% of the time) you can pick up some thread measuring wires. Something like these: https://www.ebay.com/itm/48-Pc-Thre...573189&hash=item41eb688d86:g:imQAAOSwImRYkSdG But, if it was me, I wouldn't bother with getting any and just use a nut for a gauge. 

I would start with the minimum and once you get going and learn more you'll know yourself what you want/need...

Of course you'll also need a thread cutting tool and some cutting oil. 

Ted


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## Creativechipper (Aug 26, 2018)

Thanks Ted, much appreciated, sometimes 2 steps forward one step back, still progress..lol


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## Technical Ted (Aug 26, 2018)

Add remember, as others have said, you don't need a thread gauge. I went a long time without one and just used a 6" scale to count the threads per inch... the gauge is a "nice to have" tool, but certainly not needed to do the job.

Ted


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## jdedmon91 (Aug 26, 2018)

whitmore said:


> I think the disparity is related to the tool used; if your tool is HSS (and that's the old style), the easiest sharpening
> is to make it a left-cutting or right-cutting tool, only ONE of the two edges of the tool is at the optimum height for cutting.
> With a carbide insert, a one-edged insert might have a chipbreaker on the left edge but not the right. A symmetric
> two-edge insert will cut left and right flanks equally (but it's problematic to include a chip breaker).
> ...



I use the 29 degree setting on the compound. But compound feeding is unnecessary with inserts especially the top notch style. These inserts were designed for CNC lathe threading. Witch uses multiple passes. So you can duplicate that on a manual lathe. 

Like the quote it from using 60 degree ground HSS tools where this started and for those tools the compound feed is best 


Sent from my iPhone using Tapatalk


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## Creativechipper (Aug 27, 2018)

If your like me and have never done a thread and need a place to start, this video helps....




 Goes over tools angles, etc.


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## BaronJ (Aug 27, 2018)

Hi Guys,

I forgot to mention, because I don't use a normal rear toolpost, mine is a parting off tool holder,  I use a boring bar type insert holder intended for internal threading.  Running the lathe in reverse and cutting away from the chuck the tool is at the back of the workpiece.


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