# Learning to Cut Threads



## Todd3138 (Jun 9, 2022)

I posted last week asking about thread cutting tooling and appreciate the feedback you guys gave.  I have the benefit of having access to a lifelong machinist who works for a friend of mine.  He and I have become friends over the handful of visits he has made to my shop to help me begin to understand a little bit of what I can do here  with the machinery I've got.  Yesterday, he helped me out with cutting outside threads and showed me in very meticulous detail the exact process he uses.  He brought a piece of scrap aluminum round bar because it's more forgiving of a hamfisted novice like me when I might crash my tool into the work!  I found a random nut that turned out to be a 1" - 8 and he walked me through the process of turning the bar down to the right diameter and then how to set up and cut threads.  This was our finished product and the fit was nice just like the two pieces were made for one another (which, funny enough, I guess they were!).




Tonight I decided to do it myself with a 3/4' - 16 nut.  While I'm generally happy with how it turned out, I think I got just a few thou too small on turning the bar down and ended up with play between the nut and threaded bar.  It's not enough to pull the nut off, but there is that probably 2-4 thou of play.  It does thread on as it should otherwise, but my takeaway is to be more diligent and detailed on my measurements.  I have never done a lot of precision measuring in my life so learning to properly use and read a caliper and micrometer has been a learning process.  I look forward to the day it is second nature as I'm sure it is to most of you.  Anyway, here is my finished product from tonight.


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## markba633csi (Jun 9, 2022)

You're getting the hang of it- now you should try steel- with plenty of cutting fluid
-Mark


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## snoopdog (Jun 9, 2022)

Well, If you ain't turnin' you aint lernin, good job!


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## Todd3138 (Jun 9, 2022)

markba633csi said:


> You're getting the hang of it- did you use any cutting fluid? While you can often cut aluminum dry, it does help prevent "tearing" of the metal when cutting threads.
> I like to use liquid wrench but there are many other fluids you can use
> -Mark


 Hi, Mark.  No, I did not but I get your point.  I have a gallon of threading and cutting oil sitting right by the lathe so not much excuse for not using it other than thinking the aluminum was soft enough that I didn't need it.  I did notice tonight that the threads were not quite as clean as they were on the larger piece I did yesterday.  I'll use some next time and compare.

Thanks for the input!

Todd


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## Todd3138 (Jun 9, 2022)

snoopdog said:


> Well, If you ain't turnin' you aint lernin, good job!


That's a great point!  Thanks!


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## MrWhoopee (Jun 9, 2022)

Looks pretty good for your first try without adult supervision. For cutting fluid try WD40 or kerosene for aluminum.


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## SLK001 (Jun 9, 2022)

Your thread profile looks "off".  What does your tool look like?  Looking at the thread, your crests look too sharp and your "valleys" look too rounded.  A poorly cut thread can seem like a perfect fit for a nut.  Did you measure the final product (thread mic or wires)?  You should run a triangle file in the threads to clean up the flanks and a flat file to clean up the crests.


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## vocatexas (Jun 9, 2022)

While I find WD-40 pretty much useless as a lubricant, it works pretty good as a cutting oil on aluminum.

Your first two pieces look a LOT better than my first two. Good job!


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## benmychree (Jun 9, 2022)

It does not much matter if the OD of a relatively coarse thread is undersize; there is little loss of strength with a truncated thread crest, you just have to be careful to not cut too deep and get an undersize pitch diameter.  Most threads will be undersize on the OD from fileing the crest to eliminate burrs; I did a lot of 1 - 1/8 -7 threads on disc plow axles, I purposely cut the ODs undersize to eliminate the burr problem and also to lessen the tendency of the thread crests to become damage with rough handling, I was able to finish a thread in only 7 passes, using the "oilfield thread technique".


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## Todd3138 (Jun 9, 2022)

SLK001 said:


> Your thread profile looks "off".  What does your tool look like?  Looking at the thread, your crests look too sharp and your "valleys" look too rounded.  A poorly cut thread can seem like a perfect fit for a nut.  Did you measure the final product (thread mic or wires)?  You should run a triangle file in the threads to clean up the flanks and a flat file to clean up the crests.


I'm using a  60* indexable carbide tip.  No, no final measurement with wires as I don't have any yet.  I did not try the mic though I don't have a "thread mic" either, just what I assume is a normal outside mic set (1", 2", 3").  Good idea on the triangle file.  That I DO have!


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## Todd3138 (Jun 9, 2022)

vocatexas said:


> While I find WD-40 pretty much useless as a lubricant, it works pretty good as a cutting oil on aluminum.
> 
> Your first two pieces look a LOT better than my first two. Good job!


Thanks for that idea.  And thanks for the encouragement.  I know they aren't ready for prime time work product, but I'm not disappointed with them - gotta start somewhere before you can get better!


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## Todd3138 (Jun 9, 2022)

benmychree said:


> It does not much matter if the OD of a relatively coarse thread is undersize; there is little loss of strength with a truncated thread crest, you just have to be careful to not cut too deep and get an undersize pitch diameter.  Most threads will be undersize on the OD from fileing the crest to eliminate burrs; I did a lot of 1 - 1/8 -7 threads on disc plow axles, I purposely cut the ODs undersize to eliminate the burr problem and also to lessen the tendency of the thread crests to become damage with rough handling, I was able to finish a thread in only 7 passes, using the "oilfield thread technique".


That's actually what my friend showed me and is apparently how he does it, too.  He said he likes to go a thou under for the reason you shared.  Something I did was obviously too far under, whether it was the initial turning (I believe I got a measurement of .740 just using calipers - I know they aren't the same as using a mic which didn't seem too undersized to me) or cutting the threads too deep and taking some off my major diameter in the process.  I definitely waited too long to do my first fit check with the nut and should have been a little less aggressive after the first several passes.  As has been said, practice and learn.


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## benmychree (Jun 10, 2022)

Yhere is another device for measuring threads for pitch diameter, called thread triangles, they are easier to use than wires, measured by ordinary micrometers.


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## Todd3138 (Jun 10, 2022)

benmychree said:


> Yhere is another device for measuring threads for pitch diameter, called thread triangles, they are easier to use than wires, measured by ordinary micrometers.


I've seen those used in a couple videos.  I need to read more about them to get a better understanding of how they work.  I get that they fit into the threads, but not much beyond that.


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## benmychree (Jun 10, 2022)

The little grey cells kinked in!  You count the number of teeth on the QC box input, set the QC box for that number and substitute a 30 tooth change gear for the one that was counted.  Bob's your uncle, and Tillie's your aunt!


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## Bi11Hudson (Jun 10, 2022)

Most of what I thread is much smaller than what you would use for learning. And the advice given herein on this site is on point. I wanted to bring up a couple of points that might be useful to a novice. Once you have a good understanding of threading, much of the details will become second nature.

*Theoretical* threading of standard 60 degree threads is called a "V Sharp" thread where the OD is exactly the diameter of what you are working toward. And the bottom of the thread is a sharp "V" or 30 degree angle. But as is often said in many fields, "in theory, theory and practice is the same, but in practice they are not." 

In *Practice,* the tooling will "dull" a whisker so that the V Sharp is no longer sharp. This is outlined in Machinery's Handbook, which has editions going back to 1916. I don't know the current standards for threading, they have changed several times over the years. But the final touch up of threads involves taking a whisker off the OD after the threading pass(es). And the bottom of the thread is actually a slightly rounded form for the same reason. In production, tooling will dull after a while.

In practice, the slightly undersized OD is a practical solution for the outside of the thread. When I cut a 32 TPI thread on a 3/16 rod, I start at 3/16 inch and cut to a sharp profile. Then following the threading cut I touch up the edge with a whetstone to the point that the sharp edges are gone. Essentially duplicating the "standard" given for a Unified Thread. Unified Threads are a post WW2 phenomenum. where, among other things, Whitworth 55 degree threads were reconciled with 60 degree threads. I only understand about 10 percent of all that, it's a deep subject.

The bottom or inside of a thread is a function of the nose radius of the particular tool used. It is normal for the novice to use carbide tooling to learn. Either brazed or insert types. If you look closely at the specifications for a particular insert, there is a given size for nose radius. When grinding HSS tooling, this is less of an issue. For a threading tool, the target for grinding is a sharp point. But grinding tools is as much an art as a science. 

The point is that a carbide insert for a "large" thread, say 10 TPI, is in no way suitable for a smaller thread of 56 TPI. It simply doesn't cut deep enough. Leaving the bottom of the thread too shallow or rounded. 56 TPI requires a much smaller nose radius. As grinding tools is an artform, so too is threading. There are many details to be learned, but the final parameter is "touch". Just how much to make that last pass where you don't adjust any deeper, just make another pass to smooth up the surface. That's the art part of threading. I highly recommend acquiring a copy of Machinery's Handbook. Which edition you acquire is as much a matter of what you will be doing. I have several copies, including a 1942 version and a 2005(?) version for when I need to see what the current practices are.


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## Janderso (Jun 10, 2022)

benmychree said:


> It does not much matter if the OD of a relatively coarse thread is undersize; there is little loss of strength with a truncated thread crest, you just have to be careful to not cut too deep and get an undersize pitch diameter.  Most threads will be undersize on the OD from fileing the crest to eliminate burrs; I did a lot of 1 - 1/8 -7 threads on disc plow axles, I purposely cut the ODs undersize to eliminate the burr problem and also to lessen the tendency of the thread crests to become damage with rough handling, I was able to finish a thread in only 7 passes, using the "oilfield thread technique".


”Oil field thread technique”
I’ll have to check that out.


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## Janderso (Jun 10, 2022)

Bi11Hudson said:


> Most of what I thread is much smaller than what you would use for learning. And the advice given herein on this site is on point. I wanted to bring up a couple of points that might be useful to a novice. Once you have a good understanding of threading, much of the details will become second nature.
> 
> *Theoretical* threading of standard 60 degree threads is called a "V Sharp" thread where the OD is exactly the diameter of what you are working toward. And the bottom of the thread is a sharp "V" or 30 degree angle. But as is often said in many fields, "in theory, theory and practice is the same, but in practice they are not."
> 
> ...


You bring up some interesting points Bill.
I have wondered if there is a range of thread sizes your typical carbide threading insert is designed to cut.
Mine have broken teeth. Not a big fan of carbide when threading. I have gone to HSS,


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## Bi11Hudson (Jun 10, 2022)

Janderso said:


> range of thread sizes your typical carbide threading insert is designed to cut.


To my knowledge, there isn't a specification as such for thread sizes. There *is* however, a specification for *nose radius.* Looking at the less expensive tooling I use, the "kits" or assemblies of tooling will have a fairly large nose radius. That larger radius is more suitable for general turning, having a stronger tip. But for threading small screws like I do, a 1/64 inch nose radius is too large. Think it through, a 1/64 radius equates to a 1/32 inch diameter. Try cutting a 64 TPI thread with a piece of 1/32 rod and you can see the problem. The thread becomes a spiral "U" shaped groove around the screw rather than a "V" shaped thread. In most cases, I grind threading tools from HSS. Carbide is a fair, general, turning tool. But if I'm doing threads, I need more "right" than mass produced carbide can provide. At least in my price range. There may be finer tips at a higher cost. . . I don't know. It's a lot easier to just grind it.

.


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## Todd3138 (Jun 10, 2022)

Bi11Hudson said:


> Most of what I thread is much smaller than what you would use for learning. And the advice given herein on this site is on point. I wanted to bring up a couple of points that might be useful to a novice. Once you have a good understanding of threading, much of the details will become second nature.


That's fantastic info, Bill.  Thank you for sharing!

Todd


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## SLK001 (Jun 10, 2022)

Bi11Hudson said:


> Try cutting a 64 TPI thread with a piece of 1/32 rod and you can see the problem. The thread becomes a spiral "U" shaped groove around the screw rather than a "V" shaped thread. In most cases, I grind threading tools from HSS. Carbide is a fair, general, turning tool. But if I'm doing threads, I need more "right" than mass produced carbide can provide. At least in my price range. There may be finer tips at a higher cost. . . I don't know. It's a lot easier to just grind it.



Although they probably exist, I haven't seen a carbide insert for a thread with a greater TPI than 44 TPI, so at 64 TPI, you're going to have to use HSS.  I use HSS for materials that carbide just can't handle - like threading plastic.  I can get my HSS bits much sharper than any carbide bit that I have seen.


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## Bi11Hudson (Jun 11, 2022)

For the most part, the biggest thread I have ever cut was 16 TPI. There have been a few "one offs" larger, but rare. I use HSS for threading always, I did try carbide once on 16 TPI but didn't like the results. I do a lot of 56 TPI, and some rare occasions have tried 80 TPI. For 90 or 120 TPI, I depend on taps and dies alone. I know 000-120 from memory, but my gauge only goes to 80 TPI. Optical screws are now metric, have been for some years. I don't know the thread pitch, but they are a little larger than 00-90, which is 0.047. The screws in my glasses are around 0.052. For reference, a 000-120 screw is 0.034. My lathe will cut 120 TPI but I'm not up to it. I'm just babbling, this doesn't mean anything.

.


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