Threading 4140 troubles

[Dabbler]

Yes I was on center. 400 rpm is too fast?

On a 5/8 rod at 18 tpi I thread a 160 RPM. I'm chicken to go faster as I'm getting on in years and I don't want to disengage too late. Threading works well at that speed for me.

 

[mksj]

All of the above and a crappy insert. If you are cutting a thread it should peel off, if the insert tip chips or the cutting surface is damaged, you end up with a very ragged cut and vibration. You are threading a bit fast, I usually run at around 250-300 RPM with 4140 using a carbide laydown insert. I have been using Carmex inserts, they work at a range of speeds and I get 100's of threads out of each cutting tip. I routinely thread 4140, as well as O2 steel with these inserts and they do well at a range of cutting speeds without breaking. The other point mentioned is you need to support the other end of the rod otherwise it is going to deflect and chatter. Turning and threading 4140 one needs to be a bit more aggressive with the cutting parameters otherwise it will not cut but push the metal which results in a poor finish. Check the height of the cutter tip, rigidity of your QCTP as well as the gibs, and that you have the proper angle that it is 29.5 and not 30.5. There are probably some rigidity limitations to you lathe. I have a heavier lathe, I only use the cross slide for threading. Below are some screw jacks I recently made, the larger body parts are 4140, the jack is 1144, the latter being quite a bit easier to turn. The jack threads were 1"-14, over 2" and maxed out the internal cutter depth, as well as the maximum depth of the thread cut which was in the range of 0.1".

You should be getting a nice clean curl of metal per pass, this is 300 RPM with cutting fluid. I use an electronic stop so can cut at these speeds to a blind edge, I would not attempt this without a wider relief otherwise.

 

[timmeh]

Stick out/unsupported definitely an issue, speed O.K.(I did semi trailer axle threads, 2"-3" dia. @ 250-200rpm, dry.)
Centre height maybe, but that could come back to stick out and workpiece climbing the tool, that will chip a tip in a heartbeat.
Chipped tip, even one that requires magnification to be seen, finish goes to s#@t real quick.

 

[Jake M]

I will not disagree with anyone in this thread, but I am going to share what I have experienced on my lathe, with my tools...

I bought some cheap carbide insert threading tools from Amazon with 5/8 shank. I was trying to thread 5/8 4140 stock with 18 tpi. I had 2.5 inches of stick out. I was going to use my live center, but decided not to because I couldn’t fit the tool with it. I took a scratch pass then proceeded with .005 doc. Everything was good until I hit .040 deep out of the .067 I needed to cut to hit minor diameter.

My economically oriented carbide set has a thread cutter... It cuts good threads, and would cut that thread. I have a hard time with it though, because the depth of cut that my lathe wants is where you're at. After the first one or two passes, five or so thousandths of infeed (10 on diameter, I'm using a radius dial). The problem is the cutter is not "sharp" enough to work well at that shallow of a depth of cut. Successful or otherwise, the tool isn't deep enough to work the way I wish it would. So my first reflection here is that I would be using a high speed steel cutter.

The tip of the tool broke off, but I continued because the thread looked ok and it was the very bottom of the tool.When I hit .050 it snapped even more, but very jagged,

I've broken the bottom off of inserts, but not the threading tool. (That one just breaks clean off.....) My belief is that when the bottom breaks off, even if the top looks good, feels good, and seems to work, the breaking force and deformation of the carbide that caused it came from the top of the tool. You can squeeze a few more cuts out of it. Maybe. But there's a break/crack/weak spot, or some other compromise to the structure of the carbide. It's dead when the bottom falls off. Like I said though, that's based on other inserts, NOT my threading tool, so for what it's worth.

but I’m pretty sure the thread was ruined at .045 before the insert broke the second time.

If your part is still in the chuck, the thread still has some tangible meat that needs to be removed, I've had success "picking up the thread" just a whisker behind where it was, by shifting the cross slide in and the compound out. Starting over with the infeed (on the compound), taking a few thousandths off the right hand side of the V, all the way down to where I left offf. Essentially, taking a smaller bite, and cutting off the rough finish off of the right hand side, so once you "hit" the bottom of the thread, you've still got the whole left side to cut, but you're at a fresh start with a clean finish to brace/support the tool on the non cutting side. I don't think it's a conventional method, but I've had success. You do loose your stops and your dial numbers, so either that has to be re-figured and reset, or you're doing it the cave man way with dead reckoning and a female test thread.

Just for perspective in making cuts easier/better, I know you're not doing this- I've found that I can cut 8TPI and coarser threads in tough materials by doing exactly that. Cut in until the lathe/cutter/setup isn't happy, and adjust the cross slide and compound "backwards, so that the next cut takes ONLY the right side of the thread, and go in again, right to the bottom, and repeat as necessary. That approach essentially takes your thread shaped form tool, and breaks it up to make cuts that are easier and lower pressure to take.

There's probably a science to doing that (in fact I know there is, it's called geometry, but I just haven't done the math). Just played with it. If you've got a "dead" part still in the chuck, well, that battle is lost, there's an opportunity there to see what can be done.

I was running 400 rpm by the way. My compound was set to 29deg with my qctp turned so it was at zero degrees. I advanced my depth with compound.

My question is, why didn’t this work? Was it too much stick out without support? Was it speed? Was it my process? Or I did everything wrong?

All the above, something else, and a little bit of black magic. If I were doing exactly what you're doing, based on your pictures and measurements, using high speed steel, I wouldn't need tailstock support. Using a carbide cutter with that (Which admittedly, while it's "ok", I don't think my reasonably priced carbide is a good as carbide gets...), there's more tool pressure, I'd support it. I know you're way within the "speed limit" already, but I'd probably be closer to 200 RPMs.

And I can't say enough... You've got a "dead part" there.... Trying to save it with a new tip might or might not be possible, plausible, or realistic, but you've got nothing to loose. If you save it, great. If it's still dead, thread that thing right down until there's nothing left, try different thinigs you wouldn't do to a good part. You'll burn up five or ten minutes until you run out of metal, but you'll learn something about what your material, your tool tool and your lathe likes and dislikes.

 

[benmychree]

I was referring to a recess thread relief at the left hand end of the part, but this is a double edged sword, as necking down the part will make it easier to deflect under the cutting forces, I think that support for the right hand of the part is essential and support by a center is the cure. I would be using a HSS tool, which would be able to reach in-between the center and the part, and yes, a chamfer should be cut on the part before threading, Just plain oil from the oil can does not cut it (pun intended) lubricating oil is not a cutting fluid, a real cutting oil should be used, or better yet, I like to use TapMagic cutting/tapping fluid.

 

[FTlatheworks]

I was referring to a recess thread relief at the left hand end of the part, but this is a double edged sword, as necking down the part will make it easier to deflect under the cutting forces, I think that support for the right hand of the part is essential and support by a center is the cure. I would be using a HSS tool, which would be able to reach in-between the center and the part, and yes, a chamfer should be cut on the part before threading, Just plain oil from the oil can does not cut it (pun intended) lubricating oil is not a cutting fluid, a real cutting oil should be used, or better yet, I like to use TapMagic cutting/tapping fluid.

Ya, I just wanted to test out the carbide insert thread tool, because I want to be able to use the boring bar version to cut inner threads. I did cut a recess on the left side, but did not cut an actual chamfer on the right. I often notice wd40 works way better than the heavy oil in the can I use when I don’t have tap magic or wd40.

I know I can do it with hss even at 500 rpm, but the idea was to start using the carbide insert tooling so I didn’t have to grind a long piece of hss to cut inner threads.

 

[FTlatheworks]

I will not disagree with anyone in this thread, but I am going to share what I have experienced on my lathe, with my tools...



My economically oriented carbide set has a thread cutter... It cuts good threads, and would cut that thread. I have a hard time with it though, because the depth of cut that my lathe wants is where you're at. After the first one or two passes, five or so thousandths of infeed (10 on diameter, I'm using a radius dial). The problem is the cutter is not "sharp" enough to work well at that shallow of a depth of cut. Successful or otherwise, the tool isn't deep enough to work the way I wish it would. So my first reflection here is that I would be using a high speed steel cutter.



I've broken the bottom off of inserts, but not the threading tool. (That one just breaks clean off.....) My belief is that when the bottom breaks off, even if the top looks good, feels good, and seems to work, the breaking force and deformation of the carbide that caused it came from the top of the tool. You can squeeze a few more cuts out of it. Maybe. But there's a break/crack/weak spot, or some other compromise to the structure of the carbide. It's dead when the bottom falls off. Like I said though, that's based on other inserts, NOT my threading tool, so for what it's worth.



If your part is still in the chuck, the thread still has some tangible meat that needs to be removed, I've had success "picking up the thread" just a whisker behind where it was, by shifting the cross slide in and the compound out. Starting over with the infeed (on the compound), taking a few thousandths off the right hand side of the V, all the way down to where I left offf. Essentially, taking a smaller bite, and cutting off the rough finish off of the right hand side, so once you "hit" the bottom of the thread, you've still got the whole left side to cut, but you're at a fresh start with a clean finish to brace/support the tool on the non cutting side. I don't think it's a conventional method, but I've had success. You do loose your stops and your dial numbers, so either that has to be re-figured and reset, or you're doing it the cave man way with dead reckoning and a female test thread.

Just for perspective in making cuts easier/better, I know you're not doing this- I've found that I can cut 8TPI and coarser threads in tough materials by doing exactly that. Cut in until the lathe/cutter/setup isn't happy, and adjust the cross slide and compound "backwards, so that the next cut takes ONLY the right side of the thread, and go in again, right to the bottom, and repeat as necessary. That approach essentially takes your thread shaped form tool, and breaks it up to make cuts that are easier and lower pressure to take.

There's probably a science to doing that (in fact I know there is, it's called geometry, but I just haven't done the math). Just played with it. If you've got a "dead" part still in the chuck, well, that battle is lost, there's an opportunity there to see what can be done.



All the above, something else, and a little bit of black magic. If I were doing exactly what you're doing, based on your pictures and measurements, using high speed steel, I wouldn't need tailstock support. Using a carbide cutter with that (Which admittedly, while it's "ok", I don't think my reasonably priced carbide is a good as carbide gets...), there's more tool pressure, I'd support it. I know you're way within the "speed limit" already, but I'd probably be closer to 200 RPMs.

And I can't say enough... You've got a "dead part" there.... Trying to save it with a new tip might or might not be possible, plausible, or realistic, but you've got nothing to loose. If you save it, great. If it's still dead, thread that thing right down until there's nothing left, try different thinigs you wouldn't do to a good part. You'll burn up five or ten minutes until you run out of metal, but you'll learn something about what your material, your tool tool and your lathe likes and dislikes.

I’m not worried about saving it. It was a test piece for the cheap tooling. I know I could do it with hss no problem, but I wasn’t testing hss.

 

[FTlatheworks]

Something I forgot, I was advancing depth with compound and backing off with the compound. I should have back off to reverse with the cross slide instead, right?

 

[FTlatheworks]

All of the above and a crappy insert. If you are cutting a thread it should peel off, if the insert tip chips or the cutting surface is damaged, you end up with a very ragged cut and vibration. You are threading a bit fast, I usually run at around 250-300 RPM with 4140 using a carbide laydown insert. I have been using Carmex inserts, they work at a range of speeds and I get 100's of threads out of each cutting tip. I routinely thread 4140, as well as O2 steel with these inserts and they do well at a range of cutting speeds without breaking. The other point mentioned is you need to support the other end of the rod otherwise it is going to deflect and chatter. Turning and threading 4140 one needs to be a bit more aggressive with the cutting parameters otherwise it will not cut but push the metal which results in a poor finish. Check the height of the cutter tip, rigidity of your QCTP as well as the gibs, and that you have the proper angle that it is 29.5 and not 30.5. There are probably some rigidity limitations to you lathe. I have a heavier lathe, I only use the cross slide for threading. Below are some screw jacks I recently made, the larger body parts are 4140, the jack is 1144, the latter being quite a bit easier to turn. The jack threads were 1"-14, over 2" and maxed out the internal cutter depth, as well as the maximum depth of the thread cut which was in the range of 0.1".

You should be getting a nice clean curl of metal per pass, this is 300 RPM with cutting fluid. I use an electronic stop so can cut at these speeds to a blind edge, I would not attempt this without a wider relief otherwise.
View attachment 437170

View attachment 437171

I need to make some of these for welding projects. What size lathe do you have? Everyone on this forum told me the pm1236 was plenty rigid enough for most anything. I am not happy with its quality that’s for sure. I would give anything to be able to go back and buy a used machine for the 6k that piece of junk cost me.

 

[WobblyHand]

Something I forgot, I was advancing depth with compound and backing off with the compound. I should have back off to reverse with the cross slide instead, right?

This can work, but you need to make sure you take out the back lash, or you really won't know where you are. It's a lot easier to get good results the second way. Do you have a DRO on your lathe?

I can make decent threads in both my mini-lathe and my 10 x 22 lathe, both, using the second method. My lathes are probably far less rigid than yours. I take my time to set things up, and find things go smoothly. If I rush things - stuff happens - and it's rarely good.

Think you have made a bunch of simultaneous mistakes. Try doing all of them the "better, more standard" way, and your threads should improve a lot and you will break far fewer inserts.