Threading 4140 troubles

[FTlatheworks]

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

 

[FTlatheworks]

Here the setup

 

[mattthemuppet2]

My guess is the stick out and larger depth of cut is causing the work to climb up on the tool and spring back, breaking the tip. I would expect live center support would solve the problem.

If you can't fit the live or dead center in there you can always (if you have the material) turn the part longer to mount the center and then turn off the excess afterwards

 

[benmychree]

You cannot easily thread with carbide without a recess for runout, the tools will nearly always chip or break at the tip, 4140 HT can be threaded with HSS tools at reasonable speeds, perhaps 30 to 40 RPM. The stickout easily could have been an issue also.

 

[FTlatheworks]

My guess is the stick out and larger depth of cut is causing the work to climb up on the tool and spring back, breaking the tip. I would expect live center support would solve the problem.

If you can't fit the live or dead center in there you can always (if you have the material) turn the part longer to mount the center and then turn off the excess afterwards

I thought about that. I will use the live center next time and trim the excess off.

 

[FTlatheworks]

You cannot easily thread with carbide without a recess for runout, the tools will nearly always chip or break at the tip, 4140 HT can be threaded with HSS tools at reasonable speeds, perhaps 30 to 40 RPM. The stickout easily could have been an issue also.

I cut a recess close to the chuck, but you mean a recess opposite end of the chuck? I noticed the tool did break right at the start of the work. I assumed it was because the stock wasn’t running perfectly concentric, but didn’t think to chamfer, or recess the end.

 

[SLK001]

I was running 400 rpm by the way.

WAY too fast. Are you cutting on the centerline?

 

[markba633csi]

Were you using any cutting fluid?
I agree the lack of a center support would make it iffy

 

[FTlatheworks]

WAY too fast. Are you cutting on the centerline?

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

 

[FTlatheworks]

Were you using any cutting fluid?
I agree the lack of a center support would make it iffy

I was using some oil in an oiling can