help cutting 6 pitch thread

[joe1970]

I am trying to cut a 6 pitch thread on a cnc lathe with an 8" chuck
(hurco tm-8) every time i get to minor diameter with my tool the insert chips.
Have tried all the somewhat obvious problems , Center, ridigity,and these di
ffrent style inserts laydown , top notch,on edge etc..................
Anyone out there know any tricks or tooling to achive this on a lathe of my size??
Thanks for your help
Joe

[Tim S]

Joe,

Are you using a canned cycle? Assuming that this is a standard 60 Deg V Thread, make sure your depth of cuts get smaller as you get closer to the minor diameter. It's a fast thread (6 pitch) but nothing extreme. What is the Major Dia? How many RPM's are you running?

[NC Cams]

What material too???

Some stuff work hardens as you cut it and you may find that you have to hog it rather than baby it - stainless is one material that doesn't necessarily want to be cut in ittsy bitsy amounts.

See how simple it is to cut a generic 6 pitch thread???

[joe1970]

I am using a canned cycle 4140 mat. tried diffrent rpm's settled on 450 . had to pull the job. sent out to be rolled. I could get 1 pc. on a fresh insert. then chip out and loose the pitch dia. Tried all I could think of seems to me the stress on the insert is too great. the thread height produces too much tool pressure??
Lost

[joe1970]

1.5 major

[NC Cams]

4140 can be fairly tough to machine in annealed state - rather gummy and has a tendency to work harden. Thus, each time you cut is is sort of a different steel thus wants different conditions due to work hardening.

Cuts MUCH better if it is double drawn (anneal followed by normalize - they are NOT the same process). Normalizing leaves it more in Rc28 or so, maybe as high as Rc 32 if it is force air cooled. This tends to chip better and machine to a much nicer/smoother/untorn finish.

I hear the "that's bull" comments already. All I can say is try it. We did and were amazed at the difference - then again, we're doing complex shaft turning work, not threads.

[Tim S]

I'll agree with NC Cams on the hardness factor. You definitely get a better chip with it at that hardness. To me the RPM's seem really slow (450 RPM = 175 SFM). I would program that at about 1300 RPM (500 SFM). Carbide doesn't like to run slow. What was the fastest RPM you tried?

[joe1970]

I had it up too 1100 rpm, material was anneled but not normalized . Went with a slower rpm due to chatter problems you are right at a slower rpm the material tears instead of cuts. I guess next time try again, higher surface footage , diffrent carbide grades? I am not one to give up any more tips would be appreciated.
I'll get it and keep posted
Thanks,
Joe

[Tim S]

Joe,

Can you post the code from your canned cycle? That will tell me a lot more of how you are running it.

Tim

[One of Many]

Is your canned cycle a plunge advance at 90 degrees or does it advance at a 29.5? If 90, could that be changed so that only the leading edge is cutting?

You could try and use HSS to get a better chip curl, but no angle to the advance will still load the tip from both edges. Full profile cutters may not be needed for that course of thread either. They will have a weak tip naturally at the sharp point.

DC

[RON MEYER]

joe, try contacting chris thale at hurco. 317-293-5309. he is the lathe apps guy there. he should be able to give you some help. 1.5-6 thread probably should not have a dead sharp insert. that size thread will probably show some radius in minor diameter of the thread. check the handbook. ron meyer