lathe turning

[klugster]

I'm machining some parts approximately 98in long 5.870 OD/4.740 ID(4130ht28-32rc) I have had them heat treated, pack/bored/honed, then stress relieved. chucked/steady rested/tailstock in. when making a cut the part warps out no matter what DOC,SF I run. Wall thickness is very critical on these parts. What can I do to prevent this? Is there a fixture i could make for the ID to prevent this movement?

Thanks

KLUGSTER

[HuFlungDung]

If you are turning the OD, you should be able to run your steady rest at a little over halfways down the tube. The part must already be perfectly round if running in the steady, as any egg shape running within the steady will be duplicated in the turned surface.

That means you should only be cutting a piece with an effective supported length a little more than 50" long. That amount should cut pretty good without deflection.

What kind of tooling are you using to cut with?

[Geof]

I have supported thin wall tubing by filling it with a low melting alloy, melting point is around 65 - 70 Centigrade, 150 - 160 F, called Cerrobend. Cerrobend is very rigid when solid and will make your tubes behave much like a solid shaft. But it is expensive so this may not be practical for your job

[ViperTX]

Well this tubing has a 1/2 inch thick wall....I would suspect that the warping is being caused by stresses caused by how the part is being supported.

Where along the length are attempting to turn??

[klugster]

Im using TiN coated carbide CNMG 543 AC2000 FOR ROUGHING, DNMG 432 MA (UC6010 GRADE) FOR FINISHING. IM CUTTING FROM FACE TO 67.56 IN. TODAY AFTER ROUGH CUT, TIR AT 30IN MARK WAS .065. TOOK IT TO THE STRAIGHTENING PRESS, STRAIGHTENED. RERAN ROUGHCUT PART TIR .020, STRAIGHTENED, RAN FIN. PASS, TIR .005, I CAN LIVE WITH THAT. ANY OTHER SUGGESTIONS. (I HAVE 32 MORE PEICES TO RUN)

KLUGSTER

[stupidog]

A follower rest would be ideal for these parts. If you don't have one, you could probably rig up one without a lot of trouble.

[HuFlungDung]

To back up a little, how do you know these parts are of uniform wall thickness, and straight, when they came back from the heat treater? Should there not have been a stress relief done before the honing?

Stress relieved is not the same as annealed condition, and uneven removal of stock is going to cause warpage to occur.

How do you attain initial straightness of the part before you begin cutting. Are you certain that the wall thickness is uniform?

Are you maintaining concentricity somehow, or does the part only runout after you back off the steady rest?

[Mcgyver]

this size of job is out of my league but the symptons suggest to me that it hasn't been very well stress relieved and every time you take a cut you are changing the internal forces and it does the banana. the more you cut past the outer (most stressed) area the less remaining stress there is, hence better tir. i also agree with HuFlung, your list of ops has stress relieving at the end - shouldn't it be up front?

[MBG]

edit nothing

[MBG]

nothing