Turning

[SirDenisNayland]

While I know you have a lot of experience Pixman, the things you are describing are for roundness issues, not so much tapering and runout, deflection which is the issue he has described. I do understand that runout can be a biproduct of roundness issues. While lack of material roundness and a guide that isn't tight enough can have affects on straightness, the fact that his largest diameter is flawless leads me to believe that both of those are a non issue. Inherent material stress aside, it seems pretty evident that the turned part is experiencing unwanted load in an other than axial direction which points to a tooling issue in my opinion, possibly as simple as being off center which you did mention and several of us have, so I would largely believe that not to be the issue either as I'm sure the OP knows how to set a tool.

If its not being bent from the pickoff or ejection, then it can only be happening at the point where the part is turned: a tooling issue. Or as has been mentioned, inherent material stresses which would be a real big problem but is just about the only other possibility.
Speaking of which, does anyone know of methods to check for such a thing without going through a trial and error elimination of variables such as what's going on here?

[PixMan]

Sorry about that. You are correct in that I've mis-read the problem.

I agree with you Sir Denis, it does appear to be some sort of internal stress. That material is one that should be pre heat-treated and then tempered back as a means of stress relief.

[lukehonor]

I've run a similar job on my Tsugami BE20. We run a 304 S.S. job in which we turn 1/8" bar stock down to .055" in one pass. The shaft is approximately 1.2" long. I had an out of roundess problem as well and I solved this by using a Kennametal DCMT insert with approx .004"-.008" nose radius. I ran my spindle speed up to 8000 and fed around .002"-.004" per rev (maybe less, can't remember).

WIth such a small turn diameter, it is very critical to make sure your tool is on center. If I had to bet, I would say your tool is off center.

Good Luck to you!