Changing the initial chucking won't improve on this if it already checks out within .02mm before HT.
I seriously doubt you will get rid of the runout if the part requires a heat treat process after it is turned. The 2 ends are very unlikely to maintain the same center if it distorts at all.
If the part checks good before it is HT'd, then the HT should be done first, if at all possible, then straighten if need be, then turn or grind the one end true to the spline. I think this comes down to the process sequence and methods you have chosen.
You might place this in a heat treating fixture to stablize the part to minimize warpage, but unlikely to eliminate it within .02mm. Hanging them from one end helps. Although, if these are quenched to harden, the chances of warping are much greater.
DC
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