You could be getting "spindle stretch" as we call it,Bad retainer springs slightly loose preloaded spindle bearings poor retention knob- collet ball retainer. Part lift from the vise.
One old trick (I should know as I am Old ) get a right hand cut left hand helix cutter and try it.
You will probably be surprised at the finish as all you tool preasures are into the spindle and pushed down into your part.
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