hy, you just hit into setup stability machine is performing fine on the 0.6dia, but not fine on the 0.4945dia
no, not yet, especially on gang lathes, because the problem may not be with the machine, and there are cases, when, after a service, the machine geometry is even worse then beforei am going to replace the bearings is my first thought just looking for suggestions and if most of you do your turning in one pass or multiple passes.
look, for example, a new machine may come with a geometry alignment tolerance < 0.003mm, and inside the " alignment paper" you may find that the maximum accepted value is 0.030mm
there had been cases, when, after service, machine geometry was within the maximum tolerances recomanded by the cnc producer, but now as low as when the equipment was new; this means that the result of the service intervention was bad, but officialy, it was within factory specs
this being said, let's return to your machine, so 0.6is ok, 0.4945is not :
... try to begin cutting values in between those 2, and locate the linearity of the effect, see how it changes when you continously change diameters; thus cut 0.6, then 0.57, 0.55, 0.52, 0.5
... also, try to cut a 0.4945 from a piece of material that is shorter enough to be clamped ok inside the machine, thus check contact surface with guide bushing and main spindle collet to be maximized; longer materials may induce vibrations during cutting, and those vibrations have a bigger effect on smaller diameters, because there is no room to dump the vibration
... you said that also during finishing, still the tir was bad : try to run finishing directly, thus cut a 0.4945dia starting with a smaller stock, because, if you rough, and then you finish it, you may loose contact with the guide bushing, and this means that the material is not clamped/guided as it should be
do you check the roundness inside the machine, thus right after the turning, and before cutting; do you use a dial inside the cabinet ?dia. is out of round over .001
unfortunately, my advices are general, i can not be specific, since i don't see the machine working
however, before considering the bearings, i would invest a few days to locate the issue / kindly