My 8L lathe offsets have been changing randomly. I can turn 4-5 of the same part and on the next part the OD is too small. When I check tool length offsets they have shifted by about .0025. All tools seem to be out. If I reset tool offsets and the work offset ( using a master tool) everything is ok for a while, but a few parts later specs are off again.
Has anyone had this issue and know a solution?
What material are you working with? Are the inserts worn or broken at all when the shift happens?
I am cutting brass. No damage to the inserts.
I think I have diagnosed the problem. I have a small shim befind my facing tool to angle it a bit more toward the spindle by a few degrees. I think with heavier cuts I am getting some tool post rotation, which pushed to tool nose into the part, effectively lengthening it and producing the smaller part OD by about .006.
At the same time, when I check the alignment of some of my longer drills in reference to the spindle they are about .02 away from the toolpost. That magnified variance made me think that the toolpost must be twisting, and sure enough when I sweep it I find that it has.
I previously replaced the OXA toolpost with an AXA. Big improvement in rigidity. When I did that machined the top of the mouthpiece block to get the right height and to remove some significant irregularities, so that surface is fine. I do not machine the base of the toolpost becuase it seemed fine, but I might revisit that.
I have really cranked on the locking nut and will see if that holds. If not I will try some emery cloth unrder the toolpost. I think that will work because I am not pushing the machin very hard cutting mostly brass with conservative depth of cut and feeds.