I am baffled on this one. We are tapping 3/8 -16 in a plastic material, 5/16 drill, tap held in collet, speed is slow, around 60 rpm. It will do a dozen or so holes fine but sometimes it messes up and strips out the threads. I have been watching it like a hawk and although it is hard to see because of the chips, it does look like the spindle/feed rate gets out of wack on the messed up hole. Also when I first start, it usually will mess up one of the first ones, but then do another dozen or so fine. If it is the case that the feed, speed ratio gets out of wack, why would that be happening so intermittently? Any other ideas? could it be that Im going way to slow and the machine isnt good at tracking the spindle rpm down there?
Thanks.