The way to measure backlash that is inherent in the nut/screw/table interface is to put the tip of an indicator held in the spindle on a vertical face (side of a vise, angle plate, whatever) then grab the table and shake it back and forth. You'll find out in a hurry how much slop is in the axis. I just totally rebuilt a retofit RF45 that utilizes Thomson preloaded ballscrews and servos. I bought it secondhand from a guy that didn't know the difference. When I disassembled the X and Y axes, I discovered that the builder had not locked the nuts to prevent them from rotating in their mounting block. Good components do not make up for poor implementation and attention to detail. The X and Y axes went from .008" and .006" backlash respectively to .0015". Most likely this is being introduced by the bearing and not the screw/nut interface. Regardless, I can live with it now.
Joe |