Originally Posted by
HimyKabibble
0.012" is a completely unreasonable amount of backlash for even the worst ballscrew conversion. If they're single nuts, expect backlash to be 0.001-0.005". Sources of backlash can be any or all of:
1) Backlash between the leadscrew/ballscrew and nut. This should generally be no more than about 0.005" for a single nut ballscrew, less than 0.001" for a double nut ballscrew, and perhaps 0.010" for a leadscrew with adjustable nut.
2) Poor mounting or inadequate preload in the leadscrew/ballscrew thrust bearings. This is often adjustable by tightening the nut on the end of the screw. Getting this to zero requires GOOD bearings, well-designed mounts, and proper adjustment.
3) Poorly designed bearing supports. The bearing supports must be very stiff, and must be VERY securely mounted. Cast aluminum mounts can have an amazing amount of flex. The X axis on my knee mill had 0.006" of backlash due entirely to flex in the cast aluminum bearing support. Replacing it with a home-made one made from 1" thiuck 1018 steel completely cured that problem.
4) Overly loose gibs. This allows the table to tilt as it moves, rather than sliding as it should.
5) Excessive friction due to poorly fitted sliding surfaces, poor or overly tight gibs. Generally, you want to tighten the gibs until apparent backlash increases, then back it off a bit. The apparent backlash will increase when the gibs are tightened as the added friction will cause other things (bearings, supports, leadscrew, etc.) to flex more.
Tracking down the source of backlash is time-consuming, and requires going through each piece of the system one at a time, measuring how much it's moving, and eliminating any undesirable movement.
Contrary to (apparently) common belief, software *CANNOT* correct for backlash. The best it can do is partially mitigate the effects, but it is incapable of eliminating the side-effects. Even with software backlash correction, you will still end up with "divets" where the tool stops, or changes directions in a cut, or anyplace the load on the work changes direction. The cutting forces will push the work one way or the other within the range of the backlash, leaving that much uncertainty in the actual tool position. You will see this most clearly by cutting a circular pocket. You will find you have "divets" in the circular profile at 90, 180, 270 and 360 degrees, where the cutter cuts slightly outside the desired circular perimeter. If you want to get truly good results, you MUST fix the hardware to eliminate the backlash.
Regards,
Ray L.