I'm having a problem that has me completely stumped. I have a Taiwanese Bridgeport clone knee mill that I've recently CNC'd. I used double-nut ballscrews, big servo motors, and timing belt reducers. The problem is a backlash anomoly that I can't explain. I decided to check the backlash a few days ago. I put a test indicator on each axis, and used Mach3 to move first one direction, then the other, in 0.001" increments. I found what appeared to be 0.0028" backlash in Y, and 0.0060" in X. i.e. - any time I reversed direction on an axis, Mach3 would move a few thou (and the motors *were* moving) before the indicator showed any movement. So, I dialed compensation into Mach3, and was then able to move consistently and repeatably over short distances on both axes. However, when I moved a large distance (say 10"), the move would come up long by roughly the amount of the apparent backlash.
I then tried cutting a circular pocket, with backlash compensation enabled, and it showed the signature "divets" at 12, 3, 6, and 9 o'clock. I turned off the backlash compensation, and the pocket came out pretty much perfect all the way around.
So..... How come I measure backlash when I check with two different indicators, yet in actual use, there appears to be zero backlash? I'm sooooo confused!

Regards,
Ray L.