backlash question

It seems my R2E4 series 2 has an unexpected amount of y-axis backlash. The x-axis seems perfect. Jogging the table with the incremental buttons I can move it back and forth .0001 very well. The y-axis is another story, it seems to have about .002" worth of backlash before it starts moving. I tried adjusting the backlash compensation dial, but it doesn't seem to have an effect. The screw itself is turning when I hit the buttons, but the table is delayed. Where is the first place to check it? I put an indicator on the end of the screw and it doesn't seem to be thrusting appreciably during movement.

Can anyone explain how to adjust the backlash comp on this machine? Should the dial cause an immediate change in the compensation value?

Any ideas (especially good ones!) would be highly appreciated. Thanks :)

james

A dial to see if the ball screw moves in and out is a good idea, and the first place to look. This would indicate bad bearings or perhaps a bearing locknut that has loosened up.
The nextplace to look would be the ball nut and its mounting to the table. Is there oil on the ballscrew?
When backlash is dialed in (you can watch one of the LEDs on the BMDC or go to the fist monitor and watch the Y axis. The first 2 characters are the backlash.) you need to reboot the machine. That is when the control looks at the pots and translates the resistance value to a backlash value.

George

I put an indicator on the screw as you suggested and it doesn't seem to move in and out. I next put the indicator directly on the ball screw nut and it seems to move perfectly! Table is still delayed almost .003 though... I checked tightness on the allen screws that attach the ball screw nut to the yoke, and the screws from the table to the yoke... but they all seem very tight. (I expected to find some of them loose)

Any ideas? I'm a little confused as to where the play is coming from. When I move the table back and forth, there also seems to be a slight but audible "click" as it reverses directions that doesn't happen on the x-axis.

james

It seems my gib was too tight and it was causing some sticking. I didn't think the symptoms indicated this but i couldn't think of anything else. Readjusting them seems to have fixed the issue.

I think my mistake was adjusting the gibs with the machine turned off. I believe the ball screws were turning when i pushed and pulled on the table, allowing the table to move and showing more rock than was actually there.

james

Glad you found it.
Sometimes drives that are not tuned correctly can cause issues, sometimes gibs, sometimes you get false results because the knee is not clamped.
Backlash is lost motion compensation and may have a cumulative value due to a combination of factors.
I find it best to measure backlash and tuning in a dynamic (more real) state. I increase the backlash by .005 inches, cut a circle including a spring cut, and the sweep it with a dial indicator. It is a lot easier to measure a step (backlash overcomped) than a flat (undercomp but how much), and you can see if tuning is correct by the varience at the 45 degree reading. Just subtract the step from the value in the parameter.

George
George