1997 VMC15 linear machine losing steps?

[wozzwinkl]

I have a 1997 VMC15 with Advanced Motion Controls AMP-0014 drivers and Reliance motors (smaller DC servos, ~4" in diameter).

I have been going through getting it fixed up and tuned in since buying it (new X-axis ballscrew, nut, thrust bearings, and linear rails). I am up to setting backlash and I am seeing something strange that actually confirms something else I observed while running parts. I set up programs from the Fadal manual to set the backlash. These were all set to run at 100ipm, as the manual says to set the feed during the test to something that you intend to cut at.

The basic premise is that you start with the tip of a test indicator just off of the face and below the top edge of a stable smooth block. The program starts and moves towards the block and pauses to let you zero the dial. Pressing start makes the tip move up 0.25" in Z to clear the block, then 0.25" in the positive direction of the axis, 0.25" negative, then -0.25" in the Z to bring the tip back to where it was. Pressing start at this point moves the tip away from the block so that it no longer contacts is. Pressing start one more time begins at the start of the file again, and you can keep pressing start to keep cycling through the program.

The distance from zero is supposed to be the amount of backlash required to get the control to compensate for the lost motion. Even if your backlash is set to zero, repeating this process several times in a row should still consistently go back to zero in the first step, then to whatever the backlash amount is.

HOWEVER, as you can see in the video below, this is not what is happening. Instead of going back to zero, it goes to something slightly above zero, maybe a tenth. Reducing the feed rate causes it to have less and less severe versions of this problem. At 30ipm, it is rock solid. At 65ipm, it is almost undetectable. At 100ipm, it is creeping steadily up...



Both X & Y axes do this, but oddly the Z does not.

While running parts, I will sometimes have corner rounding issues and turning my rapid from 100% down to 50% seems to make them much better. Running at slower rapid isn't necessarily an issue, but I don't think this will solve the lost motion issue, and it makes peck drill cycles take forever...

I have set the master clock and balanced the servos.

Has anyone else experienced anything like this? The machine is running OK and making decent parts, but I feel like no matter what I do I can't get rid of this issue.

Any suggestions would be greatly appreciated.

Thanks,
Jon

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[adaprecision]

I think your problem is not so much a machine problem, but the fact that the test indicator was never design to be successively slammed like this test does to it. Slowing down as you have noticed is going to cure the problem.

You also mentioned some cornering issues, if this machine was built in 97 it probably -5 control boards, and if memory serves 100 IPM is approaching the max this machine can handle, my guess is if you slow your feeds down to 80 to 90 IPM most of your issues will go away.

[carbidecraters]

If you think your loosing position you should tune the servos first them look at your couplers.