Inconsistent Axis Movement

[jeanhank]

Hello all,

I've been configuring Mach 3 to work with our machine, an old Shizuoka knee mill driven with Geckodrive.  I've been using the built-in motor tuning wizard in Mach 3 and running a simple square contour to test programmed movements vs. actual movement.  The problem is that I've been getting a trapezoid -- the machine moves the intended 5.000" in the Y- direction, but 4.9985" in the Y+ direction. 

Our mill has a dial located between the motor and the leadscrew, and the dial indicates this discrepancy as well, which to me suggests that the problem is that the motor itself is not turning the same number of steps in both directions.  If the problem were backlash, the dial would indicate 5.000" in both directions, since the error would be after the motor and dial, correct?  In any case, backlash compensation did nothing for us.

A mistuned motor should move the same (wrong) amount in either direction, right?

Anyone have any idea what the problem might be, and how to fix it?

Thanks!
Jay

[Hood]

Is it stepper or servo?
By "built-in motor tuning wizard" are you meaning the utility to set the steps per unit?
Do you have the active state of the step pins correct for the drives you are using?
Hood

[jeanhank]

Hood,

Thanks for your reply. 

We are using stepper motors.
I am indeed referring to the utility to set steps per unit.
What do you mean by "active state" of the step pins?  As far as I know, these are configured correctly, as the X and Z axes work fine.

[Hood]

Depending on the model of Gecko stepper drive you may need active low or active high set for the step pin.
I prefer to calculate the steps per unit manually rather than let the utility work it out as calculating is likely to get you closer than it is easy to measure, especially on industrial machines as they usually have excellent quality ballscrews fitted.
Hood

[jeanhank]

Sure enough, the active state was set incorrectly, but fixing that didn't solve the problem.
I'm not sure that the inconsistent movement is caused by motor calibration (see above).  Any other ideas from anyone?

[RICH]

Is the motor connected directly to the ball screw? How is it connected ie; motor shaft / coupling / trust bearing on the screw side
What type of coupling?
Is there a trust bearing on the screw side? Is the trust bearing preloaded?
Is the .0015" difference consistant anywhere along the axis? ie; not only over the 5" travel
Is the dial attached directly to the motor shaft?

A picture would be worth a lot of words if it shows the axis arrangement......
 
RICH

[Hood]

Sure enough, the active state was set incorrectly, but fixing that didn't solve the problem.
I'm not sure that the inconsistent movement is caused by motor calibration (see above).  Any other ideas from anyone?

I never meant it was, I was simply saying that I think calculating steps per unit is a better way than doing the set steps utility.
The active state can often account for the losses you are seeing but alas it seems not to have been the problem in your case.
As Rich says a pic of the setup would be good
Hood