Losing Z steps

I am using EMC on a Harbor Freight mill retrofit (using a Geckodrive G540 parallel port driver for the steppers) and while it has been working great, I recently tried to do some engraving, which has some very small motions in the Z axis- The tool position gradually gets higher and higher above the workpiece, and if I let it go, after 10 characters, it is easily 0.1" above. I tried working with the g-code to make the Z motions integral multiples of the step size (even moving to the raw step size to try to eliminate the microstepping), but that doesn't improve things. Thankfully, the error is always positive, so the tool moves away (rather than deeper) from the workpiece. This is really frustrating, and I'm not sure what is the best way to further diagnose this problem.

Regards,

Matt

I would guess your acceleration is set too high for that axis as you have to move against gravity but that doesn't make sense that your moving up... run some stepper tests on the Z to see if it is loosing steps. Check for binding in the Z axis as well or anything else that might inhibit motion.

linuxcnc.org/docview/html/common_Stepper_Diagnostics.html#r1_3

What voltage are you driving the G540 with?

John

if your using microstepping , i'd try reducing the microstep , also you say adjusting the step size to suit z motion's ?
give the indications of the type of ballscrew size , pitch , any belts etc and lets see , how the figures work out etc

The G540 step input is fixed at 10 micosteps and the step output morphs to no micosteps as the speed increases and the cross over point is settable by a pot iirc.

www.geckodrive.com/ark-2/support.html?pid=14&id=25

John

I'm driving the motors at 24V at 2.2A. The motors are rated at 2.5A. I will try running the tests you pointed out. I see that the test program points out if I have a problem, but what is the best way to choose the values? I was pretty conservative when I did the initial setup, keeping the max velocity and acceleration low (This is a hobby setup and I did not see a need to push the limits for speed, particularly as I'm also learning at the same time).

While I didn't adjust acceleration in my engraving file, I did experiment with keeping the feed rate very slow- down to 0.1 ipm, and that did not affect losing steps- I still lost steps.

Looking back at the initialization files, I may have made a mistake in my calculation with respect to adjusting for step size- I had calculated the step size multiple with respect to the X/Y, which is 32,000 microsteps per inch, while the Z is 20,000. Even with using a non-integral size, I shouldn't be losing steps, right? Particularly if I'm just transitioning between 2 different Z positions, one for cutting, and the other for moving to the next spot.

Thanks for the help. This software (and community) is pretty awesome.

Here is some info on stepper formulas to calculate voltage and other things.

wiki.linuxcnc.org/cgi-bin/emcinfo.pl?Stepper_Formulas

The speed is only a part of the equation, the acceleration and deceleration are the other part. Keep in mind that during direction reversal that the software will generate steps as fast as the acceleration setting will allow it to to reach the commanded speed. So run the test, then try cutting your acceleration in half and run again.

Is your Z axis counter balanced?

Also, 24 volts might be a bit anemic and would limit the acceleration and top speed of the stepper motors. The higher the voltage the faster you can accelerate and the faster you can go within limits.

John