Motor Screeches

Hello Forum!
I'm a newbie with a problem with my z motors making a screech sound without moving. Links to videos below.

Left motor screech: [www.dropbox.com]
Right Motor action during left side screech: [www.dropbox.com]

Initially it happened with the left motor only and the right motor moved. This led to uneven z axis and nozzle to high or low.
After some reading I tried adjusting the z mot trim pot. Clockwise just made the left side screech longer and louder, CCW made the right side screech also.
It happens at the beginning of a print and appears to be caused by the command G1 z0 F5000 in my Slic3r output G code. I need experts to advise me;
1. Is F5000 the problem? What value of Fxxxx should I try? I'm reluctant to randomly trying +/- 1k increments since I don't know what is affected.
2. What parameters in Slic3r/ Pronterface I need to change once I find a good value?

RapidBot Mega Printer, Printrboard Rev D, Marlin firmware

Thanks!

3D printing is cool stuff, the only thing cooler is a 3d printer that works!

Edited 1 time(s). Last edit at 10/20/2013 05:16PM by lvinson911.

My Prusa with M8 Z axis threaded rods is using F7800 from Slic3r, but I'm not sure if my firmware is really allowing that. I used to go a lot slower, but I put some effort into removing all binding and drag. Try going slower anyway, or see what happens without a load on the Z axis. Going slower won't hurt anything other than giving your nozzle more time to ooze. Hmm... I can't seem to find the setting in slic3r.

Have you checked for any binding on the Z axis? Are the screws oiled?

I did check for binding and my threaded rods are oiled. I did get occasional binding but that I attributed to the height difference caused by the left side not turning while the right does. I now go thru a process of setting the z endstop height then get both sides within 0.1 mm by hand & recheck z height. But as soon as the screech happens it off kilter again.
Should I lower the F value to 4000, 1000, or something in between?

Even F1000 is excessive for the Z axis. Try something like F200. You can also change the Z max speed in firmware to something similar, then it won't matter if your gcode tries to tell the z axis to move too fast.

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