[jamoore3192]

Hello friends,
I've been busy converting a G0704 to CNC, and I've run into my first real issue. The scenario is this. I zero the z axis in g54, command a g00 z-1.0, the head moves exactly one inch down. However, I then enter a g00 z0.0 and the head moves up by aprox .960". Soooooo, I reenter the g00 z-1.0, the head moves back to exactly one inch below the g54 offset... I don't suspect backlash here, because the repeatability in negative travel. Any ideas??

PS I'm using Mach 3, and NEMA 34 steppers with 5mm pitch TBI ball screws.

[CaistorAl]

try moving the z with a slow feed rate... say 5 ipm.

and back.

if it hold position, maybe your stepper is losing step.

mayb also try purposely entering a zero in the backlash in mach3, if it happens to be blank.

[jamoore3192]

Ok, I just gave it a try... Same result... I also played around with the MPG. The DRO always matches my indicator dead nuts, whenever I make a negative z move. Any positive jog, and the indicator never registers as much travel as the DRO.... But again, even after that, any Z negative move will sync the indicator and DRO back together. Bad Stepper???

[CaistorAl]

hmmm...

I think I would try swapping the z axis motor with one of the other axis motors...see if the problem follows the motor or stays with the Z.

If it follows the motor, you know its replacement time.

If it stays on the Z axis, keep searching there. Look for mechanical backlash first - I presume you've already done that.

I would swap the Z stepper driver with another axis next...see if it follows the driver.

If the problem stays on the Z, then you either have a breakout board issue or mach issue.