Brand new machine build, working on tuning and aligning it. Using the kit purchased from Vicious, with latest firmware. My Z-axis turns easily by hand without steppers engaged. When engaged, sometimes the z-axis stutters/vibrates and doesn’t move just when doing simple up/down tests using the LCD panel or default (G1 Zxxx) commands (meaning, no change to default rates). I don’t see any obstructions in the pathway and if I disengage the motors I can easily move it up and down after a vibration. If I turn the Z adjustment nut by hand I can easily cause this to happen (minimal force applied). I’ve double and triple checked my wires and they all look good, and I’ve measured voltage on each of the two coils (on the unplugged connector) while moving the stepper by hand (so no broken wire path).

I verified the voltage on the current limiter and it was as expected (about 450mv) since it was purchased from the kit. I’ve increased the current limit to 600mv and it is stronger and I’m so far able to run it up/down and not have the vibration. But I’m still concerned, because I find it surprising how little force I need to apply to get the motor shaft to turn on the stepper while it is engaged (seems much easier than X or Y, where it appears very hard to turn, though I’ve not done extensive testing/analysis on that).

I also think I don’t understand the math correctly for setting current limits. I’ve looked at the Pololu video, and read up some on the ratings math (since RAMPS is at 12V and the motor is rated at a lower voltage) and I’ve computed the following: for the KL17H248-15-4A stepper (1.5A at 4.2V) I compute (1.5a * 4.2v) / 12v = .525 amps. To set VREF on the driver (DRV8825) I use “VREF * 2 = current_limit_amps” or using the original setting of .45v that would be .45 * 2 = .9a. But my prior math said the limit of the motor is .525 amps at 12V … which means the motor was already being overdriven before I increased it to 600mv. I feel I must be misunderstanding something simple here!

So my simple questions are:

- Should I be able to move the z-adjustment nut by hand (without excessive force) while the stepper is engaged?
- Is the correct course of action to adjust the current limit, or should I be looking more at the motor or the driver (or something else)?
- Do I misunderstand the math? If so, what clues might you have? If not, why is overdriving the stepper ok?

Thanks!

Chris