Reply To: Question about GRBL's Homing function and strange Z Axis stepper behavior

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Thanks All,

Yes, right now not doing any micro stepping. I was starting with some fairly slow values.

Initially I’m starting with defaykt feed if 250 mm/min and default seek of 200 mm/min

Acceleration at 10 mm/sec^2 Homing feed ($20) which is what is being used to set the common feed rate for homing on v 0.8. v0.9 has split those out but I’m currently not running v 0.9 GRBL. The step pulse width is set at 10 usec

So I have homing feed set to 250 mm/min which is used for the 3 axis. X and Y appear to be just fine but Z is just sputtering around at that rate. If I drop the common homing feed rate to 120 mm/min then Z runs smoothly. Again in this case I’m not driving the z Axis assy, just the Z motor in the mount with the coupler pressed against the bearing as recommended in the assy instructions. But just in case I even pulled the coupler up a bit to make sure that nothing was binding. After that test I pushed the bushing back down tight on the bearing as there was no change.

I do have a Mega and there is a RAMPS board on the way so I may need to test with that.

I was just surprised that the Z Axis was behaving the way it does when X and Y (with the two steppers in parallel) appears to move just fine.

I have to compute the finally steps/mm and other bits since I’m just using something that is fairly close (as measured with X and Y axis movement.

The steppers are ones that I had gotten a couple of years ago when I first was considering building a CNC machine but had too many other projects going on to get to it until now.

They are Minebea 17PM-K402-P4V
Basic specs are:
Rated voltage 6.00 Rated current per phase .80 Winding resistance 7.5 ohms per phase Holding torque 3400 g-cm Rotor INertia 75 g-cm^2 Detent Torque 200 g-cm

If I need to I’ll change out the steppers, but it seems like these should work OK.

Currently running with a 12V power supply. Maybe I need to increase to 24V?

I don’t think that it is a bad stepper since I had tested with the steppers and a simple program on the UNO a while ago and they all appeared to perform the same. So baring a failure while the motor was sitting around I expect the issue is elsewhere. I may pull the Z axis driver and install it on a 2nd CNC shield / UNO that I have sitting around and test the stepper there, unless someone sees something that leaps out in the basic GRBL settings.