Potential defective stepper motor? Input appreciated

Greetings,
I recently completed a Kossel Mini with a hinged auto level and after initial setup got some decent prints. After starting a slightly larger print, I returned to tell tale signs of missed steps but with the delta 'twist;. The part had layer shifts stepping away from tower C and the layers began rotating ccw such that they were less and less parallel to the plate. I double checked the driver voltages and re-tensioned the belts and tried again. At this point I could no longer perform auto level (standard Marlin firmware ) or auto calibration (using Rich Cattell's Marlin firmware) without consistently having bed hits or walking off the build plate. I went back to manual calibration verification and while the geometry values looked pretty good, I immediately noticed that when sending g0 f8000 x0 y0 z0, the carriage for tower C would descend further than A or B resulting in my nozzle being several inches off center. Curiously, when sending g28, all the carriages would advance upwards at the same rate ruling out some microstepping discrepancy. Still, I swapped in an entirely different Ramps and set of stepper drivers to verify and found that the issue persisted.

So to my main question, has anyone seen an issue like this before and could it potentially be caused by a stepper motor that is behaving strangely in one direction of rotation?

Thanks for your help!

Sam

Sounds like the pulleys may be slipping on the motor shaft(s), check they're tight. If the motor shafts have a flat and the pulleys grub screws are not completely tightened against it, the shafts can rotate a bit before the grub screws catch on the flats edge. This results in backlash.

Yep, I wondered about that as well however the grub is tight against the flat.

Okay, next idea; you could try moving the motor connections around. A delta is a triangle so move the corners around clockwise, or anti-clockwise. Plug the X motor into the Y socket, the Y into the Z and the Z into the X. Move the endstop plugs around too. If tower C still moves lower etc then its the motor, if not then its something else.
Actually you might want to try it first without moving the endstop plugs around, I think that might help rule out an endstop problem, although I'm not sure what that problem could be.

That's what I love about deltas, its all the same x3(well within a couple of mm anyway).

1. Have you tried swapping the C motor cable with another stepper motor cable?

2. Have you tried removing the C tower belt, and checking that the C motor shaft revolves freely in both directions? Stepper motors sometimes have metallic debris inside, which can hinder rotation. It can usually be cleared by taking the stepper motor apart.

3. Are you quite sure that the C carriage isn't sticking, or the belt snagging?

Edited 1 time(s). Last edit at 06/10/2015 01:21PM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].