3D Printing: Erratic Z Axis Motion

From a discussion on the Makergear 3D printer forums

A Makergear M2 user, while troubleshooting other problems, had the Z axis begin stalling and moving erratically.

the random up and down movement doesnt make any sense

It’s what happens when a stepper is mechanically overloaded: the rotor can’t turn at the commanded rate.

Start by cleaning & lubing the Z axis guide rods and leadscrew. If that solves the problem, just clean and lube a bit more often. Which none of us do until there’s a problem, of course. [sigh]

If it continues to stall, reduce the Z axis speed by a factor of four. If that solves the problem, then perhaps you tweaked the speed while you were fixing other problems and never noticed.

the technical reason why the motor would move in the opposite direction

The windings set up a rotating magnetic field which, in normal operation, drags the rotor around with it. When the rotor stalls, it vibrates back-and-forth and may wind up synchronizing with the field in the wrong direction.

Old Western movies had a similar problem with wagon wheels turning faster than the frame rate and looking like their spokes rotated backwards.

The stepper may emit horrible sounds, but stalling doesn’t do any damage to the motor or its driver.

I took the bottom of the motor apart

No sugarcoating: disassembling a stepper demagnetizes the rotor. You must buy a new Z-axis motor.

The motor is assembled with the rotor demagnetized, then it’s magnetized in place. When you take it apart, the rotor smacks into the stator, which creates a localized high-density magnetic path between the rotor poles. The rotor poles can’t support the high flux and demagnetizes.

You can put the motor together and it will “work”, in the sense that the rotor will go around, but the decreased magnetic field reduces the torque for a given winding current. You can’t increase the winding current, because the motor will overheat.

The PCB traces look mangled and warped

There’s a conformal coating over the whole PCB to prevent corrosion, so what you see is perfectly normal.

Any analysis of the data from my previous posts?

You’ve been doing a lot of fiddling with the machinery as part of finding the extruder problem, so: did you, at any time, even once, unplug / disconnect the Z axis motor when the power was turned on?

If so, that likely killed a driver transistor in that channel. Order a new RAMBo board along with the new motor.

New Rambo board came today and the z axis is working properly now.

Moral of the story: never fiddle with the electronics with the power turned on!

2 thoughts on “3D Printing: Erratic Z Axis Motion

  1. Some of us who got our start with vacuum tubes got into a habit of debugging energized circuits, doing things (like shorting active elements to each other) that would be madness with solid state circuitry. There was the added danger of hundreds of volts floating around, too.

    1. Even keeping one hand in your pocket won’t save you when you’re doing that!

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