A Motor "Failure" in a 3D Printer

Troubleshooting 3D Printer Motion Problems

Stepper motors are some of the most reliable machines human beings have ever invented.  And they should be- all they are is some steel, and some magnets spinning on bearings, and some coils of wire.  There are no brushes to wear out, so as long as you don't get it so hot that the insulation burns off the wires or demagnetizes the magnets, a stepper should work for many years, especially in the relatively benign environment of a 3D printer. 

The typical driver chips on 3D printer controller boards don't have enough current drive capability to burn up motor windings of typical motors used in 3D printers, though I suppose it would be possible for one of them to fail shorted and send a lot of current through a motor.  So unless you have a failed driver, it is very unlikely that you have a burned up motor.  If you do have a burned up motor you will be able to tell by its smell, and you will surely have a dead driver chip, too.

What I'm trying to say here is that in general, if you have a motion problem in a 3D printer, the least likely cause is going to be a failed motor.  So look carefully at everything else before you start replacing motors.  Everything else includes cables and connectors (very high failure rates) and driver chips (also high failure rate, especially if you use plug-in driver modules), drive pulleys and couplers, (are the screws tight?) and configuration issues.

If you've ever built a kit-type printer or even read a stepper driver data sheet, you'll have been warned about connecting or disconnecting motors while the printer is powered up.  The problem is that when you connect or disconnect a motor while it's powered, a large voltage spike that can kill the driver chip is generated by the inductance of the motor coils.  Now what do you think happens if you have an intermittent connector or motor cable?  It will connect and disconnect the motor and destroy the driver.  If you have a dead driver, carefully inspect the cable and connectors before you put a new driver in your printer, or you may end up with another dead driver.

An Atypical Failure 

I recently did some long overdue maintenance and mods on SoM which resides at the Milwaukee Makerspace.

I rebuilt the Y axis entirely, eliminating the noisy ball screw and converting back to belt drive.  I also put in a Duet controller and set it up for high microstepping ratio.  That quieted down the Y axis, but then I noticed that the X axis was making a lot of noise that it didn't used to make.  The noise sounded like something was vibrating, and it left closely spaced vertical lines in X axis parallel sides of prints.  Ugh!

This fine pattern was showing up on X-parallel sides of prints because the X axis motor was vibrating.

When I inspected the X axis and noticed some belt dust on the motor mount, I didn't think much of it. The machine has a Gates belt and a standard, cheapo GT2 drive pulley on the motor, both of which have been on there for years.  I disconnected the belt and tried moving the extruder carriage along the linear guide- it was as smooth as could be.  No problem there.

I tried running the motor without the belt and sure enough, it was vibrating.  I verified that the screw terminals and connectors were in good condition and tested the cable with a meter and found no issues.  I thought maybe it's the driver, so I swapped driver on the controller board- nope- the motor kept vibrating.  It had to be the motor itself.

I tried turning the motor shaft with my fingers and found that it took an unusually large amount of force to start it turning.  That's not right!

In SoM, the X axis motor was mounted hanging below the X axis, with shaft and pulley pointing up.  I thought that maybe some of the belt dust I saw got into the top bearing in the motor and was gumming things up.

I replaced the motor with another, pulled from an old stratasys printer mechanism.  It ran almost completely silent again, like it used to, so I did an autopsy on the failed motor.

The failed motor, with belt dust.

Cover removed- pretty simple...  There's very little clearance between the rotor and the stator pole pieces, so it wouldn't take much to jam it up.

All the pieces laid bare. I tried spinning the shaft on the bearings and it spun freely.  So much for the belt-dust-in-the-bearings theory.

I tried spinning the bearings and expected the top one to be sticky.  Nope.  Both spun freely.  Hmmm.  I inspected the rotor and found a couple small ferrous metal flakes (sorry, I didn't take a picture) stuck to the magnetic rotor.  After carefully removing the flakes and inspecting the entire rotor under a microscope, I blew out the inside of the body of the motor with some compressed air and then put the whole thing back together.  It spun freely like it used to, with just the normal, gentle bump-bump of the magnetic detents.  I suspect that metal flakes were getting between the rotor and stators and it was jamming things up.

So where did the metal flakes come from?  Hmm.  I think the bearings are the most likely source.  The motor was industrial surplus when I installed it in the printer about 6 years ago.  There's no telling how much use it had seen before I got it, so maybe the bearings are at end-of-life.  I'll order new ones and install them.

If you ever have an axis that starts vibrating a lot, inspect the motor!