Wednesday, May 22, 2013

MXL Belts & Pulleys

The printer has been using T5 belts with printed pulleys, and they've been working pretty well.  However, we decided to investigate some other belt/pulley options.

The folks at OpenBuilds sell a belt & pulley combo at a pretty reasonable price, so we ordered some up and tried them out.
MXL belt & pulley. The belt pitch is 2.03mm (actually 0.080 inch)

The kit they sell uses an MXL belt, and obviously the pulley to match.  MXL has a belt pitch of 2.03mm, which is the distance between the center of the teeth.  The pulley has 20 teeth, so one revolution of the pulley will move the belt 40.6mm (not 46mm as I had calculated due to my inability to enter 2.03 correctly into a calculator, but enough about that).

We use a stepper motor with a 1.8 degree step angle, so that's 200 steps for one revolution.  The stepper driver is set to 1/16th micro-steps, which means on our setup, 3,200 steps will move the motor one complete revolution.

The firmware we use (Marlin) has a configuration entry for steps per unit:

DEFAULT_AXIS_STEPS_PER_UNIT = {64,64,2560,470};

This was setup for the T5 belt & 10 tooth pulley.  With these, we have 50mm per revolution, and at 3200 steps per revolution, this gives 64 steps per mm.  (3200 / 50 = 64).

The MM2 uses the GT2 belt & pulley.  Here you have a 2mm belt pitch and a 20 tooth pulley, so the steps per mm works out at 80. (3200 / 40 = 80).

With the MXL, we need 78.81773399014778 steps per mm. (3200/40.6 = 78.81773399014778).

I plugged in the new number as the 2nd value (they're in x,y,z,e order) and checked out the results by printing a couple of 20mm x 20mm boxes as vase.

I measured them with our trusty micrometer, and they both measured out to be 19.8 x 19.8.  Slightly smaller than the desired 20 x 20, but not enough to worry about.

We may try some GT2 belts & pulleys and replace the MXL's with those.  That way we don't have to worry about rounding errors, but I don't think the error is that significant anyway.

Update:  After writing this, I went back and checked the values using 2.034 for the pitch (this is what various websites give as the MXL metric pitch).  The value I used inside Marlin was changed to 78.74015748031496.  I ran a couple of test cube prints, and there was no apparent difference in the Y dimensions of the prints, so obviously a pretty small adjustment.  I'd imagine on a larger print it might be more of an issue, but I still couldn't see it being a deal breaker.

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