Geared Stepper Extruder RPM Calibration

Just yesterday, my geared stepper from MakerGear arrived.

I’ve already upgraded my electronics to Makerbot’s Gen 4 set, so for the firmware and electronics standpoint it was pretty easy.

  1. I made a 6-pin to 10-pin cable to hook up the Gen 3 stepper driver. The MakerBot wiki didn’t give much detail if it was supposed to be the first 6 pins or the last 6 pins of the 10-pin connector that needed to be wired up, but after studying the images, it looked like it was the first 6 pins that had to be wired up.
  2. Connected the stepper driver to the A-axis of the motherboard.
  3. Mounted the stepper driver in a very dodgy fashion.
  4. Copied a few lines from the Thingomatic with Strepstruder machine entry. Most importantly the extruder, A-axis and changing my driver to “makerbot4ga”.

However a few tweaks were needed to steps per RPM and A-axis scale.

First thing I tackled was the steps per RPM. And figured that since the stepper had 200 steps per revolution, and the gearbox was 13.6:1, and my driver was doing 12 steps, that my steps per RPM should have been 200 x 13.6 x 2 = 5440.

So I plugged in these numbers and things seemed to work. However, upon closer inspection, I noticed that it was almost making a full revolution, but not quite.

What was causing it? Was it that the gearing wasn’t exactly 13.6:1? Was it the ±5% per step? I didn’t really know. All I could guess was that it appeared to be consistent. I did know however that it wasn’t due to skipping.

Sure I could have slowly tweaked the steps per RPM till I got what looks like a full revolution, but that isn’t very scientific or precise.

I placed my camera on a tripod and took pictures after every 5440 steps or 1 “full” revolution (by extruding at 12.5 rpm for 5 seconds). From these pictures, I could figure out how many degrees short of 360 I was. To make it easier to take measurements later on, I put a bit of tape on the stepper shaft.

What I found was that the motor was on average about 3.49 degrees short of a full rotation. And the calculations that followed: (3.49… / 360) x 5440 = 5492.77… ≈ 5493

I stuck that number into the steps per RPM and lo and behold, the motor makes a full revolution when it’s supposed to.

I’m not sure if this number will work for every stepper from MakerGear, but I hope the process and math will be useful to anyone facing this issue.

No prints from it yet. I still have to ream the 6mm hole on the drive pulley to 8mm so that it’ll fit on the stepper.

Ps. Thanks to MakerGear for the extra chocolate. It went really well with my morning coffee before work.