(See part 1 for the beginning of the process)

Yesterday, I set out to measure backlash in the Z axis of Beaver HDZero CNC. I concluded that it had about 15 microns (0.015mm, or roughly 0.0005”) of backlash in the Z axis.

Today I’m going to measure the X and Y axes. They’re each a bit different:

  • They use larger ballscrews than the Z axis (1610 vs 1605) with twice the pitch. That means that one turn of the stepper motor goes twice as far. A single step should be 12.5 microns, and the stepper’s encoder should be able to measure about 5 microns.
  • The Y axis has a pair of steppers driven in parallel, one on either side of the gantry. In theory, there’s no reason why this should matter, but I could see extra inaccuracy creeping in.
  • I had a difficult time getting the ball nuts assembled correctly when building this; I repacked each of the Y nuts at least once, but the X nut seemed to work okay so I didn’t repack it. It lost at least a few ball bearings, though, so I probably should have repacked it. We’ll see if that matters.

The process here is exactly the same as the Z axis from yesterday. I fed the CNC a program that tells it to move from 0 to 0.200 mm, in 0.003mm steps. It paused after each step so I could take a measurement using a Mitutoyo 543-302b digital dial indicator. The dial indicator is accurate to 3 microns, which is why I’m moving the CNC in 0.003mm steps. The dial indicator is wired into my laptop via Mitutoyo’s USB cable. The cable makes the indicator act like a USB keyboard; every time I press the button in the middle of the USB cable it “types” the current measurement into the computer.

Here’s the Y axis’s motion:

Y Axis: Intended vs Measured Position.

Compared to the Z axis from yesterday, the “stairsteps” on this are very pronounced, as expected. Comparing the delta between the two lines gives this:

Y Axis: Normalized difference between intended and measured location.

This looks like around 15 microns of backlash again, very similar to the Z axis.

Now, on to the X axis. It’s a bit more interesting.

X axis Intended vs Measured Position.

Notice that it moved differently on the first vs second and third iterations?

X Axis: Normalized difference between intended and measured location.

Uhm, yeah. There’s way more backlash, and it seems to have been drifting at least a bit over time. Repacking that ball nut would probably be a good idea.

Here are all three axes on one graph; it makes it clearer that Y and Z are very similar, but X is an outlier.

Intended vs Measured Position.