Measuring Backlash, Part 2
(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:
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:
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.
Notice that it moved differently on the first vs second and third iterations?
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.