After once again figuring out how to read a vernier height gage, I measured the height of each end of the MPCNC rails:
- Position the gage near the end of the gantry’s travel
- Twiddle the knurled ring to lower the probe (a.k.a. lathe bit) until …
- It firmly captures the paper slip, then …
- Twiddle the ring the other way until …
- The paper barely moves
- Read the vernier and take a picture
So the numbers come out one paper thickness higher than the actual rail height; subtract 0.1 mm = 4 mil to get the true height:
In round numbers, the difference is under 0.3 mm along each rail.
The outer numbers on the lower sketch show the difference between each reading and the lowest value along that axis: the left rear corner is (roughly) 0.5 mm higher than the right front. The numbers inside the square give the additional height, rounded to sensible values, required to raise the low corners.
Which means you can’t plot at, say, Z=-0.2 mm to reduce the pen loading, because the pen doesn’t uniformly touch the paper across the entire plot:
These images have been perspective & aspect ratio corrected, then ruthlessly contrast-stretched to make the traces visible; the lighting isn’t that awful in person!
With the plot at Z=-0.2, the legends toward the front came out OK, but they’re missing along the far edge. The Spirograph traces go completely missing toward the left rear as the pen rises away from the paper, although I think we’re also seeing some ripples in the paper sheet.
Although such a small error probably makes no difference to a wood router, let’s see what we can do.
Manually editing the G-Code to put successive traces at 0.1 mm increments from Z=-0.3 to Z=-0.6 mm, then replotting on the same piece of paper, shows the problem a bit better:
All of the legends remain at Z=-0.2, because I wasn’t up for editing every pen-down command.
Even at Z=-0.6 mm, the pen doesn’t quite touch in the left rear corner. Previously, I’d been plotting at a nice, round Z=-1.0 mm, which worked fine. I didn’t run any tests below Z=-0.6, but I think Z=-0.8 would draw a complete plot.
That agrees reasonably well with the height gage measurements.
It’s obviously impossible to re-level the rails by dinking around with the corner post lengths, because I can’t move the EMT in precise increments and it’d never stay in that position anyway. Instead, I should slide shims under the three lowest corner feet to raise them enough to match the left rear corner.