Plotting the backlash / calibration target on both the CNC-3018XL and the MPCNC quickly showed, contrary to what I expected, the MPCNC was dead-on accurate, albeit with some wobbulation and a trace of backlash:
Although it looks ug-u-lee, the (lower speed) drag knife cuts come out nice and, because the entry and exit moves match the main cut, the minimal backlash wasn’t a problem.
Turns out only the X axis on the 3018XL had a problem:
Apparently the longer leadscrew I installed as part of the “XL” conversion has a small thread pitch error: about 1 mm short in every 250 mm of travel. I don’t have any (definite, non-handwavy) method to measure the pitch directly, other than by running the follower nut and measuring the results, but it’s consistently short.
Quite some time ago (after blowing up the OEM controller board), I set up the Protoneer CNC board in 1:8 microstep mode, making the GRBL
$100 setting a nice, round 400 step/mm for a two-start leadscrew with 2 mm pitch and 4 mm pitch:
400 step/mm = (200 step/rev * 8 µstep/step) / 4 mm
After a few more measurements suggesting the leadscrew actually traveled 249.2 mm, the correct value will be:
401.28 step/mm = 400 step/mm × 250 mm / 249.2 mm
To verify I understood the problem and solution, I set
$100 to a few integer values around the goal:
The top image shows the leftmost line at the 10 mm mark on the scale, because it’s easier for me to match the ink line with an engraved line, rather than the non-line at the end of the ruler.
The other images show the results for
$100 set to 399, 400, 401, and 402 step/mm, respectively. The results last two results bracket the desired 250 mm outcome, with 401 step/mm being Close Enough™. GRBL accepts a floating point step/mm value, so I set
$100 to 401.28, but I was unable to convince myself the result came out consistently different than 401.00.
Plotting both the tick marks (green) and the knife path (red) on the 3018XL, then cutting the bare paper on the MPCNC, showed the two machines now agree on where the knife should fall. The outer end of the tick marks extends 1 mm beyond the cut line to ensure small misalignments do not produce an obvious white gap around the edge of the deck.
The Y axis continues to match:
And now the X axis looks just as good:
The drag knife corners are rounded, as you’d expect. The cut seems slightly offset from a small origin touch-off error, but the scales now match.