OK, so this is what I’m doing. I have about a 32"x50" cut area. I made a grid of 3 rows and 5 columns. Each column is 12" apart (for 48" wide) and each row is 10" apart (for 20" total). I cut a 1/2" helical drill in each location. I cut those into the spoil board, and I used the depth guage of some digital calipers to measure the depth of each cut. Since it’s all from the same gcode, any difference in depth should be because I fail at calipers, or errors in Z.
I expected to see three rows of nearly the same numbers, but with deeper cuts in the middle than the outside, because I expected the tubes to bow in the middle. I only cut three rows because I wanted some more data points.
What I actually found is that the difference across the 48" axis is actually as much as the difference across the 20" axis… Here’s some raw data:
average depth of each column (in mm, this is the platform moving across the pipes):
Left 0.96 1.55 1.37 1.30 0.23 Right
average depth of each row (in mm, this is the gantry moving on the wheels):
I wish there was a better way to visualize the data, but any plots I’ve made just don’t tell the story.
All in all, I’ve very happy with these results. The complete range of z values over that entire area is 2mm (48in is 1200mm…). That’s about 1/16th of an inch over 48inches. But the “standard deviation” in that area is 0.6mm, since except for the corners, it’s very consistent, indeed.
The measurements for the columns makes a lot of sense to me. Shallower on the ends, deeper in the middle. The rightmost column is shallower than the left, and I can only figure that the pipes are bent, or the vacumn tube is causing some error.
The measurements for the rows don’t make sense to me. I don’t know why the rear of the machine would be cutting shallower than the front. I’m going to look at the machine a little closer with this in mind, and see if I can figure it out. Some ideas I have are that the wheels aren’t completely flat, and the machine landed on a different side for the rear and front rows, or maybe the table thickness is slightly different, and at the front, the bottom wheels are pulling the gantry lower, whereas in the back, there’s less downward pulling of the wheels, so the top wheels flex a little less. I might cut a groove in the spoil board from front to back and measure the depth along it to see if I can tell if it’s a smooth sloped pattern, a discrete one, or a wavy one.
At any rate, I definitely think these numbers are better than I could have hoped. I am sure they are better than the MPCNC I made, because I was adding more to the depth of cut just to consistently make it through material. Ryan said at some point that in the size range of a smaller low rider, or larger MPCNC, that the low rider might be the better choice. I think this supports that. I would be curious to see what other folks find with a test like this.
BTW, here are all 15 measurements, if you’re curious:
Left 0.66 0.84 0.79 0.77 0 Right
1.03 1.72 1.5 1.39 0.24
1.19 2.09 1.81 1.74 0.44