It fits on top of the sprocket and has a space for a bubble level to be glued into it. It also places the bubble level 3 inches away from (the center of) the sprocket for increased accuracy of the level.
I also wanted to mention that you want to set the 12 o’clock position in relation to the top beam (so the the top tooth of the sprocket is exactly perpendicular to the top beam). So if your top beam isn’t perfectly level, then that could also be a source of inaccuracy when using a bubble level to set your 12 o’clock position.
My experience is using a level to get a good horizontal potision of the frame is fine. And once it “looks” level, it is good enough. In any case, a fair top beam level will not warp the workspace distances.
But adjusting the sprocket 12 o’clock to a known exact position with the level will make a significant contribution to the MaslowCNC acccuracy through the entire workspace as shown in the above color diagrams.
When I created it my thoughts were exactly what @MartinJ says above.
But now that you ask and I think about it, 1 degree is 1 degree. Whether the level is half and inch away or 12 inches away, the level is only going to rotate 1 degree. But it does give you the benefit of being able to better visualize the position of the sprocket. When you move the motor 1 degree, the tip of one of the “teeth” moves 0.065 mm. But at 3 inches you see it move .2 mm. It “amplifies” the movement by ~33%.
I’m going to print one as well. I’m only about half way through configuring anyhow, but I may as well go get it all as close to right as I can. Mind you I have to wait for the garage to warm up above the sub freezing we’re at now for anything to happen, but having that hanging where I can see it every day will be a great motivator for when I can get back out.
Hope you don’t mind i’ll check the licence an other time
Would never sell something that will give me an advantage to test fork firmware and GC with the best start for a manual calibration with my 3rd sled.
Edit: is this 0.17 or dev 0.18?
Edit 2: The workflow ->inkscape ->freecad would interest me.