I thought you guys and gals might like a little spreadsheet that can be used to fine tune the movement accuracy.
For instance, you might cut a 100x100mm square and end up with a 100.25 by 98.65 square.
This sheet allows you to type in the actual versus modeled dimension of a feature and get a corrected value for the steps per mm setting in Grbl (your machine's control firmware). You may want to run two or three test, but I find that a couple adjustments is about all you need to get within about 0.1 to 0.15mm consistently.
See attachment and feel free to share it wherever fellow CNC nerds are found.
Edit: Don't forget to tighten the V wheels so they sit snugly against the rail and to square the machine and plumb the router for best performance.
So I followed the procedure here of CNC a 100x100mm contour to check for travel. And it turns out that my X = 100.57 adn Y = 100.25. I followed the spreadsheet and adjusted, using the console in UGS, X using $100=39.772 & Y using $101=39.900. But it seemed to have scaled my WCS. I tried to run the same 100x100mm NC program and I got a scaled down version of what the part should look like.
Looks like you start to get into this over shoot under shoot scenario eventually, I'm getting about a thou (.02mm) and decided to call it close enough since the sep/mm isn't approaching a limit... I guess that's about within the machine's tolerances. I also figure I'd mention you can do the same with the Z axis, just make sure to calibrate to 80 steps/mm. $$ lets you check what the machine is currently set at. For the Z if you can attach a dial indicator it's super easy to calibrate since you can read without cutting.
"Pumb the router" also called "tramming" can be done by following this guide. Check the tram of the router and decide what direction needs adjustment. Essentially you adjust the tilt (forward and backward tilt) and roll (left and right roll) of the router by loosening connection points and move just slightly those points. The left/right roll adjustment is usually made at the router bracket. The forward/backward tilt has to be done at the X axis 2040 V Rail Extrusion connection to the side of the M3 base. Additional adjustment may be found by adjusting the bed Y axis mount at the front and back of the M3 base.
Want to make sure I'm understanding the calibrating procedure. The X and Y adjustments are absolute values, right? They don't build on the current (or previous) value? So if I cut the 100mm square box, make adjustments but I'm still off, I don't need to worry about the previous adjustment, just use the spreadsheet and calculate the adjustment based on the new measured value?
Post by Derek the Admin on Jul 10, 2019 2:03:23 GMT
You need to make sure the spreadsheet "knows" your current value. If, for instance, the steps/mm had been changed to 6 from 5 (just making up a number) and the square ended up 100.5mm, the spreadsheet would need to know that the value in grbl was 6 when the cut was made so it can give you a good adjustment value.