If I cut a 10mm x 10mm cube, I’d get a result of 10mm in the X axis and about 8mm in the Y axis. It’s not losing steps because it continues to cut everything properly. Just all Y dimensions are short by about 2mm. 1mm on the top and 1mm on the bottom. Is it possible there is slack in the head somewhere? I try to physically move it, but there’s no looseness. Any suggestions on what to look for or do? All the belts are tight, and test runs with no load result in perfect distances (i.e. mark 500mm, move the head, mark and head align). Thanks
So are you cutting short only when cutting?
What happens if you run Y 10mm With a load?
What happens if you run Y 10mm Without a load?
same for 20mm?
With the router unplugged (always think safety) can you move the tip of it? in the Y and X.
is it firm or flimsy, use a bit of force it should be firm
10mm x 10mm was an example. I guess I should have explained a little more 
10x10 no load cuts 10x10
10x10 load cuts 10x8
100x100 no load cuts 100x100
100x100 load cuts 100x98
Yes, when I trying to move the tip it is very firm in all directions. I use force short of pushing the motors to skip.
What struck me was that you are short 2mm in both the 10 mm trial cut and the 100mm trial cut,
so its not the GRBL setting…
Can it be the thickness of the cutter?
Are you cutting inside or outside or following the middle of the line?
Make a 100mm line in the Y axis lift then jog it away and then make a line in the x axis, so Your eliminating the corner which has the thickness of the cutter.
Or cut a square using the setting “outside” see if that helps
Sorry thats all I got… good Luck
regards Neal
Ok, I’ve fixed 90% of the problem and actually found it was happening in the X and Y axes.
The belts, though tight were stretching and were causing it to be 2mm off. So when it was on the inside pushing, the belt stretched and the it would be 2mm shorter. when it pushed on the outside, same. So depending on the cut direction, i’d get diagonal misalignment by about 2mm.
Now, I’ve made the belts super tight. I actually worry they are too tight. It was a nightmare to keep the belt from slipping on the ends. But now, it’s cutting much much better, except in the X i’m still seeing .2-.3mm misalignment. I don’t know if I can tighten the belt anymore, so I’m wonder if the head isn’t moving that much.