Dimensions off on small objects

Every once in a while I check out my laser to make sure things are correct. Today I was verifying the dimensions. I etch a 400mm x 200mm rectangle on a piece of paper. It’s dimensions are correct. I cut a 25mm x 25mm square and it’s dimensions are off by .2mm. If I scale that error up to 400mm the error would be just over an 1/8 of an inch.

Any ideas on why my step count over a long distance seems correct, but off quite a bit on a short distance?


Have you tested at a lower travel speed ?, the distances may be too short to slow down at too high a speed.

Kerf - Your beam has thickness. When you cut something, you’ll shave about 0.1mm off either side (depending on your tube power and actual beam width - usually 0.08mm to 0.2mm for 40w to 100w). Your 400 x 200 mm rectangle is probably also off by 0.2mm, but it’s much less noticeable at that scale.

I thought about kerf, but the kerf is much smaller than the error I am measuring. I did another etch with a 400x200mm box and the 25x25mm box in the center of it, so this time both boxes done the same way. Using the same measuring device, the 400x200 is right on and the 25x25 is off. Also, my error was closer to .4mm on the 25x25mm box.

@bernd - I was etching at 100mm/S, not terribly fast for my machine.

It could also be calibration / step size error. If you calibrated your machine without accounting for kerf it would be exactly right at the size you calibrated, but wrong for all other sizes, instead of off by a fixed amount (the kerf) for all cuts.

I thought about calibration too. That’s why I always calibrate over a long distance. Any error should be smaller over a shorter distance. If I would have calibrated for a 25mm square, any error I have should be 16 times that at 400mm. If I calibrate for 400mm, any error should be 1/16th that at 25mm.

I have time today to look at this more. I’ll see if I can figure out what is happening.

I think I found a couple issues, The biggest one is my metal meter stick is a POS. I checked it against my micrometer in 100mm steps and at 400mm the stick is off just over 1mm.

I also found my Y belts were a tad loose, tightened those up and the Y axis seems better.

I guess it’s good to check your machine once in a while and really think about what’s going on.

I built my machine just over a year ago now. I expect things like belts should be settled out now, but still something to check once in a while.