Fixing accuracy issues on a cheap laser in software

Cheap laser cutters from ebay are, well, cheap. But there are better machines out there. The one I've got came with air assist and a platform that can be adjusted in height. It works well enough but the parts I cut have slight geometrical defects.

The aim of this tutorial is to correct dimensions and skew of cut parts.

For example a square ends up with angles that aren't quite 90 degrees and its height is different from its width. With more control on the machine firmware, these issues could be fixed once and for all in it, but we paid for a low end machine, we'll have to deal with that some other way.

You'll need a caliper or some way to get reasonably accurate measures.

Start by cutting a paper or cardboard square, 100x100mm is ok for a 150mm caliper.

You'll end up by something like this. Measure the length of a, b and c.

Now we'll find the angle we need to skew our square in order to really get 90 degrees angles on the cut square.

A bit of maths (cosine law) tell us

Your gamma angle should be close to 90 degrees (if you end up with 1.57, you found radians instead of degrees, multiply it by 57.3)

The angle we want equals gamma - 90 and it sould be close to 0. It'll probably need adjusting because it's hard to get accurate measures.

I'll show how to do the corrections with Inkscape (you can use any software that can apply skew and scale transformations).

Open the transform panel (Object->Transform... or Shift-Ctrl-M), set the horizontal angle in the skew tab to the angle you just found, select your square and click Apply in the transform panel.

Now cut the square again, the diagonals should be equal. If needed, adjust the angle using 0.1 or 0.05 degree steps until you're happy with the result.

Fixing the size is easier, measure a and b on your fixed square. If you used a 100mm square, the height correction is 100/a and the width correction is 100/b, enter these values in the Scale tab of the Transform panel, select the square and click Apply.

If you cut that image, you should now get a perfect 100mm square, note your angle, width and height correction values somewhere and apply them before cutting.

If you know what the laser kerf is and want to account for it, use: (100-kerf)/a and (100-kerf)/b