I've seen DXF files screw up the scale as well. When I trade files with someone I usually put in a 1"x1" (well, 25.4mm x 25.4mm) square somewhere in the design so it can be checked and then the artwork can be scaled appropriately if something didn't come out quite right. Typically it's an inches to mm thing, or it's a 72ppi versus 96ppi thing.

There the 3D scanner (https://hackaday.io/project/164454-cheap-3d-scanner-for-makers )using >50 pieces of acrylic , to construct the mechanical platform.... you remind me the difficulty on that ..

It's not been your week has it..  :)

I'm still waking up from the winter slumber.

I would suggest using a 2D CAD package and sending a DXF. I have never had a problem with scale. For very fine tolerances you should allow for half the laser beam width, which is about 0.1 mm i.e. (0.05 mm offset). Just double check that the exported DXF can be re-imported. Some DXF files are a little crazy when exported from their native format.

For my service provider, red is cut, blue is engrave (lines) and black (for enclosed shapes) is raster engrave. I usually have a green box (not recognised by the service provider) around my design for the board size and location, and make the bottom left corn the origin.

@agp.cooper that's very useful know-how. This might be worth provide an example with comments, e.g. on GitHub.