I was trying out exporting a Fusion 360 sketch to DXF to get it into Inskape to make an SVG. I know that Fusion 360 has a CAM module, but I am not there yet. I have just started experimenting with the software.
My Inkscape document is set up in mm and the dimensions of the SVG look the same as the sketch’s. I then import it to jscut and get some test gcode. I cut it out and it looks smaller. I test some moves to make sure my $100, $101, $102 are not messed up and they are spot on. I then measure the piece and it looks… suspiciously 90/96 of the original 
I bring it back to jscut, change the ppi from 96 (Inkscape 0.9.x default) to 90, check the gcode’s X max and it is exactly my piece’s width plus the cutter radius.
So, I know what’s wrong, I know how to work around it… but I am really interested in knowing why. Any ideas about the DXF export resolution in Fusion 360… and how come Inkscape shows the right dimensions but exports in a non native resolution?
UPDATE:
I tested with Makercam, just to take the CAM out of the picture. The same happens as in jscut: 90/96 smaller.
I generally use Inkscape directly for vector editing. I was just fooling around in Fusion 360 cause I wanted to draw around projections of 3d solid models. This is how I ended up in this kind of complex experiment.
Btw, I tried with an online DXF converter, then opened and edited the SVG in Inkscape. The document properties and “bounding box” (i.e. group of paths) are identical between the two test files (Inkscape DXF conversion and online DXF conversion). One is 90/96 shrunk, the other is fine. So there’s definitely something wrong with DXF conversion in Inkscape. Or at least the particular export I fed it. Go figure…
The header in the DXF is defined in mm, so no problem there either.
$INSUNITS
70
4
As you mentioned, when you open a DXF in Inkscape, a scaling value is expected either manually or automatically. This is where I suspect there is a bug. It seems that Inkscape wants to draw pixels and needs to apply a resolution value. My assumption is that a value of 90ppi has been left hardcoded since a pre-0.9 release, when the native resolution was indeed 90ppi.
What is crazy though, is that once the DXF has been converted, the displayed size on the canvas in mm is correct.
Anyway, just in case someone runs across the same situation the solution is to
- either convert from DXF to SVG with another tool
- or use 90ppi in jscut/Makercam
