On opening a document out of fusion 360 (my main use case) or elsewhere Inkscape defaults to px for the unit, which would be fine if by converting back to mm it didn't turn them into different measurements.

Is there a way to stop this behaviour?  I'm trying to use Inkscape to design &/or prep prior to using the laser software for output to the laser cutter.  If I can't be confident about the measurements Inkscape is unusable to me.

There are templates in Inkscape to use mm or inches. You can save a document you have set up as your default to open every time.

I use Fusion and Inkscape together frequently for precision laser work.

I've tried saving over the template with a document that has my laser bed dimensions and units set to mm.  It works great for new documents.  Doesn't do anything for opening an existing dxf in inkscape.

Have you tried importing the dxf to the template?

So open inkscape and choose import then the dxf?  Just tried that and a 416.2mm object is now 10622.537mm.  Any idea what would cause that?

It's a bit of a bummer that means you can't use file associations at all if you have to manually import them.  Rdworks has no issues with importing them from fusion.

I'd adjust the import scale to 25.4.  1 (one)

It should be remembered on following imports.

Just tested a 100mm sq dxf imported into Inkscape with a scale of 1. Size looks right.

Not sure what changed, but I imported again, and imported close, but larger than normal.

There's the file if you want to have a look at it.

That's weird. But Sheetcam shows the same extra 2mm height, so it isn't Inkscape.

Edit: LibreCad also shows 418.2.

A bit more is shown in the Inkscape display for line thickness, but stroke can be turned off, or geometric bounding box can be set in Preferences.

Would this help then?  Here's the fusion file.  https://a360.co/2GSILQd

It seems the sketch needed to be fully constrained to export properly. Fusion is weird sometimes. 

My revision to the fusion file made some assumptions on dimensions and constraints. Inside the zip.

If it's not exporting properly, why would rdworks be picking up the right dimensions fine?  Same with QCADCAM.

Beats me, but even Fusion imports it at 418.2.

I would report the bug to Autodesk.

It's supposed to be 418.2

That's what I'm saying inkscape is the only one importing with incorrect dimensions.

The file provided at the link has the long sketch figure at 416.2mm.

The screenshot in the post #8 also shows 416.2mm

Inkscape has no problems with the dxf. Fusion reports one distance, but exports another.

So, after you constrain all the sketch figures, then save the sketch as a dxf. All is well.

Have a nice day.

What about this then?  It's a 10mm square from Fusion, units set at mm, but letting inkscape read the scale from the file it thinks inches, if I set it manually, it imports it at 10.265 per side.

It's adding a pixel per side.  Same happens if I import it at 10" it shows up as 10"+1 pixel.  So 254.256mm

There can be apparent size difference due to stroke width (usually 1px) when the bounding box is set to visual rather than geometric. 

The setting can be changed, or the stroke turned off (use fills and/or view outlines). 

I always set the scale/units manually. 

Thanks for going to the effort of making the gif.  Definitely helped!

Sweet, that seems to be the issue.  Inkscape adding to the shape with the stroke.

Do you know if there's a way to save a template for when opening a dxf so that I don't have to set it up every time?

Most of the dxf import parameters are retained from the last use.

Keep in mind Fusion exports dxf in mm*, so if you are using imperial units in F360, a 10" line exported will be 10mm in Inkscape unless you set the scale to 25.4 in Inkscape, or switch to mm in Fusion before exporting.

*This is despite the units flag in the dxf file being set to any other units. 

I'm doing it all in mm.  Only reason I quoted the 254mm square was because reading the scale from the file resulted in a 254mm square (+ 1 pixel).

I know you said most, what isn't?  What does it get it from?  Because I'm getting basically the original default.svg parameters on bringing a dxf in through a double click in windows explorer.  It's not retaining anything from the default.svg I saved.

Sorry to not be more clear. The import dialog is independent from open-file. 

I would open Inkscape first with the mm default template. (Yes, I understand you want the file association, but this is only an extra click.) Then import the dxf. The dxf dialog box should have your last settings.

You might also be interested in the Fusion add on "DXF for Laser"... it has kerf compensation.