Summary: With ‘skirt’ turned on, Pronterface will always report the object size to be (2 * skirt offset) mm bigger in X and Y than the final object will be.
A short technical post.
When you load a G-Code file into Pronterface for printing, you get messages of the form
the print goes from aa.a mm to bb.b mm in X and is cc.cc mm wide
for each axis, describing the expected dimensions of the object in X, Y, and Z. When I finally started looking at these figures, I thought the X and Y estimates were seriously out. Pronterface reported the 20mm test cube as 32mm wide. And a Sim City 2000 building that is 60mm wide was reported as 72mm.
I was about to start changing E-steps in firmware when I stopped and thought (always a good idea). The variation was constant, not scaling with the object size. My printed objects match the dimensions in the STL files. But not what Pronterface was saying. Much reading of source-code later I finally twigged.
Pronterface includes the ‘skirt’ in its object size. Of course it does, or I wouldn’t know if printing the skirt was going to send my nozzle bashing into mounting bolts, or off the printing deck. My ‘skirt’ is set to 6mm from the object. So with 6mm of skirt on each side, Pronterface consistently reports objects as 12mm wider in X and Y than the object will be when I hold it in my hand.
A trivial discovery, but one that I’ve just spent over an hour trying to understand. Nothing was wrong after all! Which made it impossible to find an explanation on the web – no one else thought it was a problem. Now anyone who searches may find this explanation.