[chbot] Laser Cutting

[Helmut Walle]

On Mon, 15 Dec 2014, Volker Kuhlmann wrote:

[...]
> You say if inkscape uses 90dpi and illustrator uses 72dpi, illustrator
> to inkscape would give you 80% (72/90) - illustrator puts in 72 points
> to get a full length, but for inkscape 72 points is only 80% there.
> Inkscape to illustrator would be 125% (90/72).
>
> In my case this would mean that inkscape created the DXF at 72dpi,
> whereas the laser cutter imported it at 90dpi. Worth checking up on,
> if there is a dpi setting in DXF.
>
[...]

Hmmmm, a part of the problem might be that neither Inkscape nor 
Illustrator are actually meant to be used as CAD/CAM software... they 
are both primarily there to create content that can then be presented 
on some kind of visualisation hardware, for the human eye to enjoy. As 
all of these technologies are rendering in terms of pixels, a DPI 
value makes sense.

For CAD/CAM applications, however, what matters most is that design 
units are correctly translated into real-object dimensions. While this 
equally involves the same arithmetic of scaling, the respective 
software products and file formats have actually originated in their 
respective distinct domains. And for that reason, file formats 
belonging to the visual presentation domain often can be specified 
with a DPI value, while CAD/CAM files often cannot.

In this specific case, where DXF is used, I believe there may not be a 
way of including a DPI value in the DXF file. But it is quite possible 
that Inkscape scales the output on DXF export, based on the DPI 
setting of the original Inkscape document. For this, and other 
reasons, I would not want to use Inkscape for CAD/CAM applications but 
would always go with a product actually made for that purpose, e.g., 
Qcad or Librecad. These products feel and operate much more like CAD 
software and are therefore much easier to use for designing parts that 
need to be manufactured, too.

Just some further thoughts... oh, and if you are keen, this is the DXF 
spec: http://images.autodesk.com/adsk/files/acad_dxf0.pdf (not sure 
whether it is the latest version, but in any case it does show the 
metric/imperial measurements distinction).

Kind regards,

Helmut.