On May 17, 2012, at 11:13 AM, Csanyi Pal <csanyi...@gmail.com> wrote:

Well, I think it can't be done that way, because an involute of a
circle is actually a spiral. In BRL-CAD is not such shape out there,
right?

Not as an implicit object, but you could describe one with NURBS surfaces, sketch+extrude objects, and volumetric or extruded bitmap geometry (albeit aliased). 

On this link are .dxf files that I can easily open with LibreCAD.
 
You should be able to open those with our dxf-g importer too.  It won't import reference data like datum planes, but any 2D sketch geometry should import as a BRL-CAD sketch object.
 
I imagine that that one could draw this way in BRL-CAD too a sketch,
right?
 
Yep.  Our sketch editor can be used for that purpose using bezier curves.

However, on my Debian GNU/Linux wheezy/sid I can't use neither in Mged
nor in Archer the sketch function to draw circle. I can draw only
lines.

If I could draw such a sketch then I could extrude it to a cog, right?
 
The GUI sketch editor in mged isn't the easiest to use but does work.  You have to select the "create" button for every arc, curve, line you indend to create.  The status line should give some basic instructions on how to use it. Alternatively, you could use a 2D editor like qcad to make your dxf, then import that as a sketch into BRL-CAD.

Cheers!
Sean


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