Machining on an existing surface

[Thom Dudley]

I know there's a way to do this, but it's stumping me. The hive mind here is excellent, so here's the problem:
We have a scanned model of a skateboard which we want to machine on a 3-axis PRS - the scan is rough (as you can see in the picture - sampled at .5" increments) as it's mostly a guide for the odd heights & contours of the skateboard deck. How do I make toolpaths that ultimately ignore the initial model of the board. Or to ask another way, how do I use my reference board to create the cuts? We have yet to determine WHAT will be cut, whether pockets, full 3d machining or V-carving will be necessary. We have MoI, a fairly basic "version" of Rhino, which does not recognize DXF files.
http://tinyurl.com/cc4l7p9

[knight_toolworks]

Are you making the mold for the skateboard?

[Thom Dudley]

No, though making molds was a big selling point for the bosses when we got the shopbot. We have boxes of blank decks left over from a project, so we're trying to find ways to mark them. Laser engraving works well but goes out of focus (and we laser cut stuff all day, we're bored of lasers), so I wanted to try carving 'em on the shopbot - without carving the board shape its self, just using the model to follow the shape.

[adrianm]

If you had Aspire you could use the project onto 3D feature of the 2D toolpaths to carve onto the top of the model once you had created it within Aspire.

No idea how to do it without Aspire though.