A while back (the patents have expired) I invented a game in which plastic pieces interact with plastic tiles contained in a tray, that make up the board.

As an example I added part of a page in the Patent showing a square tile with a round central blind hole, at the bottom of which there is a sloping surface. The piece that interacts with this is circular, with an arrow on top and a stem below that fits in the hole in the tile, and is cut off at an angle matching that of the sloping surface in the tile, so that when the piece is rotated to the orientation such that the sloping surfaces are meeting, the piece is fully inserted into the tile, and the arrow on to points in a specific direction relative to the tile. Thus the arrow on the piece indicates the direction encoded in the tile, which is effectively invisible to the player until a piece is moved onto the tile.

This is intended to be a concept for the actual game, and the prototype I made using silicone rubber molds and casting plastics used a somewhat different concept. The stem is nominally 1/8" in diameter, and is cut away to a 1/16" diameter over a quarter inch of its total 3/8" length, using two helical cuts from near the top of the stem to almost the bottom of the stem. A similar contour is made in the hole in the tile, which makes the cam action far superior to the sloping stem bottom as it ensures that under gravity the piece will rotate to the correct orientation. The arrow on the piece then indicates the direction associated with the tile.

I have so far been able to construct a piece in ViaCad 3D by lathing a set of lines that define the top face (not including the arrow), the outer diameter stem, and a short section of stem at the inner diameter. What I don't know is how to model the two curved half-turn helical cuts necessary to create the cam described above. Similarly it's easy to define a square tile with a hole at its center, but how can I put the matching helical cam contour inside the hole in the tile? In case anyone thinks this wouldn't work, it has worked very well in the prototype I made, referred to earlier. An additional point is that in most scenarios the tile would need to have other material hollowed out from its underside to make for a relatively constant thickness of the plastic part that would be created by 3D rapid prototyping.

Any suggestions welcomed!