Hi Nick, also looking at the underside of the paw pieces, the surface underneath (the one that you're trying to mate with the top surface of the larger piece with the boolean union) seems to be fragmented into a couple of surfaces:

I turned on control points for those surfaces, and it looks like the underside pieces are coming from a surface of revolution?

I turned on control points for this piece here:



And it appears to come from a surface of revolution like this:



The piece you're trying to mate it to with boolean union also is a surface of revolution but it is rotated to a different angle, the surface control points look like this:




Having that small piece at a different rotation than the large piece especially with the closed seam edge of the revolved surface running right through the middle of the piece (making it actually have 2 separate surface pieces for each face on the bottom of the paws) is going to make things more difficult for the boolean code to figure out - the booleans try to intersect objects with each other so it has to figure out that those pieces are supposed to be coincident - if they both came from the same original surface (instead of different rotations) that would probably be easier for them to figure out.

A couple of different methods which you could use that would probably work better would be to make the paw pieces actually push a little ways through the top surface instead of making them coincident - that will avoid complications in coincidence determination.

Also another possibility for stuff like this where the intersection is difficult is to use the Trim command to cut pieces by each other and then join the results together.

- Michael