Building for a 3D Print: UNION of manifold meshes produces non-manifold geometry

Howdy –

Some background: I’m helping a local bagpipe band obtain new cap badges. They have some hoary old pewter ones and need some new ones. So I’m thinking, “this is the perfect project to help me learn blender - I can create a 3D printed master that a pewter artists can use to cast badges.” Yay!

But no. I must have some kind of jacked-up workflow, or am doing something hideously wrong. My general understanding of the workflow for creating an .STL for 3D print is this:

  • Create meshes
  • Ensure each mesh has no non-manifold elements
  • Ensure each mesh overlaps
  • Use the UNION boolean to sequentially merge the meshes, checking for non-manifoldness after each merge

So I’ve created a hand (eventually to be holding a dagger) that consists of a palm, a thumb, and four fingers. When I try to merge the pinky with the palm, I end up with thousands of non-manifold vertices. “Remove Doubles” doesn’t help: it does remove doubles (with the threshold set at its default of .0001), but doesn’t actually fix the non-manifoldness.

I thought perhaps that the pinky and palm’s position respective to each other might have something to do with it. So I created a “test union cube” that would engulf the backs of all of the objects, thinking that if I merged stuff to that, it might work more cleanly. If anything, it was awful. I read some articles on non-manifoldness (e.g., over at Shapeways) and one of the articles mentioned that, in essence, blender doesn’t like to merge really complicated meshes with simple ones. So I used Ctrl-T to triangulate the test union cube, then applied the subdivision modifier (set to View:6) with all of its edges set sharp, to make the cube ferociously complicated too. No dice. While it did reduce the number of vertices removed with “remove doubles” (from something like 80+ to 6), it still resulted in essentially the same non-manifold geometry.

I’m either having a forum issue, or am still too brand new, to post images. So I’ve put up some images (and a downloadable .blend file with the hand elements – watch out, it’s 37MB) on my blender page:

I’m half-suspecting that it has to do with the way I built the individual meshes (they’re all cubes that have been modified using the subdivision and/or the mirror modifier). But I’m at a loss at this point. Help?