Update 3: More updates to the design, particularly housing for a ball bearing in the flange.
Update 2: It seemed like a Wiki was actually more appropriate, so I am retiring the Google Group in favor of a new Wikia page. Please join in!
Update: I created a Kitchenaid DIY Google Group to try to get together some of the handful of people who have been hacking on this. Also posted a listing of known projects.
Substantial updates to the earlier prototype. Here I've added an adapter flange that holds it in place against the Kitchenaid adapter hub. This was also supposed to house a sleeve bearing but I made the dimensions too tight so that will have to wait for next iteration. Also I found some nice, small, cheap, sealed ball bearings from VXB so will probably design for that instead. The coupling between the square hub and the 3/8" drill shaft is essentially the same as before, only shorter to leave room for the flange.
Here's a video of this version in action. Note that now I'm using it to drive the older Hario conical burr grinder model, rather than the Hario Slim. (We have both, one for home and one for my office. Kitchenaid only at home though ;).)
One advantage of the older Hario is that the coupling from the drill adapter to the grinder is simpler. The Hario Slim uses a pentagonal (AKA 5-point) drive in a small nonstandard size (~3/16"). I tried a 3D printed adapter for that but I don't think the Shapeways "Strong, Flexible" material is strong enough. I'm trying a lasercut acrylic piece next.
For the older Hario, the situation is much simpler. The drive has a nonstandard shape (cylindrical with flats on two opposite sides) for hand cranking, but it's also threaded as M6-1 so you can just screw something down on top of it. Actually, you can even just clamp the drill adapter directly down on this, as jeffkobi did in his original design. I wanted to try something simple and off-the-shelf to avoid clamping directly to the grinder, so I looked for M6-1 coupling nuts that would fit into the 3/8" drill adapter, but unfortunately they are all slightly too large. However, a 1/4-20 coupling nut fits snugly into the adapter (you may have to test a few to find one with clearance) and screws smoothly onto the grinder drive for the first few turns. I don't have any MechE, so can't really tell you the relative benefit of either clamping directly to the drive or screwing an unmatched thread onto it, but as long as I include some washers so that the 1/4-20 is only riding the M6 threads a little way I think this might be a little nicer. An M6@1/4-20 adapter nut or just an M6 with a smaller outer diameter would be ideal but I haven't found such a product.
The previous hub coupling has already worn out, so it's still not clear whether these 3D printed parts are viable. The first thing to go was the set screw holes; on the failed piece the screws get pushed back through their threads so the coupling no longer grips the 3/8" shaft. But I think the mistake there was to tap the holes and use nylon set screws before switching to steel set screws. On the new piece I just screwed steel set screws through the untapped hole so the fit is tighter and it seems to be holding up better so far. Probably a clamping adapter design would be better, although it would be a tight fit and I'm not sure how well 3D printed plastic would handle that kind of stress.