Here's a PDF of the Service Manual .
Attachment Hub SpecsEdit
There are a lot of specs to be posted, but the key one is just the size of the male square coupling that fits into the attachment hub. 0.45" is a pretty close fit, although a 3D printed piece at this size has a small amount of play.
According to aka47's ball mill project page, a 12 mm square bar will fit; this is potentially a much simpler solution.
The closest standard part size in imperial units is imperial 7/16" = 0.4375". This is the typical width of a square head nut for a standard 1/4-20 imperial bolt; these nuts will fit somewhat loosely into the attachment hub. The hexagonal head of a 1/4-20 bolt is also typically 7/16" wide along the smaller axis (diameter of the inscribed circle), but the longer axis (diameter of the escribed circle) is about 0.5052" (see WP:Apothem), so the hex head will not fit into the hub squarely, but it will fit askew. Basically no standard off-the-shelf part is a good fit for the hub, hence the tendency towards custom parts, but the 7/16" parts can probably be used in a pinch.
Thingiverse has some OpenSCAD models you can use to base dimensions of other parts of the mixer off of. Here's a general search of Thingiverse for "KitchenAid" .
These models have been used and are known to fit well.