I'm contemplating a similar problem at the moment, so I thought I'd add it in here rather than start a new topic.

Here's a corner joint, with three 15mm plates (Eco Cast, so should be 15mm consistent across):

http://i1120.photobucket.com/albums/...rner_joint.png

Idea is to get the edges aligned perfectly and easily. Plates are held using M5 bolts, with 5.5mm through-holes on the plate faces to allow some wiggle room on the edge threaded holes. This must come up quite often in various CNC machine builds, so I'm curious what you normally use.

So far I've got the following solutions on my list:

- pocket cuts to match the plates against each other

- corner jig to align plates, so no further machining

- locating pins

Dead square box is needed, I'm open to any suggestions.

Regards,
dsc.