I just got in from the shop after 7 hours of tweaking the shaper for lock miters. It has been 15 years since I have done lock miters and those were in solid maple. Back then I made 3.5" x 3.5" square corner columns for our staircase railing that fit over the 1.5" x 1.5" square steel tubing I used as the skeleton for the main stair case railing in our house. I guess 15 years is long enough to forget how perfect the setup has to be and what a pain it is.
This time I was using the lock miters to make boxes out of plywood. Plywood is even tougher than hardwood because it doesn't machine as easily and there is no rounding over the edges, the face ply's have to meet perfectly.
A good couple of those hours were making a new fence face piece for the shaper and tweak it in to be a perfect 90. The rest was dialing in the lock miter setup and practicing how amazingly slow I can push a piece of stock through the shaper so the plywood ply's do tear out.
Anyone else use lock miters? Anyone else use them in plywood? 30 years ago I took a class in cabinet making and they presented the lock miter as a way to join the cabinet exposed side to the face frame with a nice clean look. (I didn't actually see anyone attempt this)