Rough milling of the yellow birch is completed and I've gone ahead and milled the frame members for the mirror. For the frame corners I've chosen to use open mortise and tenon joinery. This is a very strong joint, also, it gives me a chance to let the beautiful endgrain of the yellow birch be expressed on the sides of the frame. Here I'm doing a dry fit of the frame and cleaning up the corners of the rabbet with a chisel and plane iron. Cleaning up these corners now, before glue-up, lets me verify the fit of the mirror glass against the wood stop of the frame. The mirror is very flat (I don't need to check that), and fortunately, this wood has proven to be very stable. I'm finding that the fit is closer than the thickness of a sheet of paper, so, a little touch-up with a card scraper and its ready to go.
This close-up shows the corner joint at the rabbet where the shaper cutter left a rounded cutout at the end of the pass.
Here is the end result, following some hand work with the chisel and plane iron. After gluing the joint, I'll do the final preparation of these surfaces, including softening of the sharp edges.