Not much time for planemaking or blogging lately, but I did make some progress on the mortise.
Chopping the mortise was not really more difficult than with an unskewed plane--just more time-consuming.
As usual, the first step was to make a guide block for chiseling. I just used a piece of southern yellow pine scrap. I laid out the lines, cut it with a handsaw, and then did lots of careful planing, checking frequently with my two bevel gauges (set at 38° and 20°) and a straightedge, until the angles were perfect and the surface flat.
Here's a completely phony, posed shot, taken after the fact, that shows how the guide is used.
I generally chop freehand until I get within about 1/4" of my front and rear layout lines, then use the guide to finish off. Here's the mortise roughed out:
Then I use a flush cut saw to widen the bed and define the abutments. The bed cuts are simple--i just lay the saw on the bed and cut 1/4" deep. For the abutment cuts, it helps to make a spacer block. This was the first point in the project where the geometry got a little tricky. My first attempt was a failure, but it helped me figure out the correct the angles. The main angle is 10°--a standard wedge angle. But the skew is not 20°, because I'm referencing off the 38° bed, rather than the sole of the plane, and this decreases the skew angle. I suppose I could have figured this out with trig, but in practice it was easier to just do a little trial and error. The final skew angle turned out be 15°. The sides of the spacer block are angled at around 10° (relative to the bottom of the block) to match the sides of the mortise. Anyway, here's the spacer block:
I shot some Spray 77 adhesive on the back of the block to temporarily hold it in place for the saw cuts. After I was done, the block was easily removed with a few hammer taps.
Finally, I chiseled away the waste between the saw cuts. Here's a shot of the more-or-less finished mortise:
I've got some stupid cosmetic scars on the bed (from chiseling the abutments), but the cutting iron won't care. Overall, I think it's looking pretty good. The next thing is to deal with the iron.