This is the 18th post in this series about the construction of a complex 19th century French sawhorse, by Mazerolle. The study of the braces connections to legs rotated in various ways serves as a vehicle for some types of roof carpentry. Previous installments are found in the Blog Archive to the right of the page, so if you are new here or haven't visited in a while, you might want to check the archive first.
The last set of mortises for the faces aplomb legs were the blind mortises, which, like the other mortises in these two legs, are oriented roughly along the diagonal axis of the leg. In the text, no mortises are indicated at these positions, and due to their difficulty of cut out, I suspect that the braces were simply spiked in position at these spots. It's not possible to use a through-mortise at these lower brace locations as the long side braces overlap the exit locations and their tenons would partially interfere.
I figured I could form mortises with the aid of a jig. So, I figured out the geometry of a jig block, and here is the rough-cut piece:
The inside of the block is cut so as to fit directly against the corner of the leg:
I needed to make some final passes with a couple of planes to get the block shaped just right:
Checking from time to time that I was making a 90˚ angle internally:
I then brought out the secret weapon, something to which Mazerolle would have had no access - double stick carpet tape:
The tape allowed me to mount the block directly against the leg:
Next, I put a few clamps on to keep the block from squirming anywhere, and used my router to cut the mortise, referencing to the side of the block with the router fence:
I then slid the block along a little further and removed a bit more material. The tricky bit done, I then could commence paring the mortise to shape:
This one is mostly cleaned out now, just a few more pares and it is done:
With only a little trimming of the lower mortise wall still needed, here's a view of the mortise in the leg's normal orientation:
As far as mortising goes I'm done with those two legs. That leaves the remaining pair of legs, each of which is unique, since their rotations relative to the plan are each unique. That means eight more mortises and I'm, er, through. Fortunately, both of the remaining legs are aligned to one or the other x-y orthogonal axes of the plan, and that means that the mortising for those pieces will be more or less normal to the faces of the pieces, unlike the obliquely-oriented mortises just done for the faces aplomb legs. I'll have to go back to the drawing and carefully work out- again - the positions of the mortises on the legs and the slopes of the upper and lower walls. I do this again, because the more I practice the drawing method, the more it soaks into my thick skull.