A solution to the mortise chopping problem
bill tindall
>I have the need to chop mortises occasionally, which means I don't maintain a proficiency. The picture shows an arrangement that I have found helpful for chopping straight(orthogonal) mortises. This arrangement is the sum of the many helpful suggestions I received from you all. One can see the "daylight" between the stick and the chisel. It is very easy to adjust the chisel to achieve uniform daylight in this space, which in turn ensures a vertical chisel orientation. In practice I put a piece of white paper behind this space for better contrast with the black oxide coating on the chisel. The clamping arrangement for the rail worked beyond my fondest expectation for convenience and sturdiness.
I drill a pilot hole at the far end of the mortise layout. This hole provides clearance for the chisel to penetrate better at the start. Then I move the chisel forward in 5/32 increments (approx) and chop as deep as I can drive the chisel, prying chips out after each setting. In about 1" I can drive the chisel to a full depth, in this case 1 3/8". I found that I got smoother walls when I used smaller incremets, with the 5/32 (approx.) an optimum of smoothness and speed. I found that I could chop a 6" mortise in 12 minutes, with about half this time spent, removing chips, tidying up the ends of the cut and going back to deepen the first part of the mortise where I couldn't get full depth of cut. This pace is nearly as fast as a tortise, much faster than a snail, and easier to set up than the hare-like pace of the router.
Thanks again to all who contributed to this success.