I’ve been reading about chopping mortises, and thinking through my experience last weekend. I think I have a better perspective on mortising now.
A couple of things I’ve been looking at:
First, watching a few videos is helpful. This one from Paul Sellers is pretty good, he demonstrates chopping a mortise with both a heavy mortise chisel and a standard bevel edge chisel, starting from one end of the mortise and working toward the other. The first pass he has the chisel vertical, and the bevel facing in the direction he travels across the mortise. By the time he gets to the far end (maybe a little sooner) he’s at full depth. Then he turns the chisel so the bevel is facing in the other direction and works back – starting just past where the mortise isn’t yet at full depth. But this time, he holding the chisel so the bevel is vertical, so the the chisel is at maybe 30 degrees.
Another good source of information is Jeff Gorman’s web site, the section on mortising has some great pictures. It does a really good job of illustrating some of the dynamics associated with chopping mortises. He illustrates really well where the force is concentrated, it’s worth a careful read.