David,
I built a standing desk from Roy Underhill’s book “The Woodright’s Apprentice”. There are intersecting mortices in the legs. Roy gives this advice:
When chopping the intersecting mortices through the posts, make the first mortice only as deep as the intersection. That way, the chisel cutting the second mortice will not break out into thin air and splinter away the insides.
I have used this method on several projects and cut the mortices with a mortice machine and it works great.
I hope you can visualize what is meant. If you are making two mortices that are 1” deep and 1/4” wide, then make the first mortice only 3/4” (1” – 1/4”) deep. Then when you cut the second mortice to the full 1”, it will be in solid wood all the way down.
-- "Too much hurry ruins the body. I'll sit easy … fan the spark" - I. Anderson
