Cut a through mortise on the diagonal

[Joe Petersen]

I'm trying to get some side work making mailbox posts. I think I know how to make them but I am unsure about the arm joinery. The arm is a 2x6 that fits through a mortise on the diagonals in the 4x4 post.

I will make a cradle on the tablesaw holding the 4x4 at a 45/ degree angle and saw a rectangle flat on both sides with a dado blade. But then I need to mortise the flat and this is were I am stuck. I have no mortising machine and can't think of an efficient way of making several at a time.

[Roy Harding]

Try making a cradle for your drill press, holding the 4 X 4 at the same 45 degree angle - then you can hog out most of the mortise with appropriately sized drills.

Perhaps the cradle you have already built for the TS can be adapted to the drill press after you've made all the cuts?

[Alan Schwabacher]

If your arm really is the way you drew it, how are you going to fit it through the mortise? The answer you already got seems like the way to go if you have the post. Another option is to make the post by gluing up two pieces of triangular cross section. That way you can cut the mortise with a dado blade before glueup.

Actually, rather than triangles, you could make the post from a couple of 2x4s, and cut the mortise diagonally into both pieces before glueup. You'd need a cradle to hold the pieces at a 45 degree angle while cutting dados, but that could also give the through mortise you want. The mortise would be missing just a little bit on two edges, but that might be acceptable.