If you follow me on Twitter, you may know that I spent most of #blizzard2016 hand-cutting mitered half lap joints for a cherry side table. The finished piece will have eight such joints, and although I’m only about halfway through the first of two frames, I’d like to reflect a bit on the process.
First off, some defined terms. For the duration of this post, I will refer to the lower piece in the picture above (the E-G piece) as the “Angled Piece”. The upper piece in the picture (the E-F piece) will henceforth be known as the “Recessed Piece”. I’m sure they have proper names that have no relevance to what I’m about to say.
On each Angled Piece, there are three cuts to make. The first two apply to all lap joints: a shoulder crosscut and a cheek rip cut. The third is unique to the mitered half lap: a 45° miter cut across the face. My preferred order is (i) cheek, (ii) shoulder, (iii) miter, but whatever you do, always make the cheek rip BEFORE you cut the miter. Otherwise, you will lose your guiding kerf on the rest of the rip as you get to full depth.
Once all three cuts are made, and after the cheek is flat and parallel to the face (whether off the saw or by router plane), it’s time to square the shoulder to the reference edge by paring down to the knife line. Then is the most critical step: true the miter to 45° using the same reference edge to which the shoulder was squared.
When I cut the first joint, I thought to just straighten the miter, transfer the mark to the Recessed Piece and leave well enough alone. After all, it doesn’t have to be perfect; it just has to be consistent. Since my shoulder was square, this would have worked fine, assuming I sawed perfectly and my full knife line was intact. But alas, the saw jumped out of the kerf and took part of my knife line with it. So fitting the joint became trial and error angle finding with a shoulder plane.
If you instead true the miter to 45°, go ahead and still use the Angled Piece to transfer the mark to the Recessed Piece (it’s easier than using a combination square). But if you saw less than perfectly, you can always fall back on just truing the mitered recess to 45°. With a straight edge on the Recessed Piece and a square shoulder on the Angled Piece, everything will come together perfectly either way.
One more thing: depending on how much material you remove from the miter on the Angled Piece getting it to 45°, you may need a couple passes on the shoulder to bring it back in plane with the end of the miter. Otherwise, your inside corners won’t meet right at the miter.