Of course, it’s not always easy to keep everything aligned perfectly as you make the keys and occasionally the shape of a key turns out a little off kilter toward the bottom. In that case - if the key isn't horribly misshapen - you can salvage it by flipping it over onto a piece of heavy paper and tracing around the top edge of the key. When you're done with that, you can cut the shape out, flip it over, and use the paper pattern you've just made to establish the outline of the mortise.
After all of my proselytizing over the merits of making perfect butterfly keys, I spent a good chunk of a Saturday morning making sure mine came out just right. And, even though I had to make adjustments with a rasp in a couple of spots, they came out acceptably well. How do I know? I checked them with a try-square by setting the flat edge of the square on the top surface of the keys and looking at the gap between the blade of the square and the bottom edge of the key to make sure that it was fairly even overall for the whole key.
Satisfied with the shape of the keys, I set them face up on the piece of jatoba in the spots I'd picked out for the mortises and traced around them. After that, I scored the outline of the mortises with a few taps on a chisel. The purpose of that was to make sure that when I rough out the mortises, I didn’t run the risk of chipping the non-mortise surface of the jatoba.
The next step was to cut the basic shape of the mortises with a plunge router. That part was a little nerve-wracking because it's surprisingly easy to let the router cut slip past the edge of the mortise. An ideal cut, in my opinion, leaves 1/16" to 1/32" of material left between the router cut and the outline of the mortise. I always route the mortises in three passes - 1/4" at a pass - if I'm using 3/4' material, as I am here.
The last step in this phase of the project was to trim off the edges of the mortises. I put an edge on the bevel edge chisel that I used for most of the trimming two or three times during the process and tried to keep the chisel at a 90 degree angle to the material at all times so the edges of the mortises wouldn’t tip inward or outward when I was finished, which could cause problems when I install the keys.
The final operation in this phase, as you see here, was to finish off the corners of the mortises with a skew chisel.
All that's left now is to get the keys into the mortises and trim them off flush with the surface of the jatoba. I'll let you know how that went next Thursday.
Original article and pictures take http://www.rockler.com/how-to/butterfly-key-joinery-part-iii site