The first step in getting the keys ready is to get the stock ready. To have any strength, the keys need to be cut so that the grain of the wood runs the lengthwise on the key. An easy way to do that, if you don't mind having all of your keys the same length, is to cut a strip that's equal to the length of the keys from across the grain of a fairly wide board. Doing things that way, you'll be able to get three or four 2-1/2" long keys out of a strip cut from a 1 X 6 board. I usually bevel the keys slightly on all sides so that when they're done, they taper slightly inward from the top to the bottom of the key. When I cut the strips, I set my miter saw to a 1/2 degree bevel and cut the piece so that I end up with a piece that tapers inward on both sides.
The angle of the outward flare of the key, as far as I know, is a matter of personal preference - at least I've never heard otherwise. If there is a standard for the angle of butterfly key flare, I am unaware of it and have probably been in recurrent violation of it over the years. If anyone knows of a reason for always using a certain angle for the keys, please let me know, so that I can adopt the practice, pass the information on. Until then, I think that a 76 degree angle (relative to end-grain edge of the stock) looks good and seems to make a strong joint.
To mark the keys, I divide the length of the key in two, and mark the stock so that I have a reference line for the center of the keys. Then, I set a T-bevel to the desired 76 degrees and mark the keys from the centerline out so that I end up with approximately 3/4" of material across the center of a 2-1/2" key. Next, I set the T-bevel to a 1/2 degree angle (relative to the edge of the stock) and carefully mark the ends of the keys so that I have a cut line that produces a slight inward taper from the top to bottom of the key. Finally, I reset the T-bevel to 76 degrees to mark the cut lines on the bottom side of the keys.
The marking process seems to be one of the most crucial stages of the entire project. The key’s shape needs to be the same on the top and the bottom for the key to fit into its mortise correctly. When I reset the T-bevel to 76 degrees, I check the setting against the cut lines I've made for the top of the keys to make sure that I am using the exact same setting for the top and bottom of the keys. Using two T-bevels and leaving one set a 76 degree angle (or whatever angle you choose) wouldn't be a bad idea, in fact.
I use an Ikedame dovetail saw to cut the keys. An Ikedame saw is a variety of Japanese saw with a short, fairly stiff blade that's very easy to direct. The fact that the saw cuts effortlessly and is easy to steer helps with the important task of getting the cut established on the correct plane as early as possible. I start the cut by scoring the end grain of the stock along the cut line and then begin to angle the cut downward along the cut line on the top surface of the key. I like to check the direction of the kerf frequently in the beginning stages of the cut to make sure that it’s in line with the cut lines on the top and bottom of the stock.
If all goes well, I end up with keys that have a nice, straight-lined shape on the top, and an identically proportioned, slightly smaller shape on the bottom. In that case, all I have to do is set the key on top of the workpiece and trace around it to get the shape of the mortise, as I'll do next Thursday in Part III. I'll also route the mortises and trim them to fit the keys. Along with that, I'll offer an alternative method for getting the shape of the mortise right, in case (as sometimes happens) the keys don't come out as beautifully as they were originally envisioned.
Original article and pictures take http://www.rockler.com/how-to/butterfly-key-joinery-part-ii site