Welcome to the final post in the butterfly key joinery project series. If you missed the first three entries:
In Part III, I laid out the mortises, rough-cut them with a plunge router, and cleaned up the edges with a chisel. This week, I checked the fit of the keys in the mortises and installed the keys. In the process, thought of a couple of things that I should have clarified on the subject of cutting the mortises: I said that I routed the mortises in three passes, 1/4" at a pass, to get them ready for the 3/4" tall keys. I routed the mortises in three passes mainly because it's easier to control the router in a shallow cut - I doubt that it would have been a burden for the 3 -1/4 HP plunge router if I'd have cut the entire depth in one pass. Also, it would have been more accurate to say that I routed the mortises in two passes that each took off roughly 1/4" of material, and one pass with the router set to depth of 1/16" less than the height of the keys. I made the mortises a little shallow because the corners of the keys near the very top break easily, and I think it's asking for trouble to cram them all the way down flush with the surface of the receiving material.
The first step in installing the keys was to get an idea of how well they fit into the mortises. To do that, I checked to see that each key would slide down into its corresponding mortise about half way without my having to put much pressure on the top of the key. With the keys in the mortises, I checked to see whether there were any large gaps between the key and the mortise that might show up in the finished product. Part of what tapering the key does (see Part II and Part III of the series) is make it so that the key gets wedged into the mortise at the top of the joint. The mortise / key joint is a "compression fit", in other words, and very small gaps and inconsistencies between the shape of the key and the mortise get forced into alignment by the pressure of the fit when the key is finally installed.
Any gaps that are the thickness of a matchbook cover or more, on the other hand, can cause problems in the finished fit of the joint. Usually, if I've taken care to get the right shape of key, the right taper from top to bottom of the key and I’ve laid out and cut the mortises using the procedure I described in Part III, I end up with mortises that are a little on the tight side to begin with, and can make slight adjustments in either the keys or the mortises without ending up with a loose fit. It's important, I've noticed, not to cram the key into the mortise to see how well it fits - it's been a challenge on a couple of occasions to get a key out of a mortise without damaging either.
After a few minor adjustments to the shape of the mortises, I had a key / mortise fit that I was reasonably happy with. I made sure that I hadn't left any chips in the bottom of the mortises and made sure that I'd gotten the corners of the mortises as square and clean as possible (I always round off the bottom corners of the keys anyway, to make sure there won't be any problems where the corners of the keys meet the corners of the mortises). Finishing that, I was ready to glue the keys in place. If parts fit together well, you don't need to use lots and lots of glue, (if they don't fit well, glue won't cure the problem anyway). I just coated each side and the bottom of the key with a film of glue and tapped them into place, using a block of wood to protect the top of the key. You can see here that even though I used the glue sparingly, I got a fair amount of squeeze-out around the top of the joint.
After the keys were glued in place, I wiped off the excess glue and set it aside to dry. The next day, I took off most of the 1/16” or so of the keys that sat proud of the surface of the jatoba with a block plane, and finished leveling the surface with a scraper. The only thing I'd mention here is to be sure that the plane is sharp and that you are careful to keep it flat on the surface of the key. You also need to understand that wood almost always planes better in one direction or the other, depending on the direction of the grain. If the plane is leaving a rough surface, or it’s hard to get the plane to go across the surface, flip the piece around and plane from the other direction. The purpose of using a cabinet scraper to finish off the job is that at some point in the process, you'll have to run a tool diagonally across the grain of both the key and the receiving material. With a sharp cabinet scraper you should be able to go straight across the grain of a piece of wood without causing damage.
That's the butterfly key process, essentially (or at least my process). All I have left now is to think of a way to use a split piece of jatoba that I've spent some time repairing.