The World’s Oldest Joint

I decided to re-examine the problem of cutting the mortises on the lower aprons on the mortising machine.  As I said in my previous post, the mesquite is so tough that the 1/2″ chisel just wouldn’t cut through the piece without burning and jamming.  That bothered me: mesquite is tough, but the steel of the mortising chisels is tougher. It should have cut without so much of a fight.

I examined the 1/4″ chisel that I used to cut the leg mortises, and all four corners were blunted.  Sharpening the chisels requires a very specialized tool, available from Woodcraft, Rockler, and similar places). I had to go ahead and shell out for one.  Fortunately, it’s fast to use, and works extremely well.

I decided to touch up the 1/2″ chisel at the same time, and I discovered that it had become clogged badly – no wonder it wouldn’t cut properly. After removing the clogs and touching it up, I tried the apron mortises again, and it cut well enough. Still slow, but likely better results than I would have gotten with a coping saw.

The other half of the joint is the tenons.  Mortise and tenon joinery is very strong, and one of the world’s oldest construction techniques.  It appears in finds thousands of years old, including the Khufu Ship, which dates to about 2500 BC.  Don’t let that surprise you; the Egyptians knew a lot about woodworking that we still use,  including this type of joinery, the bow saw, intarsia, barrel hinges, and plywood. Kari Hultman’s wonderful blog, The Village Carpenter, discusses ancient Egyptian woodworking at some length.

The tenon is a stub that protrudes from the end of the (usually) lateral pieces, and inserts in the mortise. The purpose is to present surface grain to be glued into the mortise.  End grain will not take glue at all; it just absorbs it, and you can snap it apart with your fingers.  A properly glued mortise and tenon joint is stronger than the original wood, and the stress that would cause it to fail will usually rip the wood completely apart. Note:

The tricky part of the tenon is the shoulder, where the surface of the apron meets the leg.  It has to be smooth, square to the tenon itself (called the “cheek”), even on both sides, and free of chip-out.  I’ve cut this kind of joint before, but only on the table saw and band saw. This is my first excursion cutting them by hand.

The cut line has to follow the 3 1/2″ angle of the legs, but the angle gauge I discussed in a previous post isn’t big enough <sigh>.  I made it about 3″ deep, but these are 5″ aprons. Yes, that’s really annoying.  The solution was to use the angle gauge to set my bevel gauge, and mark the cut lines with a marking knife.

The cut line follows the apron all the way around.  The lateral sides of the tenon are marked the same way, and run down the apron to the crosscut line of the shoulder.  Then the crosscut is made with a small joinery saw, to 1/3 of the thickness of the apron.

I’ve learned a lot of my woodworking by reading, and this was no exception.  I read several things on the subject before attempting these cuts, and I’m glad I did. I found one particular technique that improved the results greatly.  I started the cut line across the tenon with a chisel first – much like starting the initial chisel cuts in a dovetail joint.  That creates a sharp shoulder edge to start the saw against, and keeps the initial crosscut of the wood fibers clean. Then, because the saw starts slightly below the face of the wood, the sharp chisel line is what shows against the leg. Once that’s done, the cheeks of the tenon are cut, and the ends trimmed to the correct width. The flat surfaces of the tenon are trimmed square with a shoulder plane.

I had four of these to cut, two for each of the side aprons.  Honestly, my first attempt wasn’t all that great. The fit in the mortise was too loose. The second was better, but one of the shoulder cuts had wandered slightly, and the two shoulders weren’t quite even. The third was better yet, and the fourth was close to perfect.

Fixing the loose tenon was easy – all you have to do is glue a small piece of veneer to one side of the tenon (the side that was cut too deep) and plane it down gently until it’s tight. The fix for the second one was more difficult, because both sides have to be exactly the same length between the tenons – so after squaring the edge on one, the other had to be brought down slightly with a shoulder plane to match. But when that fourth tenon tapped perfectly into place with a mallet, that made my day.  These appear to be square, and will glue up tight.

Each adjoining face is marked, so that I know which tenon was fitted to which mortise.

Cutting the tenons was labor intensive, but felt really good. Slow and clean was better than fast and full of dust. When it was done, I had a small pile of sawdust and a handful of shavings – not a face full of choking dust, and I spent the day cutting wood instead of setting up the table saw. Ask me which one I like better.

Next time: Tenons for the back apron and drawer stile, and starting the curved lower aprons. And (maybe) back to the top, to test out the copper filler.