Thread: Relative mechanical strengths of various Mortise & Tenon Joints

Here is my understanding:

1. Side shoulders prevent racking of the joint
2. Top and bottom shoulders hide the mortice and stop dirt etc from entering the joint
3. Haunches prevent twisting
4. Through tenons allow maximum glue surface

One reason for using wedged joints is that you can assemble the joint dry and just belt in the wedge. Then if the wood shrinks over time, you can belt the wedge in a bit more to tighten it up. They can also be disassembled. I built my lathe bench this way.

Silent it all makes sense, bar the Haunches prevent twisting? Because the haunch is much smaller then the larger tenon face, and sits on the same axis as the tenon itself, and therefore the forces the haunch would handle and cope with would be the same as the tenon itself. Having a full width tenon with no haunch would seem to have an even greater capacity to resist twisting. If the axis was different, it would seem as a feasible explanation.

I have seen a Japanese M&T in the shape of the cross, now that would stop all twisting.

So that the strongest M&T joint would have a small cosmetic should, with solid primary shoulders, and would be a through M&T. Maybe someone can point more light on the haunch's function.

Idea: Could the haunch just be a good way of being lazy, because a haunched tenon, means a smaller mortise and less chopping.

If you have a larger piece of timber with a haunched tenon, that means you do not have to chop out a correspondingly large mortise to take the super sized tenon from the large piece of timber. While at the same time to sacrificing much of the joints strength.

A haunch is usually only used when the rail is at the very top or bottom of the stile. It allows you to have the full width of the rail supported in the mortice without having to remove too much wood from the stile. Being near the end, you want the wall thickness in the top of the mortice to be as thick as possible so that you don't weaken the joint, so you cut a normal depth tenon with a haunch so that you get the full support against twisting without weakening the wall of the mortice.

Difficult to explain, hope that makes sense.

Edit: You also often see a haunch used with a grooved frame, where the haunch fills the groove.

I just had a quick flick through a book I've got here:

A through tenon exposes end grain which makes the tenon more susceptible to taking on moisture, which can cause the glue to fail, so it is common to wedge through tenons to avoid this problem.

A haunched tenon is used at the top of a frame to extend the twisting resistance all the way to the top of a leg.

Silent - thank you, that makes allot more sense.

Originally Posted by silentC

A through tenon exposes end grain which makes the tenon more susceptible to taking on moisture, which can cause the glue to fail, so it is common to wedge through tenons to avoid this problem.

I had planned to used draw-boring in conjunction with my through M&T, would this have the same effect as the wedges in stabilizing the joint more?

That I couldn't tell you. I've normally associated the draw bore with a stopped tenon. Once the pin is in, there's no adjustment to be made, but the joint wont come apart. I suppose a through tenon could shrink away from the walls of the mortice and you might end up with a loose fit. That's where the wedge is good, but in truth in over 3 years and having moved it a couple of times, I haven't had to belt those wedges in on my lathe bench since the day I put it together.

For workbenches, I think it's more common to use one or more fixed wedges in the ends of a through tenon which is cut flush. With mine, I used fixed wedges to hold the feet to the legs, a bridle joint to hold the legs to the rails at the top (what the bench top sits on) and I used these wedged through dovetail tenons to hold the strechers to the legs.

So there's plenty of different ways to skin the cat and it depends on what you have to work with.

I think the ideal joinery for the stretchers of a workbench is what I have on my own bench; see the diagram below. The stretchers have two blocks glued and dowelled to each end. The legs and the blocks have holes bored for 3/8 x 8" carriage bolts. This setup yields a rock-solid joint, which has no chance of racking.

Rocker

I like what you have done there Rocker, it is very interesting.

When I built my bench, I used 1/2 rebates with a bolt throuh them. The old woody that was advising me at the time told me to cut the rebates tight & plane down the rails to be a hammer fit. He also said NOT to use glue. He recons that the glue has a tendancy to let go after years of hammering etc., on the bench, & you are better off not using any.

I don't know if he is right in the long term or wrong, but 5 years down the track, & he is dead right sofar. not a creak, squeak or wobble so far.

Steve

I have no drill press, or even an electric drill, or facilities to align dowel accurately. I have a chisel and a hammer. I intend to use west systems epoxy resin, with draw-bored dowel drilled with a hand brace. If it fails then it is god's will.