details on making through-wedged mortise and tenons?

[Adam White]

I have some 2 inch square pieces of hard maple into which I'm cutting

1/2 inch wide by 2 inch long through mortises. These pieces make up a base for a workbench top. For added strength I'm planning to wedge the tenons, which is something I've heard about but never done.

My understanding is that I need to taper the mortise so that it is wider on the "outside" side of the stock -- something like 1/8 inch wider per inch of thickness, so that's 1/4 inch wider for my 2 inch stock. Therefore the "inside" length of my mortise will be 2 inches while the "outside" length of it will be 2+1/2 (1/4 wider on both sides). Then I can either: a) drive my wedges between the inside of the mortise and the outside of the tenon (on both sides), sort of squeezing the tenon; or b) cut two kerfs into the tenon and drive the wedges into these kerfs.

My understanding is that driving the wedgies into the kerfs produces better results because the tenon actually "bends" outward so that the wedged end is larger than the "inside" end ... locking the tenon in place much like a dovetail joint. BUT ... you have to do this without splitting the tenon when you drive the wedge in.

Questions about all this (keeping in mind that my tenons will be 1/2 inch wide by 2 inch long by 2 inch deep):

1. How far from each side of the tenon woud you make the kerf? 1/8 inch or so?
2. How LONG would you make the kerf? 2/3 the length of the tenon, or about 1+1/3 inch in my case?
3. What do you think about this guideline of "1/8 inch for every inch of mortise depth" for the question of how much to enlarge the mortise on the "far" side? It sounds fine in theory, if my kerf is 1+1/3 inch long I just don't see how I can push the end of it it 1/4 inch sideways without splitting it?

If anyone has experience -- good OR bad! -- making these joints, I'd sure appreciate hearing your story. I realize that different wood will have different characteristics, but I might still find your experience helpful even if the wood is different.

Thanks in advance. Adam

[George]

Bore a hole across the tenon and kerf down to it to reduce possibilities of splitting. Use a 3/16 bit, 1/4 from the edge, less from the bottom.

Make your wedge so it is as wide as the difference you need to make up in about 3/4 its length, or a touch more. Compression'll give you some extra penetration, but you should not risk bottoming.

I use cherry wedges in hard maple for "pretty."

You didn't happen to see Ol' Roy's knockdown bench did you? I'm going to try that joint soon. Mortiser makes a lot of things easier (gloat).

[Phil Crow]

For a tenon that wide and thin, use two wedges, each about the thickness of the tenon from the end. This way, you still "bend" the tenon outward on the top and bottom as well as not putting too much stress trying to deflect all that wood.

I don't know that I would make a saw cut at all. Just start to "split" the wood just a little with a chisel and tap the wedge into that split.

Can't say. I've only done this on a few joints, and I made them about

3/16" wider. Then again, I was using oak, and from what I understand, it's a little more forgiving than maple in this regard.

My preference is to use contrasting wedges for your tenons. If all you wanted was a tight joint, then why bother with the wedges? You can get a tight joint with a square mortise and tenon and some titebond. With the contrasting wood, you add a little element of "trick" to your project.

Good luck. Let us know how it goes.

-Phil Crow