Mortise and tenon question

[Tom Porter]

I’m planning a workbench with mortise and tenon joinery, with which I have very limited experience. I’ve been looking into tutorials on the various ways to cut the mortise. In one instance, the instructor cut them with a router but did not square the round corners with a chisel. He considered the step to be unnecessary, reasoning that there was sufficient surface area for the glue to make the joint strong. This doesn’t seem to be the prevailing opinion and seems dicey to me. I’m curious as to whether this practice is common or a bad idea.

I’m not particularly considering making the mortises this way; I’m just curious.

[Brian Holcombe]

Mark out the mortise to match the tenon (or vice versa) and cut to exact size. A router works for big mortises, I keep away from the knife lines then cut to them with a chisel.

If you make through mortises they’re easier and stronger because you can cut from both sides toward center.

[Simon MacGowen]

He considered the step to be unnecessary, reasoning that there was sufficient surface area for the glue to make the joint strong.

He is right from a structural point of view. Those ends are endgrain to facegrain, and the critical part is the facegrain to the mortise wall.

Festool dominoes (loose mortise and tenon) have curved/pointed ends, not square, and again, the critical glue surfaces are the faces and the walls.

Squaring the routed mortises is not difficult (it is good chisel exercise), or you can round the tenons to match the mortises. Or, just leave them alone as the instructor suggested.

Simon

[Chuck Nickerson]

Lots of people make mortise and tenon joints with rounded corners. After all that's the Festool Domino is. Dowels are an abbreviated version al well.

FWIW- I 'm a square-corner guy myself.

[David Eisenhauer]

While the proponent of the non-squared corner mortises may be correct in that there is sufficient glue surface such that it is not necessary to square the corners, I do not believe that practise is widely used. Large mortises (such as in workbench base construction) do often have the bulk of their material removed by a router or drilling, but the final sizing to the line is accomplished by a chisel as Brian says.

[John TenEyck]

I've been using routed mortises and loose tenons for over 20 years and have yet to have a joint failure. That's proof enough to me that they work. Aesthetics might dictate the use of square mortises on a through joint. Otherwise, I favor routed ones with loose tenons for their speed, simplicity of process, and well mated surfaces.

John

[Andrew Seemann]

I don't know that I would use a rounded mortice with a square edged tenon. Rounded mortices with rounded tenons work just fine. Personally I like the idea that there being some physical bearing surface on the end of the tenon, even if it isn't as good of a glue joint. Theoretically it probably has enough glue surface on the sides, but theory doesn't always work well in actual woodworking practice.

Myself I always use square tenons and mortices. Mostly because my mortiser is much quieter than my router.

[Simon MacGowen]

I've been using routed mortises and loose tenons for over 20 years and have yet to have a joint failure. That's proof enough to me that they work. Aesthetics might dictate the use of square mortises on a through joint. Otherwise, I favor routed ones with loose tenons for their speed, simplicity of process, and well mated surfaces.

John

Frankly, joint failures are less common these days because of the modern glues we use. They do happen, but a lot because of poor execution (filling gap with glue as a fix to an undersized tenon or tail for example). I have seen only one domino joint breaking apart due to overleading and undersized members, not the fault of the dominoes (or their non-squared ends).

I still use the router and loose M&T but only when the job is too big for the dominoes.

Simon

[Pat Barry]

A mortice with rounded ends, ie drilled with a Forstner bit and a matching tenon is just as strong as one that is squared off with a chisel. A good fit and adequate adhesive are primary concerns.

[David Eisenhauer]

Agreed. I was under the impression that the OP meant that a rounded corner mortise (created by a router) could be mated to a rectangular cornered tenon because the sidewall glue up of the tenon to the mortise alone was of sufficient strength to negate the need for a snug square to square matchup (assume it also holds for a round to round matchup). As I guessed above, probably the glue up would be strong enough, but I just don't think many folks are going that route. I bet that most folks match their square to square and round to round corners for a snug fit.

[John TenEyck]

Agreed. I was under the impression that the OP meant that a rounded corner mortise (created by a router) could be mated to a rectangular cornered tenon because the sidewall glue up of the tenon to the mortise alone was of sufficient strength to negate the need for a snug square to square matchup (assume it also holds for a round to round matchup). As I guessed above, probably the glue up would be strong enough, but I just don't think many folks are going that route. I bet that most folks match their square to square and round to round corners for a snug fit.

I intentionally make the tenon at least 1/16" narrower than the mortise so that I have some room for adjustment to align the parts during glue-up, in case I didn't get the mortises perfectly matched. The glued side walls are what hold the joint together.

John

[Brian Holcombe]

They hold the joint together but the end grain abutments are important to the structure. I don’t agree that an intentionally loose fit at the ends is the same structurally as a tightly fitted tenon.

[Simon MacGowen]

The round mortise-square tenon is structurally strong for regular furniture projects. As I pointed out in my last post, if such a round mortise-square tenon is going to fail, it is most likely due to something else (loose fit on the sides/walls) other than the rounded ends in any typical furniture piece. Bear in mind that we are talking about a difference in the shape of the ends (with a slight void on the corners), not a huge gap between the mortise and the tenon. If a mortise is 2" wide/long and the tenon is 1-3/4" wide/long, of course, it is a structurally weak joint.

Some even make their own dominoes that are square (while the mortises cut by the joiner are curved): https://www.wwgoa.com/article/money-...domino-tenons/

Having said that, I would not intentionally leave a gap of 1/32" between the mortise and tenon, let alone 1/16", even for alignment purposes. Any misalignment (difference on the surfaces) due to a snug fit joint can be easily planed away.


Simon

[John TenEyck]

The glued side walls are what hold the joint together. Open space at the ends is irrelevant. If the glue fails it doesn't matter whether the tenon fits tight or loose, the joint will come apart unless it was pinned or wedged.

20+ years, no failures, including chair joints. Works for me.

John

[Brian Holcombe]

Maybe the glue will fail becuase the joint is being worked constantly rather than having an end grain abutment to fixture it.