Tenon question?

[keith micinski]

SO I have always made the typical tenon that has a recessed top and bottom. My question is, why? It is an extra setup and I don't see any advantage to it. I recently made some tenon's for a table that were flush with the top of the apron and it made me wonder why do I need to recess the bottom? The only thing I can think of is it covers up the bottom of the Mortise hole if it isn't perfectly square but since I use my hollow chisel mortiser the bottom is always nice and square. Any ideas on why tenons are usually done this way. I am probably going to make a set to see unless someone has a glaring reason why I shouldn't.

[Mike Henderson]

That's the reason I've always assumed for shoulders on tenons. If your mortise is not perfect - if you bruised the top a bit and made it too wide there, the shoulder covers it. And bruising the top is easy when you chop out a mortise by hand. Otherwise, you have to make a perfect mortise from top to bottom.

No, I'll keep putting shoulders on my tenons.

Mike

[keith micinski]

I agree that doing it by hand it would be hard to make a nice socket but I think with my Mortiser I could make it perfect. One thing I did think of is that the tenon has to be exactly perfect if you don't have shoulders. I usually over size the mortise hole just a hair and I wouldn't be able to do that with out a bottom shoulder.

[Frank Drew]

Keith

I'm not positive I understand your question, but if you're asking why tenons usually have shoulders top and bottom in addition to both sides, as Mike explained, the bottom shoulder (can be as little as an eighth or a quarter) hides the bottom of the mortise; as for the top of the tenon, in your example of a table where the apron's top edge is flush with the top of the legs when they're trimmed to final length, you always make a shoulder there, though sometimes with a small stub or haunched tenon, to keep the joint strong.

Same thing on door work, the tenons (so also the mortises) always stay away from the ends of the stiles/outer edges of the rails, otherwise you'd just have a version of a bridle joint.

If I misunderstood you, apologies in advance.

[John Coloccia]

I always assumed tenons had shoulders because:

a) I think it makes for a stronger joint that's less prone to wobbling

b) it's 10 times easier to assemble a piece seating everything to the shoulders as opposed to measuring and somehow clamping everything perfectly square or making a dead flat mortise bottom and indexing off of that, hoping that glue doesn't get in the way.

[Stephen Cherry]

The top and bottom shoulders can be under compression, so you need some surface area to compress.

[Steve Griffin]

I'd also add that shoulders make gluing easier, as it is much less likely to splooge out and need to be removed.

-Steve

[Dave MacArthur]

What John said. The shoulders of the tenon resist flexing of the joint. A slightly loose tenon without shoulders (a stick in a hole) will merely pivot on the lip of the mortise and transfer all the resultant leveraged force into breaking the glue joint inside, or expanding the mortise and thus weakening the joint.

With shoulders, any torque of the tenon will result in a compression force against the outer face of the board holding the mortise, and this will stop or resist the levering action of the buried tenon breaking the joint.

There was an article I read once of the relative strengths of merely embedding a stick in a hole vs. true tenon--both hold weight equally well against gravity, but the shouldered tenon is massively more stable and strong against the torques and rackings that come with furniture. Sorry, don't have the link to said article. But, consider from a mechanical engineering standpoint the function of diagonal braces at a square corner, and consider that all tenon shoulders provide the exact same diagonal bracing for a 90 degree member, alllll the way around that member. Then it's pretty intuitively obvious.

[keith micinski]

I will still make the cheek cuts and form a tenon. I am just saying why the need for the top and bottom shoulder. I do think it would help with glue up and positing to have a bottom shoulder. As far as strength goes I stand on my tables all of the time so I am not to worried about the strength of them holding themselves together. i think I am going to make some test ones today and them perform some tests and see how strong they are.

[Will Overton]

The real test will be how they hold up over time. Most of the joints we use today have been developed over a long, long time, with strength of the joint in mind.

That said, today's glues are so much better than in the past, taking some shortcuts might not be too detrimental.

Report back in 20 or 30 years and let us know how the tables are doing. :)

[glenn bradley]

I'm in agreement with those who say the shoulders are there for strength against racking. the non-shouldered side would be your weak link in something like a table apron. Think about leaning against one side of a table and you can picture what happens to the apron joints.

[Lex Boegen]

The mortise and tenon joint is a cross-grain joint, so wide tenons actually weaken the joint, which is why multiple tenons are recommended on wide rails. Putting a top and bottom shoulder on the tenon reduces the cross-grain area so it's less prone to the glue joint breaking apart from the stress of wood movement. The rule-of-thumb is that a tenon should be no thicker than one-third the thickness of the rail material, and if it is wider than ten times the thickness of the tenon, it should be made into multiple tenons. There's still plenty of long grain glue surface left after cutting the top and bottom shoulders, so the tenon isn't weaker, and with less cross-grain glue surface, might even be stronger in the long run.