How do you size mortise & tenons?

[dennis thompson]

I am building a kitchen table.
The top will be 60" long & 42" wide.
The rails will be 1" thick & 3 1/2 " wide.
The legs will be 2 1/2" square
It is made of ash.
What size would you make the mortise & tenons on the rails?
Thanks
Dennis

[Jim Matthews]

I'm building something similar.

I was taught that the tenon should be 1/3 the thickness of the leg.

The real determining factor is the size of the mortice. If you are hogging out by hand, make the tenon match the thickness of your chisel nearest to 1/3 the leg thickness.

If you're doing it with a router, you can make more than one pass.

One note, if you're rails are really high, they should have an upper and lower tenon.
I was told this helps the joint resist fracture when the top part of the tenon is in compression, and the lower part of the tenon is in tension.

(When you push down on the table, the legs try to splay outward.)

[bob hertle]

I'd make them 3/8 thick by 2.5-3.0 wide. Length depends somewhat on setback but my rule of thumb is to make them as long as possible, even if I have to miter the ends for clearance.

Regards
Bob

[Stephen Cherry]

I'll vote for 5/16 or 3/8 by 2.5 inches, with no shoulder at the bottom of the tennon. I cut tenons first, mark each tenon with a number, then use a chisel to mark the top and bottom onto the leg from the tenon itself.

For speed and accuracy, you really want to develop the tenon cutter and mortising tool as a matched set.

One more rule of thumb- snugg on the sides, and tight top and bottom.

[Tony Bilello]

The rule of thumb is the tenon should be from 1/3 to 1/2 the thickness of the apron and the length of the tenon should be about 1/2 to 3/4 the thickness of the leg. Again, this is not carved in stone and sometimes instinct will tell you which end of the scale to lean toward.
In your case, I would go with the 1/2 figure which would be a tenon thickness of 1/2". This will leave plenty of shoulder on the tenon. Since the legs are 2 1/2" thick and I will assume that the rails will be centerded I would go with the 1 1/4" long tenon. The end of the tenon might have to be 45'd.

[Frank Drew]

I was taught that the tenon should be 1/3 the thickness of the leg.

The real determining factor is the size of the mortice. If you are hogging out by hand, make the tenon match the thickness of your chisel nearest to 1/3 the leg thickness.

Jim, roughly one-third the thickness of the rail (the tenoned piece), not one-third the thickness of the leg.




One note, if you're rails are really high, they should have an upper and lower tenon.
I was told this helps the joint resist fracture when the top part of the tenon is in compression, and the lower part of the tenon is in tension.
I'm not sure what "really high" in this context means, but with a wide rail, like the bottom rail of a door, the tenon is ideally interrupted, so as not to weaken the mortise. This really isn't applicable with a typical table mortise and tenon.

[Frank Drew]

I'd make them 3/8 thick by 2.5-3.0 wide. Length depends somewhat on setback but my rule of thumb is to make them as long as possible, even if I have to miter the ends for clearance.



I agree with Bob, tenons should be longer rather than shorter for more glue surface.

Also: Except in very small tables with thin legs, table rails are rarely centered on the legs; in almost every case, at least in traditional furniture design, they cheat towards the outside, with maybe a 1/4" or 3/8" reveal (setback).

The tenons should be haunched at the top, to protect the short grain at the top of the mortise, and they should have at least a small shoulder at the bottom, to hide the bottom of the mortise.

Corner bracing is an excellent idea and really helps over the long term.

[Kent A Bathurst]

Jim, roughly one-third the thickness of the rail (the tenoned piece), not one-third the thickness of the leg..

This is the one that has always left me scratching my head - maybe you can help with the logic/rationale.

My approach is to make the tenon as thick, tall, and long as I can, while always protecting the integrity of the wood surrounding the mortise. So, on a 1" thick rail into a 2-1/2" leg, I might go as thick as 5/8" - 11/16", leaving the shoulder along the cheek at 5/32" - 3/16" [but - the shoulder on the ends of the tenon would be greater, to protect the short section at the top of the mortise].

The design of the piece defnitely comes into play - if the rail is "cheated" to the front of the leg - as noted above - this can limit the thickness, of course. I don't have a hard-and-fast rule on how much leg to leave on the mortise cheek, but when I'm doing something like, say frame & panel, I use the "one-third" guideline - not for the tenon's sake, but for the integrity of the wood along the mortise. So, for 3/4" thick stiles + rails, that means 1/4" either side of the mortise - which is always how I have interpreted the "1/3 rule".

As the scale of the piece gets bigger, I make that dimension bigger as well. It's basically a judgement call balancing the design and the dimensions, and I find a point where I can say "OK - I'm comfortable with that".

And the length is usually defined by a 45* at the end, where it meets the adjacent incoming tenon, as noted above

But - absent these restrictions, why not make the tenon as thick as possible?

[Jim Matthews]

Quite right on the rule of thirds; the tenon should be roughly 1/3 the thickness of the rail rather than the leg.

I like to leave at least a 1/8" reveal from the shoulder of the rail to the edge of the leg.

http://www.diyinfo.org/wiki/Constructing_Mortice_And_Tenon_Joints

Perhaps it is needlessly complicated, but the interrupted tenon is what I use on wide rails and I have not had one fail yet. This has allowed me to build without lower stretchers.

[John Piwaron]

Zero

This is a kitchen table, right? I've built one of those. It's in my kitchen now. The legs are removeable so I can get it in and out of the room. If they were fixed in place it'd have to have been assembled there and would never leave.

In my case, the aprons don't meet at the corners, the leg fits in there (as if it were mortised) but the aprons are joined by a piece at 45 degrees. There's a special threaded fastener that screws into the leg and the other end passes through the 45 degree piece with a washer and nut on the other side to draw the leg tight.


FWIW, if this were some other table that could fit through a doorway, I'd go for at least a 1/2" tenon. Maybe 5/8". YMMV

[Frank Drew]

This is the one that has always left me scratching my head - maybe you can help with the logic/rationale... why not make the tenon as thick as possible?

Kent,

In general terms, I don't think we disagree; you'll note that I said that the tenon should be roughly one-third the rail's thickness. If you want to make it a bit thicker than that, no problem; I'm not sure much is gained but I also don't think it hurts anything, as long as there's enough shoulder on the tenoned piece, and thick enough mortise cheeks.

I think that when we're talking about a typical table joint strength and integrity over time, a bit longer is more important than a bit thicker; tables rarely, if ever, fail because the tenon isn't thick enough, but they often do come loose because the tenon isn't long enough, or perhaps because the table didn't have corner bracing (don't ask me how I know that :cool:).

[Kent A Bathurst]

Frank - thanks......We're on the same page - I just was always wondering "huh?"

I guess my basic design approach boils down to "OK - how big can I make those suckers?" :p. I definitely agree about length + width & glue surface area.

I am making a suite of A+C-style furniture in QSWO. So - something new to me - got a LN dowel plate and some sets of drawbore pins. After pussy-footing around with shoulders and tenon jigs and mortisers and chisels and planes and handsaws to get the M&T joints "just so", there is a certain tactile, neanderthal-ish, satisfaction about picking up the large-size Japanese chisel hammer and beating the living crap out of the piece of furniture I'm building [well, the pins anyway]. :D Now - THAT's a joint that won't fail.

[Sean Tracey]

The rule of thumb is the tenon should be from 1/3 to 1/2 the thickness of the apron and the length of the tenon should be about 1/2 to 3/4 the thickness of the leg. Again, this is not carved in stone and sometimes instinct will tell you which end of the scale to lean toward.
In your case, I would go with the 1/2 figure which would be a tenon thickness of 1/2". This will leave plenty of shoulder on the tenon. Since the legs are 2 1/2" thick and I will assume that the rails will be centerded I would go with the 1 1/4" long tenon. The end of the tenon might have to be 45'd.

Yes, 1/2 the thickness for the tenon is stronger for the most likely critical loading of a table.

A table is unlikely to fail due to weight placed on it. This would put the joint in shear. Racking as in leaning back on a chair is what causes most joints to fail. I experienced this as a kid. I didn't lean back in the chair, I picked it up to move from under the table and the back leg caught the carpet stopping the back legs and I stumbled back on it. It snapped instantly.

You can also see this illustrated with respect to tables. Watch a cowboy movie bar fight. A cowboy can stand on top of a table and not break it. But if a cowboy get's punched and falls into the table sideways, it comes apart. Of course, those might be special Hollywood tables designed to easily break apart.

Finewoodworking actually tested various joints in racking. The joints made with 1/2 thickness tenons were much stronger than 1/3 thickness tenons. This makes sense knowing that you have two mortise walls and only one tenon so the joint will be stronger when the tenon is as thick as the combined mortise walls.

Parts of the Finewoodworking test actually tested the glue rather than the inherent mechanical strength of the joint. For instance, the half lap joint was the strongest. This may be true for a door frame, but not for a chair or table.