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Its been a while since I have done mortise work. Whats the consensus on mortice/tenon sizing. I was always told the tenon is twice as long as the thickness of the board, and the thickness of the tenon is half of the board thickness. Is this correct? Thanks for your input.
Check out Mike's 2nd reply in this thread. It mirrors the rules of thumb that I have followed in the past.
Just my opinion, but I'd rephrase a few of Mike's points.
1. The tenon is 1/3 the thickness of the mortised piece.
Since we're not just working with similarly thick rails and stiles all the time, I think a better way to put it is that the tenon should be roughly 1/3 the thickness of the tenoned piece; you're not going to make the tenon on a 1" rail 1/3 the thickness of a 2" or 3" table leg that gets mortised, after all.
2. The width of the tenon is 5 times the thickness of the tenon, not to exceed 4" or the tenon is then divided. Dividing tenons does give more glue area,but it also relieves the amount of stress that a single long tenon could place on a board. Something to do with the leverage principle here.
Dividing very wide tenons actually gives less glue area, but increases the strength of the joint by strengthening the mortise cheeks with a solid wood interruption in the middle of the mortise's length.
3. The length of the tenon is 2/3 the width of the mortised piece. The longer the tenon, the stronger the joint .
Some tenons go all the way through, so a 2/3 formula isn't especially useful here, IMO, but I agree generally that longer is stronger.
3. Mortises are started at least two board widths away from the end of the mortised piece.
I think the important principle here is that tenons at the end of mortised pieces should be haunched, so as not to weaken the mortise by wedging forces. No tenon at all, however, might allow the tenoned piece to twist.
I was going to add some info to Mike's post but when I came back to the thread I see Frank has covered all my concerns and a couple more.
I guess I'm not quite understanding #3. I'm making a small coffee table with 3" apron that will be attached to the legs by m/t with 3/4 material. If I started the mortise at twice the thickness of the apron from the top of the leg which is 1 1/2", there is no way I could get the width of the tenon. I thought the top shoulder on the tenon should be the same as the sides.
Randy
Each piece requires some "flexing of the rules", they're not etched in stone. They are also more or less for blind, or a basic M&T joint.
The important attribute to accommodate is: How will the joint see stress. If I'm reading your post correctly there will be no angular forces applied to this joint.The stresses, or force will be primarily applied in the vertical plane. The purpose for Frank's response is that the mechanical advantage of the apron would be more than enough to "blow out" the end of a leg it were subjected to rotational forces.
You can get away with not following the "rules" here, by haunching the tenon or using a "haunched stub tenon", with the haunch cut at a 45 degree angle.
I know this sounds complicated, but it really isn't. There are many books and info on the net about the M&T joint and how to execute one properly.
The M&T joint is the almost literally the "cornerstone joint" of furniture building, and there a many ways to do them.
The "Japanese" M&T methods are an artform unto themselves. They have an amazing number of different joints. Beautiful work.
PS
Frank is right anout the thickness of the tenon. I screwed up in the post that is linked.
So I'm confused. If I have a 3/4" apron member going into a 2" leg would 1/2" or so mortise be ok?
Hi Russell, a 1/2" tenon on a 3/4" apron only leaves 1/8" shoulder on the apron.
A little small in my opinion, I would use a 3/8" tenon maximum..........Regards, Rod.
Quote:
Originally Posted by Randy Henry
Randy, you don't need a shoulder on the top of the tenon, but reducing the length of the top, say, 3/4" of the tenon so that it's just a short stub tenon (a haunch) both insures against blowing out the top of the mortise by lever action against the leg's end grain, while also providing resistance to the rail's twisting that might occur if you entirely removed that much of the tenon. A minimal shoulder at the bottom of the tenon helps hide the mortise.
Google "haunched tenon" and you'll get lots of hits which explain and illustrate the idea. Note: some writers, esp. Brits, call any tenon that doesn't go all the way through a "stub" tenon; for our purposes, a stub tenon is short (like 1/2" - 3/4"), much shorter than a normal tenon.If the face of the rail is going to be flush with the front face of the leg, then you'd want at least a 1/4" shoulder/cheek on the outer side of the mortise and tenon, so with 3/4" material a 1/2" thick tenon (that's what you meant, right?) would be too thick, there'd be no back shoulder to the tenon.Quote:
Originally Posted by Russell Johnson
A lot of times, though, the rails are set back from the face of the legs, which not only looks better, IMO, but it allows a thicker tenon, for strength, plus a wider (thicker) outer cheek to the mortise; the tenon shoulders don't need to be any particular depth, as long as there is some shoulder on both sides of the tenon, and sometimes in fact you can offset the tenon, giving a deeper shoulder on one side but still having at least a minimal shoulder on the other.
It's really with like-sized frame members that the 1/3 rule for tenon thickness is most appropriate -- you want a thick enough tenon to have some strength, but you also need thick enough mortise cheeks to be strong enough.
Here's a link to a picture of a haunched tenon (outer cheek removed for illustration):
http://americanwoodworker.com/userdo...d_Tenon_00.jpg
If you're making a frame with grooves for a panel, the haunch can be sized to fit the groove, but IMO it's not good practice to go the quick route and make the tenon only as long as the groove is deep.
Frank, Mike, Thanks for you posts. Makes more sense now.
The 'rules of thumb" are good, but sometimes you just have to stop and look and see if it looks right. Change that, if its about right it will look ok. It is very obvious when it dont look right.
The key is maximising strength in both pieces, since the weakest piece will break first. Generally speaking you want as big a tenon as you can manage within the rules of wood movement. To balance that you also need at least that much strength in the walls of the piece being mortised.
The "one half" or "one third" rule makes more sense if you consider it to apply to the piece being mortised, not the tenon. Consider that a 1" apron going into a 2" leg could easily have a tenon that is 1/2" or 3/4" thick and still have 1/4" or 1/8" shoulders. This would be significantly stronger than a 3/8" tenon.