Mortise & Tendon Advise

[woody_newbie]

Hello to all;

I am seeking advise on the general rule of thumb on cutting the width and length of mortise and tendons. For example if I am making a chair and the leg stock is 2 inches wide what width and length should I make the tendons on the strecthers. I am assuming that the mortise should be cut to the same length and width.

Thanks in advance to all who replies to this question.

[Popeye]

Hey Woody, Someone who has done alot more of this can slide in here, but let me give you what I know and have used. The general rule of thumb I've always used is thirds. So on 3/4" stock say... the tennon would be 1/4" wide and that leaves 1/4" on each side of the mortise.
Recent reading in Bill Hylton's book on power tool joinery, he thinks in terms of halfs. So tennon thickness equals 1/2 of stock thickness. So on 3/4" stock the tennon would be 3/8".
I've used thirds when I've done mortise and tennon joints on some doors without problems. Because in the past I've always used a router to make the mortise's the depths have been limited to what I could do with a straigt bit, mostly a max of 3/4". Hopefully this is a start for you. Pat

[Tom Hintz]

I know there are all sorts of ideas on sizing mortise andtenons but I try to keep it as simple as possible, and the tenon as big as I can, especially with smaller stock.
I often reduce the shoulder cuts to 1/8" or so, maybe a bit deeper on larger stock. The ends are usually deeper but I seldom go more than 1/4" there. since the tenon is not visible, (unless we did something really wrong), I'd just a soon have a big tenon to get all the strength and glue surface I can.
I have a story on making mortise and tenon joints at the link below that might be of help.

http://www.newwoodworker.com/tipsmortenon.html

[LCHIEN]

I am faced with the same question for a coffee table I am building.
The legs are about 2" square and the aprons (Stretchers) are goping to be 3/4" thick.

Generally I think the apron will be inset from the edge of the table legs by 3/16th or 1/4" or so, so thickness of the wall of the mortise is at least 1/4" which is probably adequate.

The obvious thing is to keep the tenon reasonably thick for strength; I like the rule for 1/3 to 1/2 the thickness. Which makes mine 1/4 to 3/8" thick. Either will do since I have a mortising machine with bits in those sizes. An in between size would be silly - more work and less accurate. I think 3/8 sounds best.

I like to cut the tenons on the bandsaw. I have trued my BS table trunion to make the blade run vertical - important for the stopped cuts for the cheek and shoulders. Easier and safer to feed horizontally into the BS blade than to have a 4+ foot stick waving in the air on a tenoning jig. Altho that is fine and so is a router jig (like Rod Kirby's.)

As for the length of the tenon. Probably about an inch long. There'll be some interference with the two right angle apron pieces into one leg and I might have to miter the ends of the tenon a bit to prevent this.

Shoulder - probably about a 1/4 to 1/2 inch depending upon apron height. I'm leaning toward 2.5" apron so 1/2" shoulder will leave 1.5" tenon.

[ChrisD]

The 'halves' or 'thirds' rule is pretty much what you'd find on any article about the subject. Loring makes a very good point about the resulting thickness of the mortise wall(s). You don't want to make it so thin that it can blow out when the glue swells. Ultimately, the size of your chisels, router bits, or drill bits will determine the size of your M&Ts. It's not an exact science but should be within mechanical tolerances.

[vaking]

I believe rule of thirds (or halves) applies when mating pieces are same thickness. When you join 2" square leg with 3/4" apron it does not play. I have a book by Tage Frid where he advocates making as few shoulders as possible. In the case Loring describes it would probably translate into:
small shoulder on the front side of the apron just to make sure the entry to mortize is not visible, 1/4" thick the most. Small shoulder at the bottom of the apron, no shoulder at all at the top and back of apron. With the coffee table as Loring describes you will have 1/2" thick tenon with mortise walls being 1/2" on the front and 1" on the back. Tenon may end up being 2" wide because there would be no shoulder on top.
One moree thing - people usually make mortise a little deeper than length of tenon to let extra glue room to go(1/16 to 1/8" deeper).

[woodinville guy]

For a couple of projects, I have used the beadlock loose tenon system. It is very easy to use and provide nice strong joints. It is basically a doweling system but the dowel is a special shaped piece of hardwood.

-Dave

[drumpriest]

Here's another vote for beadlock. You can't use it in all situations, but for something like a coffee table, it's easy as can be. You create the mortise using a funky doweling jig, and the tenon is profiled at the router. You can purchase the tenon stock in beech. It includes shims for offseting your apron from the legs.

[LCHIEN]

quote:Originally posted by vaking

I believe rule of thirds (or halves) applies when mating pieces are same thickness. When you join 2" square leg with 3/4" apron it does not play. I have a book by Tage Frid where he advocates making as few shoulders as possible. In the case Loring describes it would probably translate into:
small shoulder on the front side of the apron just to make sure the entry to mortize is not visible, 1/4" thick the most. Small shoulder at the bottom of the apron, no shoulder at all at the top and back of apron. With the coffee table as Loring describes you will have 1/2" thick tenon with mortise walls being 1/2" on the front and 1" on the back. Tenon may end up being 2" wide because there would be no shoulder on top.
One moree thing - people usually make mortise a little deeper than length of tenon to let extra glue room to go(1/16 to 1/8" deeper).

Well, Alex, no shoulder at the back of the apron - that makes sense, but no shoulder at the top? That makes no sense because then there'd be no strength to the joint except what force the top could exert. Usually the top is relatively loosely attached to allow for shrinkage and therefore will not keep the tenons in the mortise if there's no top portion to restrain it.

[woodinville guy]

You got the router bit for beadlock? Can we comission you to make us tenon stock?

-Dave

[vaking]

Loring,
Firstly, there is no right or wrong here, there are many ways to make this joint and all will be correct. Tage Frid advocates very different approach - a very utilitarian one. If you don't have to make a cut - don't make it. An approach of a professional working to make living and looking for ways to cut corners and get away with it. Like Dell computers, mass production type approach. Some of it we agree with, some - we don't. I understand your concern about strength of joint but I think it is not justified. Mortise and tenon joint is very strong because it provides a large long-grain glue surface. Modern glue is stronger than wood fibers, it is a known fact. The fact that you replace a fraction of an inch of wood at the end of a leg (effectively an end grain section) with longer tenon does not really weaken the joint. More importantly, let me give you few more examples of this approach:
Biscuit joint is much easier to make than mortise and tenon. People ran some tests on comparitive strength of those joints. Mortise and tenon is stronger but by a thin margin. You probably saw some of those tests. I would go further and say that mortise and tenon joint 50 years ago (glue was not as good as today) was not stronger than biscuit joint is now. That means all the things that were made with M&T then can be made with biscuits now. We shall see a lot more of biscuits used. Drawer lock joint and even simple rabbet joint for a drawer holds well now because the glue is good. Dovetail drawers are now used more to show off craftsmanship skills than practical reasons. It was the only strong joint for drawers 200 years ago. What I am saying is that with today quality of glue mortise and tenon joint will be plenty strong as long as you don't screw up the thickness of a tenon and provide the glue surface. You don't need to rely on pressure from the top.
Not long ago I picked up an old headboard from a dumpster and took it to my workshop to dissasemble. It was a mass produced headboard probably dating back to sixties. It had no art value, I wanted to brake it apart to see how it was made. Horisontal pieces were joined to posts with M&T. I was surprised to see that tenons were cut with square corners, mortises were cut with the router. Nobody bothered to chisel out mortises or round over tenons, they just oversized mortise length and made bigger shoulders on ends to hide it. The glue was not very strong - few strikes with rubber mallet and it gave in. The glue I use is a lot stronger. Still - joints lasted for 50 years and the headboard was still intact when I got my hands on it. This mentality was in use then already, it is prevalent now. That is why we are commenting on decline of skills. The joint Tage describes is practical. I don't know if this is the best joint, but strength will not be an issue.

[Imadunatic]

All good points,

but my take is that the shoulder hides any discrepencies in the fit, this is especially true for green guys like me, or if you use a router, you can roundover your tenons instead of squaring out the mortise,
as of yet, i dont do it for the money and just do whatever i find the most enjoyable, as long as im making dust i'm happy, lol

kevin

[Tom Miller]

quote:Originally posted by Imadunatic

...the shoulder hides any discrepencies in the fit....

That, and also the shoulders are what square the joint up, since the end of the tenon is not supposed to bottom out in the mortise.

Regards,
Tom