I have a general question about mortise-and-tenon joinery. I know the traditional way to construct M&T joints is to do a “closed” mortise which houses the tenon completely on all sides. But since I don’t have a mortiser, it is more feasable for me to do open M&T joints (I also typically add a pin or two at each corner).
From a strength perspective, it seems that both methods are equal, since the majority of M&T strength comes from the face-grain joinery. Both open and “closed” M&T have the same amount of face-grain surface area. The only thing an open M&T is lacking is a little bit of end-grain surface area, which doesn’t affect strength much anyway.
So, are both open and “closed” M&T’s equal in strength, or am I missing something?
Replies
Matthew,
If you only look at the functional glue surface, an open mortise is as strong as a closed mortise. The advantage of a closed mortise comes from its mechanical strength.
A closed mortise and tenon joint has considerable strength even without glue, especially if it is pinned. The mechanical strength of a closed joint plus the added attachment from glue would make it stronger than an open mortise that depends on glue only.
John W.
Matthew --
I think what you're calling an "open" M&T joint might be the one more commonly called a through mortise -- that is, the mortise goes all the way through the lumber, but it does provide two facegrain-to-facegrain gluing surfaces. This M&T is just as strong as a closed M&T joint with the same dimensions. However, if by "open M&T" you mean a lap joint -- in which there is only one facegrain-to-facegrain gluing surface -- that one is not as strong as a true M&T joint. You only have half the glue surface, and the piece with the "mortise" in it is weaker.
Jamie,
Actually, I am referring to something in the middle. I'm talking about mortise-and-tenon joints where the mortise has three sides -- two face grain and one end grain. I used this, for example, when I construct a cabinet door or a mirror frame. Kind of like a lap joint with another face-grain surface.
Matthew,
Are you talking Bridle Joint?
Now I'm confused.
If you're talking bridle joint, it's stronger than a half lap, and has been used in light door frames occasionally.
Strength of these is less than a through mortice.
Cheers,
eddie
Edited 9/5/2003 3:59:48 PM ET by eddie (aust)
I also think he means bridle joint.
Matthew
Agree with eddie, your talking about a bridle joint. Getting ready to put them on the top cross-stretcher on a new work-bench. Using through mortices pinned on the bottom one's, but the top will have about 160 pounds resting on it. It will have shoulders that butt the legs.
It doesn't have as much strength as a closed or through mortice, but should be fine for the application you mentioned. You could pin them from the back side as not to expose the pin normally.
Good luck...
sarge..jt
If you open your Architectural Quality Standards 7th edition, section 1400, they consider this joint to be acceptable in rail and stile doors (full size entry) from economy to premium and custom. They call it a slot mortise and tenon but bridle joint would be a good description. I'd be curious to know what others are using as a source of reference in their asessmenst of this joint?
The AWI is a good professional standard quide used in the architectural woodwork industry by some small and really big shops to quote jobs.
Rick,
I made the first post on this question and I think that my logic is sound, a joint that has both mechanical strength and glue strength is stronger than a near identical joint that depends on glue alone.
Is an open mortise and tenon joint adequate for even demanding applications such as full size doors, according to AQS (7th ed.) the answer is yes, but that doesn't mean the joint is as strong as a closed mortise and tenon, and that's the question I answered.
John W.
John,
To get to the bottom of this, I went to Barnes and Noble yesterday and bought a copy of Ernest Joyce's "Encyclopedia of Furniture Making." Page 159, illustration #2 is exactly the joint I am talking about. Joyce calls it a Bridle Joint at one point. He also calls it a "slot/open mortise" joint.
Interesting, though, he does not include this joint in the chapter entitled "Mortise and Tenon Joints" (Chapter 17). Instead, he puts it in the chapter entitled, "Housing/Dado, Halving and Bridle Joints" (Chapter 16).
I use this joint for cheval mirrors, and cabinet doors. I see what you're saying about its mechanical strength. If I were building an entry door, I would definitely opt for the full mortise and tenon.
I started reading one chapter of this book last night, and ended up spending a few hours on it today. Seems like a great book! So from now on I will know how to properly name my joints on this board!
Thanks everyone for your input.
Matthew, You'll be fully conversant with our British terminology anyway, which sometimes confuses the hell out of Americans, ha, ha. Yep. You described a bridle joint.
Just wait until you get to the chapter on dovetails. Joyce doesn't mention half blind dovetails at all, but he does discuss lap dovetails. There's also an error in his description of setting out and marking the tails. When you find it, let me know, and I'll correct it for you.
Still, Joyce is the first book I turn to for technical solutions. It's known as the bible in the UK. Slainte.Website
Relax John...wasn't directing anything to you. I don't think it's as strong either but the AWI considers it to be acceptable in the application Matthew was asking about. It would have been nice if they included the bridle joint in the test FWW did on joints a few years back. We tend to over build stuff in our shop but I'm always researching professional sources such as Forest Products Laboratories and professional tests of products and materials rather than just speculating or I'll run my own test to see how well an application is suited to the task. There's many ways to achieve the same results in woodworking.
I find this a fascinating discussion -- the whole idea of how strong a joint is, and how strong it NEEDS to be. If two types of joints are compared, and joint A falls apart after 800 PSI, and joint B falls apart after just 400 PSI, joint A is obviously stronger. But what if you're only using it for an application that generally requires 350 PSI? It becomes a moot point. Of course, that doesn't change the technical fact, as John pointed out, that joint A is in fact stronger.
Instead of blanket statements about the virtues of various joints, or the evil of others, I would like to really develop a better understanding about how much strength each type of joint provides, and then match that up with the strength that is required in a given operation. One type of joint in particular that has come under heavy criticism is the biscuit joint, loathed by many traditional woodworkers. I have only used biscuits in a few operations myself, and I recently sold my biscuit joiner, but now I'm wondering whether I did that for the wrong reasons. Are biscuits appropriate for more joints that I at first thought? I notice that more and more woodworking plans are calling for biscuits in case work, and recently Fine Woodworking ran an article that all but assumes biscuits are part of every woodshop these days.
I read that Fine Woodworking article a while back on various joints' strength -- when they fail and how totally they fail. It would be great to develop a comprehensive listing of all joints and their general strength. Of course, the actual strength would depend on how well the joint is prepared, what glue is used, etc. But you could probably develop a benchmark for comparison. Then, you could have a related chart of "typical" strength requirements for various operations. For example, entry doors, cabinet doors, drawer fronts, etc. I know this would have to be calibrated for the size of each thing, but again you could probably develop a benchmark that would at least be good for comparison. Having this kind of system would allow woodworkers to see which joint provides necessary strength for the operation at hand. There would of course be borderline cases, but this system would at least be able to inform you of joints that are definitely too weak for the operation.
Sometimes I feel that woodworkers, just like many other professionals, develop a sense of what is right and what is wrong without providing a basis for that assessment.
I've done it myself, and not just in woodworking!
I use biscuits for face frames, and have found them to be plenty strong in all but certain circumstances. I use a #20 biscuit for 99% of my joints.
It's fairly easy to hide most biscuit joints by putting the excess out of eyesight. Frame corners I let them fly top or bottom, and dividers I fly below the piece in most cases. If there is beading inside the opening, the joint gets covered, so it doesn't matter.
I have had joints pop when moving large, tall cabinets by myself, but never break apart or separate.
One time I dropped a frame from about 6', and it landed on a corner. The corner that hit, and the opposite corner broke, but not directly in the joint. The other two joints just cracked the finish.
Matthew,
As mentioned, many people call the joint that you are referring to as a bridle joint.
You asked about this joint because you don't want to use a traditional mortise and tenon joint due to a lack of a dedicated mortiser. There are several ways to cut a mortise without a dedicated machine: by hand with mortise chisels, drilling then cleaning up with chisels, plunge router etc.
Not to start a hand tool vs. machine/ Neanderthal vs. Normite type debate, but the mentality that you need a specific machine to perform an operation (or else you can't do it) is what drives the hand tool crowd nuts.
When it is most appropriate to use a traditional M&T joint, just layout the mortises and cut them with the tools you already have. You will find that it is not difficult, and you won't have to avoid them.
Kyle
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