Joint design and wood movement...analysis and new thoughts

[Mark Singer]

Mark.

As someone that was not really an advocate of even loose tenons. I'd like to get back to the original intent of the thread. That being, the technique, and design of the loose tenon that you use in making and assembling your loose tenons joints.

There is one sentence in that final paragraph that caught my attention though.

"In fact, because of the properties of modern glues, many times the wood will fail before the biscuit does when the joint is put under enormous stress."

If I remove the word "biscuit", and simply substitute "joint". It emphasizes a point I was trying to make. Too much reliance on the adhesive properties alone in a joint. There needs to be mechanical integrity in the joint. Having the glue line as the failure point mechanism is not good in my opinion.

I think we have been convinced that an adhesive, of some form or type,is all that is required to join two pieces of material. We still need a good interlocking mechanical joint. I don't care if the glue is stronger than the wood. If I can make a joint that won't break at the glue line. I'm better off. Tenon joinery, loose or integral, accomplishes this.

If I M&T a table stretcher to 2/3 the depth of the leg. That joint cannot fail at the glue line, or the intersection of the two pieces. I have to break a significant amount of material both inside the joint, and outside the joint to completely fail that joint. It's a stronger joint, no matter how we look at it.

Back to your method though. I can't find your original post anymore that demonstrated how you were making the loose tenons. I have a picture, but I personally think that it is something that would be of benefit in this thread. A picture is worth a thousand word and all that stuff. If you could post a pic of the tenon, and the joint assebly, or link to it. I think it would be a good visual for everyone.

Mike,
This is a pic of my door construction. It is not what I am suggesting in this thread which is to oversize the tenon , removing a lot of material for movement allowance. The tenon on the sofa was this larger size, but I don't have a pic...my drawing at the begging of this thread shows it... Interesting though this layout works well for big doors and I have never had a problem

[John Stevens]

John,
I found an article that really supports the use of biscuit joinery....They state clearly it is not the correct choice for chairs and the joint will fail without warning....see the last paragraph

[skip to quotation from article]

But know this: when biscuit joints fail, it is usually sudden and quick. When a traditional joint fails, it is usually a slow process. So biscuits are not the right joint for chairs or other furniture that is subjected to repeated and constant strain. You want a chair to become loose and wobbly before it falls apart with someone in it. But for the vast majority of other joinery applications, biscuits are an excellent choice.

Mark, that appears to be an excerpt from Jim Stack's book of biscuit joiner projects. As far as I know, Jim Stack has not participated in any quantitative analysis of biscuit joint failure. Of the articles I've read, only one article ( Wagner) reported a sudden failure of a biscuit joint. Gray's article states the opposite, when discussing the graph of load vs displacement of a biscuit joint:

It is surprising to note that although the plastic region (in which irreversible damage occurs) is relatively small, the load curve is gently rounded and the tail is long. This indicates that failure, as defined in this article, is not as sudden as others have suggested. Clearly, the joint is able to sustain significant load over a wide range of joint movement, even after irreparable damage occurs.

So who do we believe, Wagner or Gray? Well, Wagner's article describes some tests performed by Grant Taylor, who runs a custom door business. There was no mention of any expertise Mr. Taylor might have in testing failure in wood products. The fact that Mr. Taylor only tested one sample of each type of joint suggests a problem with sample size, and there may well have been other problems. The Gray article described tests performed in a laboratory facility of the Wood Science and Technology Center of the University of New Brunswick. Six samples of each joint type were tested according to guidelines established by the American Society for Testing and Materials.

Each reader can decide for himself (herself) which article is more likely to have reported reliable info. But even in Wagner's article about Taylor's "home made" experiment, Taylor found that the biscuit joint that "failed with a dramatic explosion" under 2800lb of pressure had exhibited a gap at 1700 lb. So one might wonder whether that sort of gapping would have given an audible warning (creaking) to a user of a biscuit-joined chair long before the chair actually failed.

In addition to that, we should think about how chairs fail. My stepfather is a furniture repairman who works on lots of stuff, from antiques to high-priced Scandinavian furniture. This doesn't make me an expert by any means, but I've seen quite a number of broken chairs. Those that broke at the joints were chairs that were made before the era of modern glues. The joints in those chairs failed because the glue deteriorated. Glue deterioration is a gradual process, unlike loading a joint to failure in a matter of seconds. In the latter process, the wood itself always breaks. In the case of glue deterioration, the joint loosens and begins to creak and show gaps. The creaking and gapping are the warning signs that should alert a reasonable person to the fact that one or more of the joints has failed. But even if the wood breaks or otherwise fails, the odds are slim that it will occur simultaneously in more than one joint.

However, we should keep in mind why we're even talking about using biscuits in chairs. After all, if you look at almost any mass-produced chair today, it's made with pocket screws because they make a very strong joint that is usually aesthetically acceptable because the holes and screws are hidden, and they're very cheap to make in terms of labor costs. The only reason I mentioned chairs was because you mentioned that Sam Maloof doesn't use biscuits because he had problems with them. Maloof makes a lot of chairs, so I replied that Thomas Stender (a studio furniture maker) has used biscuits in chairs for years and never experienced a failure.

But Maloof, Stender and chair-making are only tangentially relevant to this thread, because they deal with the question of how strong a biscuit joint is. The central topic of this thread is about using loose tenons to join wide boards. Specifically, loose tenons mounted in mortises whose cheeks are about 1/4" thick. Based on my limited experience and the articles I've read, I believe that a biscuit or two, set 1/4" below the surface of the joint, would be strong enough to serve as the floating tenon(s) in this type of joint. I understand that you and many people here disagree. Thank you for taking the time to engage me in a fact-based conversation about this subject. Even if I haven't changed my mind as a result of our exchange, I still think you've helped improve my understanding of woodworking.

[Mark Singer]

John,
The bisquit ar a 1/4" setback would not achive the same result of reducing the potential for differential expansion of elements whose grain is perpendicular.... It was actually 3/8" setback....a 3/4" tenon in 1 1/2" element....
It seems strange to me to even consider bisquits for chairs...I have always used mortise and tenon and there is a part of me that says this is a higher level of woodworking and it should be done correctly in a proven manner. I am not saying the results of the testing is wrong.....it really doesn't feel right....and that is a big part of woodworking as you do lots of projects , you tend to stick with what has worked in the past...and thats just the feeling inside , if you know what I mean....like Mike Cutler, I like the mechanical aspects of a joint to add integrety and not depend as much on the glue..

[Mike Cutler]

Mark.

Thanks for posting the pic. I think it will help with a visual, even though it was not specifically what the this thread was about. Thank you. The "divided" loose tenons in the door pic would seem to work in the same manner as traditional divided tenons. Relieve the stress of one single large tenon and divide it into two. The same amount of area is in the joint, but the stresses are individually diminished. I essentially used the same physical joint for my Arts & Craft door project, but the tenons were integral divided tenons that were mortised ino a floating tenon and then joined to the door.
In your original post the oversized loose tenon is able to be a single width, instead of divided, because the loose tenon by nature removes the stress component of the large integral M&T joint in my opinion. I like it. I've seen some books on oriental joinery methods that are very similar to what you have presented. The floating tenons went very deep into the mortised member. They too were wider than western convention.

John.

It's not a matter of changing minds. I have always advocated that someone should stick to the methods and materials that they are comfortable with. If you are comfortable with the results you are obtaining that's all that really matters. I have read your posts in this thread more than a few times. You have presented the biscuit joiner in a different lite for me. I admit that I used to view the biscuit joiner as a pretty limited tool, that was being overused due to marketing hype. Heck.. You even had me in a Borders Bookstore until 9:30 last nite reading books on biscuit joinery. I don't doubt the initial strength of a biscuited joint. I'm just uncertain of the long term reliability when referencing chairs as the model. I think that a chair and a door present some of the more difficult obstacles to overcome in woodworking. One, the chair, sees tremendous stress and abuse( I'm referring to a dining room chair) in it's lifetime. The other (the door) has to remain perfect in a single plane, with no mechanical forces to aid in a perpindicular direction.

[Mark Singer]

Mike and John,
I really appreciate the tone of our discussion...it has remained objective and thoughtful....unlike some of the "my brand is better than yours" threads. In woodworking each of us finds techniques that work and we tend to use as a default....if they work , that is probably a good thing.
Finishes are much the same way....if you have one you like stick with it..
I enjoy looking at all the ideas and keping an open mind...then going with what feels right..for me that would either be true mortise and tenon or loose tenon consruction for all frame joinery but, panel or sheet good construction ,or large surface of solid wood construction where biscuit joinery works really well and I am comfortable with it

[Mark Singer]

Mike the dimensions on the expasion relieved joint I used on the sofa were 1 1/2" x 8" member....3/4" thick tenon ,cut square not rounded by 6 3/4"....It is important to leave enough wall thicness to resist shearing through. Although my premise is that the mortise walls and tenon become a composite...here the adhesive was West Systems epoxy....which is the best for Teak wood and insures the development of a true composite...when I say composite, the analogy would be like plywood...each layer by itself is weak and flexes....together the surface resistance and developed section modulus produces a stable and strong group that acts together