Why not biscuits?

[Jay Knoll]

Hi everyone

I was looking through "Practical Design Solutions and strategies" by Taunton and read John Wagner's article "Choosing the Strongest Joinery for Doors"

To make a long story short -- he found that a two biscuit joint failed at 2,700 pounds of pressure in his test, while loose tenons and m&t joints failed at 2,600 pounds. Three biscuits raised the failure threshold to 3,000 pounds.

So, do you think this translates into furniture? For example, I am thinking of making a small writing desk. Should I bother with tenons when I have a perfectly good biscuit joiner languishing in a drawer in the shop?

Jay

[tod evans]

jay, the tests done where performed after only a week or so of glue cure time. give thought to the seasonal movement of wood...a mechanicly strong joint will survive lots of seasonal wood movement and remain intact. i`d be interested in seeing the results of the test if the samples had undergone several seasons and the related abuse most joinery takes.. i`ll stick to the tried-n-true myself.....02 tod

[Charlie Plesums]

I get suspicious of tests that don't make sense; this one doesn't make sense to me. A mortise and tenon has much larger glue area and much more wood between the parts, so it is hard to believe it is weaker than a couple biscuits. I will continue to use mortise and tenons.

When I first got the biscuit cutter, I used it for everything. Then as I realized that I could glue up panels just fine without biscuits, it started to gather dust. Now it is coming back into use, both for alignment (hardwood edges to a veneered table top) and for joints such as connecting pieces of plywood. Nothing evil about a biscuit, but (as I said), I don't believe they are as strong as a mortise and tenon.

[Jay Knoll]

Charlie

Actually it does make sense, because they were VERY careful about controlling variables. They standardized as much as they could for example -- "the glued surface area of the dowels used in one joint was the same surface area as the loose tenon or mortise and tenon used in another"

So, in "real life" the actual tenon might be much longer/larger than those used in this comparison. Thus, perhaps, leading to your conclusion that the tests "don't make sense".

They also compared the "dramatic failure of the biscuit joints" with the gradual failure of the M&T which they note even when the joint fails the pieces are still attached to each other.

So they conclude that "the best joint" is the loose tenon principally because even while it opened up it still showed resistance to deflection and overall good strength

But the question still remains, if I were going to attach a table rail to a leg, would I be gaining that much more by making a loose tenon or MT joint rather than using a couple of biscuits? After all, isn't a biscuit just another form of a loose tenon, albeit a bit shorter and thiner?

Jay

[Mike Cutler]

Jay.

I think I would be more inclined to compare a biscuit to a spline, and not a loose tenon. A loose tenon still follows the M&T joint formula, ergo it has to end up physically much larger than a biscuit.

I don't like the test method that you have described. If I want to intentionally weaken the M&T joint to make it comaparble to the biscuit, I've defeated the purpose of the M&T joint, but a properly sized M&T joint (loose or fixed) is much stronger than a biscuit. The proper comparison should have been a properly sized tenon, versus a properly located biscuit.

The advantage to the M&T joint for the joining of the table leg to the stretchers or apron is that it can be adapted to the size of the material involved. It can be haunched for extra strength, pinned for increased mechanical properties, split to accomodate different needs for shear load. It is a very adaptable joint.

I can see the use of the biscuit for aligning, and maybe adding some shear strength, but for the joint that you are refering to, I would use an M&T joint, or a sliding dovetail.

My .02 fwiw.

[Kirk (KC) Constable]

I don't like the loose tenon idea because to me, it's just a big biscuit. Kinda. I much prefer an integral tenon, and particularly on an apron/rail to leg joint. If I sell a table and someday it comes apart for whatever reason, I'm gonna feel a lot better if the buyer doesn't see a biscuit.

KC

[Mike Cutler]

I don't like the loose tenon idea because to me, it's just a big biscuit. Kinda. I much prefer an integral tenon, and particularly on an apron/rail to leg joint. If I sell a table and someday it comes apart for whatever reason, I'm gonna feel a lot better if the buyer doesn't see a biscuit.

KC

KC.
Have you had a chance to check out the loose tenon that Mark Singer uses?
I've never been a loose tenon fan either, but Mark's method, and design are worth checking out.

"Integral Tenon". That's the expression I was looking for instead of "fixed". Too much time spent sniffing Kerosene, WD-40, and Kroil today. Lost a few brain cells. Thanks for the correct terminology.

[Kent Fitzgerald]

I don't like the loose tenon idea because to me, it's just a big biscuit.

There is an important difference: an M&T joint (even with a separate tenon) will maintain mechanical strength even if the glue fails. A biscuit joint will fall apart under the same conditions.

[tod evans]

jay, when something such as the mortise-n-tennon joint has proven to withstand the test of time why go looking for a new way to skin the proverbial cat? are you looking for speed? or cost effectiveness? or perhaps the tool manufacturers induced magazine test has caused you to rethink the tried-n-true?
.02 tod

[Don Baer]

When I (along with several other Creekers) took the workshop with Sam Maloof and he was showing us how he did his Glue ups for making chair seats and table tops I asked him why he didn't use biscuts and he was quite specific that they weren't strong enough. I figure if Sam won't use em then niether should I.

[Jerry Olexa]

My instincts would say that M/T joints are stronger than Biscuits. (I guess I'm wrong again) . I love my Bisc joiner but would be hesitant to use on a really important, stressed joint

[Mike Cutler]

John Stevens.
You bring up a lot of good points in your post about stress, and determining the amount of force, or stress a piece will see.
One source of stress induced on furniture that has probably caused the most problems, is simply moving it.
Moving furniture is hard on joints. A great deal of leverage can be applied, with very little effort to a joint. In the example of the writing desk that Jay is considering ,there probably wouldn't be too much stress because of the size and weight, but for furniture to stand the test of time. The joints have to be mechanically strong.
I'll admit that I'm sort of a purist, or snob if you will. I believe in the strength of the M&T joint( integral). Properly done, that desk will still be moving around 200 years from now.

The "biscuit" joint is not new. These types of joints have been incorporated in furniture, in one form or fashion for many years. Splines and dowels can be found in all furniture from different eras and countries. The biscuit is unique only in it's shape and the machine to cut it. I think it definitely has a place in fine furniture. I just personally wouldn't use it to attach a leg to an apron or a rail.
More that .02.

[Mike Parzych]

I'm not a biscuit user myself. The Father of All Biscuits, Norm Abrams, went wild with them when it first hit the market, then seemed to back off. I had heard that they can be problems when joining boards for a table top for instance - the glue swelling the wood around them initially followed by an eventual "un-swelling" as the glue cures, resulting in an eventual visual "bleed thru." Am I right about this?

[John Stevens]

One source of stress induced on furniture that has probably caused the most problems, is simply moving it.
Moving furniture is hard on joints. A great deal of leverage can be applied, with very little effort to a joint.

Mike, thanks for your comments--they're all good for me to keep in mind. Regarding the one point I quoted above, here's my thought. Any piece of furniture with a flat top, whether it's a desk, a bookcase, an end-table, etc., can be expected to undergo two big stresses. One is the stress of a move. The other is that of a fully-grown adult standing on it instead of getting a step-ladder. The first situation is one that only results in property damage. The second situation is one that may result in serious permanent bodily injury or death, so I think that's the more important one to design for.

In the second situation, the load of the adult is borne pretty evenly by all of the joints. So if we're considering the case of a four-sided end table or desk with long thin legs joined to stretchers by one biscuit on each end of each stretcher, the load is distributed among eight biscuit joints. If the stretchers are 3/4" thick give or take 1/16" and the slots have been centered in the stretchers, and the stretchers and legs have been butted nice and tight so there's no perceptible gap, then each of the eight biscuit joints are pretty much all exposed 1/8 of the total load, which is mostly tensile and shear forces. I don't worry about biscuit joints failing in this kind of situation.

In the situation where furniture is moved, all bets are off, IMO, because the risk of furniture being exposed to damaging levels of force is very hard to pin down. (I say that based on my experience of having worked from time to time with my step dad, who is a furniture repairman, and on my experience with having moved myself over a dozen times in the past 20 years, and having moved many of my friends, by virtue of being the owner of a pickup truck.) On the one hand, it stands to reason that the stronger the joint, the lower the odds that it will be broken during a move. So, putting aside aesthetics and other intangible values (because they're irrelevant to the narrow issue we're discussing here), the cost-benefit analysis between biscuits or a stronger joint like a traditional M&T is whether the cost of the additional labor involved in making the traditional joint is less than the difference in the cost of repair of damage that will be suffered by each type of joint, multiplied by the probability of that damage occurring during a move. Based on that analysis, I still wouldn't hesitate to go with biscuits.

However, once you figure aesthetics and "tradition" back into your choice of joinery, I have to admit that biscuits are way down with pocket screws. There's nothing like the look of a pinned M&T or through M&T joint, and my hat is off to you folks who have the skill and patience to make joints like that. It's even more than looks--its the fact that more of the "life" of the builder has gone into the piece, and people who truly appreciate furniture realize this. If I had the ability to do make joints like that, I wouldn't use biscuits.

[John Stevens]

When I (along with several other Creekers) took the workshop with Sam Maloof and he was showing us how he did his Glue ups for making chair seats and table tops I asked him why he didn't use biscuts and he was quite specific that they weren't strong enough. I figure if Sam won't use em then niether should I.

Thomas Stender is a furniture maker who wrote a book on how to make several models of tables:

http://www.amazon.com/gp/product/157...Fencoding=UTF8

In that book, Stender says he's been using biscuits in his chairs for years and has never had a chair fail. No disrespect intended to Sam Maloof, but Stender is basing his opinion on chairs he's made and sold; Maloof isn't.