Biscuits do not add to the strength of a joint, just aid in aligning the pieces.

How many times have you read/said that?

Intuitively I thought that a biscuit had to add strength... However, I think because I've seen the statement I don't know how many times... I've actually started to believe it myself. In fact I may have even said it before.

So...sitting here eating my lunch... I did a search which pointed to a web page that compares the "biscuit" joint to mortise and tenon. Here's a quote... the "Authors compare the two kinds of joints in force-versus-deformation response, tested to failure. Data from a limited number of experiments indicates that biscuit reinforced joints nearly equal the strength of the mortise and tenon."

So I'm thinking...have you ever heard anyone say "Adding a mortise and tenon joint doesn't add to the strength of a joint" ? I think that my intuitive feelings have been vindicated...but I'm not structual guy. I wonder if I'm way off track? Is there anyone out there that can explain why I would be wrong?

See " TESTING OF PLATE (BISCUIT) JOINERY AND ADHESIVES FOR APPLICABILITY IN CONSTRUCTING ARCHITECTURAL PRODUCTS"
http://www.woodworking.org/WC/GArchi...abstract1.html

I too have seen that so many times and just quit feeling like I had to correct it. It is true for edge gliung or face gluing solid stock where you are gluing long grain to long grain. The glue line is already stronger than the wood.

BUT If you are gluing anything related to end grain, plywood, mdf,particle board, then the biscuit makes a tremendous addtion in strength. Steve

Glenn -

I knew there would be some kind of response to the statement "Biscuits do not add to the strength of a joint, just aid in aligning the pieces."

Given the context of the question - involving the top and edge banding requiring long grain or side gluing - the answer remains the same and is supported by your reference (see 1. Introduction, second paragraph, 1st sentence). So I stand by that.

Do I own a biscuit joiner - yes. Do I use it - yes! To assemble face frames, quick butt joint boxes, and even an occasional shelf within a carcass. They are great for tasks like that. But when gluing up panels, the only addition biscuits provide is alignment. The glue joint by itself is stronger than the wood.

All tools have a good use when properly applied.

Ted

Quote:

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Originally Posted by Glenn Clabo

How many times have you read/said that?

Intuitively I thought that a biscuit had to add strength... However, I think because I've seen the statement I don't know how many times... I've actually started to believe it myself. In fact I may have even said it before.

So...sitting here eating my lunch... I did a search which pointed to a web page that compares the "biscuit" joint to mortise and tenon. Here's a quote... the "Authors compare the two kinds of joints in force-versus-deformation response, tested to failure. Data from a limited number of experiments indicates that biscuit reinforced joints nearly equal the strength of the mortise and tenon."

So I'm thinking...have you ever heard anyone say "Adding a mortise and tenon joint doesn't add to the strength of a joint" ? I think that my intuitive feelings have been vindicated...but I'm not structual guy. I wonder if I'm way off track? Is there anyone out there that can explain why I would be wrong?

See " TESTING OF PLATE (BISCUIT) JOINERY AND ADHESIVES FOR APPLICABILITY IN CONSTRUCTING ARCHITECTURAL PRODUCTS"
http://www.woodworking.org/WC/GArchi...abstract1.html

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Hi Glenn,

It all depends on application. Norm used to use the biscuits all the time to reenforce an edge to edge joint in a panel glue up. In that type of joint, the glue surfaces are long-grain to long-grain and the clamping is ideal. Flat panels glued up in this fashion are seldom structural joints. With this in mind, the biscuit is only there to help in alignment and the added strength isn't needed.

When glueing end grain such as the case in a butt joint, the end grain doesn't create a strong joint at all. This is why we use mortise and tenon joinery or biscuits. Using a tenon (either a true tenon or a biscuit) creates a face grain to face grain gluing situation - even if joining two boards end grain to end grain.

So, the strength is really dependant on need (not needed for a panel glue up) and the application of the bisuit. In the case of a side by side comparison of biscuit to mortise and tenon, this would, without the joint, be a butt joint - terribly weak even with the best glues and clamping.

Hope this helps.

Chris

Glen. In reading the link in your post I came to a few conclusions about biscuits, and biscuits joints. To make a carte blance statement that a biscuit adds no strength, is I believe wrong. The actual question is does the biscuit provide added strength to the adhesive process? I realize that the strength of the glue joint comes from the total surface area of the joint and the application, but that biscuit is none the less there adding a form of shear stress protection. An improperly sized,and or located M&T joint can actually result in a structurally very weak joint, just as a properly sized, and located M&T joint results in additional strength. In the arguments presented on that link, I more or less concluded that they were making the same inference to biscuits, and that an improperly located biscuits could result in an overall weaker joint, regardless of the adhesive application process. I think in fairness they should have compared the biscuit to the dowel,or the spline, more so than the M&T joint. I was taught in wood shop many years ago that the joint is more than just glue.I don't really care if a joint breaks at a point other than the glue line, all that proves to me is that you have strong glue and a structurally flawed and poorly executed joint.It is more important that the joint doesn't fail at all! We used to "screw and glue" alot, and the screws were considered to be "Disposable Clamps", but they were always there distributing the forces over a total greater area of material, and reliveing the glue line alone of providing all the strength. If I can distribute the shear, torsion,compressive and lateral forces of a joint over a greater total surface area and incorporate more of the inherent strength characteristics of the material being utilized. I achieve a better "total Joint" and that is what I look towards, not just trying to make the "strongest" glue joint. Sorry to be so long winded, it just irratates me that so much emphasis is placed on the adhesive component of a joint and not the proper design and execution of the joint as a whole. More than my .02 FWIW on the subject.

Apples and oranges. When people say biscuits don't add strength they're talking about long grain to long grain, not long grain to end grain. Clearly a long grain to end grain joint (such as where M&T's are used) will gain strength through biscuits. I haven't seen any long term data on biscuits so I won't use them to replace M&T. The study in the link above, as well as the one published in FWW a few years ago only look at maximum strength, noy cyclic loading and not the effects of years of expansion and contraction with humidity changes. I put enough effort and expensive materials into my projects that the additional time to use traditional joinery is insignificant. Maybe time will prove biscuits to be sufficient, but I'm not chancing it with my furniture.

As far as their strength in panels, the others are right that they don't add anything. Glue up a panel with no biscuits or dowels. Set it up on some bricks so that the edges are supported and jump on the center. I've tried that experiment a couple of times and the panel always broke somewhere other than the glue joint. If the glue joint is stronger than the surrounding wood then biscuits can't add anything. Personally, I never found biscuits to add in alignment either. It's just one extra step which is one more opportunity to mess up. I think some people use them to align improperly prepared lumber - something they shouldn't be doing. I won't glue up boards that aren't perfectly straight. If you've properly face jointed your boards and do the glue up on a flat bench then alignment isn't an issue.

I have to disagree that they don't aid in alignment. I work with a lot of mesquite, which tends to not be flat 10 minutes after it's prepared...in other words, when it's time to glue. The biscuits will without any question help in aligning boards during a glue up...and they'll actually take quite a bit of bow out and hold it. Are they necessary? Of course not...and I don't use them on everything. But to characterize them as a complete waste is a bit harsh, IMO. We'd all like to only glue up panels with boards that are all perfectly flat, but I've been working wood a long time...and somewhere along the way I decided I'd get a helluva lot more built once I realized everything's not gonna be perfect all the time.

KC

I certainly didn't call biscuits a complete waste. I don't use them very often but there are places where I do, such as for face frames on utility cabinets. I've never worked mesquite so I'm not familiar with how unstable it is. I pretty much stick to the stable hardwoods available in my area (walnut, maple, cherry, etc) and I've only had two situations where wood didn't stay flat until glue up: if it was improperly kiln dried or if it was a particularly warp prone wood such as flatsawn elm or sycamore. I buy nearly all my wood air dried to get around the first situation. I avoid the second by not using flatsawn elm and sycamore.

Gee these worms are good!

Sorry guys, I didn't mean to open a can of worms with my earlier post, but it's nice to see people who believe in their ideas enough to make these explanations. There's a lot of interesting thoughts here. I never thought of the long grain/short grain angle. Interesting.

Pat,
I don't know if it is a bag of worms or just a good old fashion discussion of experience. There are many newer woodworkers out there that miss out on using some tools because they get the wrong impression. I know that Ted didn't actually say that biscuits don't add strength ALL the time...but even I who knew better heard it so many times I began to wonder if I was wrong. Ted just got me off my duff to prove it to myself. I've been making lots of sawdust over many years and I still like to hear people explain how they do things...and take their experience to the shop with me. I use biscuits all the time...just because I find that it works for me. Does anyone remember the torture of dowels?

Sometimes seeing/hearing how people figure things out is as interesting as seeing the end product....for me anyway. Unlike many of the more productive folk on the forum...I still read and think more about working wood that I actually do.

IMHO, I think biscuits do add some sthregth, but not as much as, say, a mortise and tenon joint. One of the woodworking magazines recently did a test. They built two boxes - one using bisucits and one using mortise and tenon joints. Then they dropped an anvil on each to assess the comparative strength. The biscuit box broke apart on the first hit. It took two hits to break the other box. I don't know how "real world" the test is (can't really imagine a situation where one would drop an anvil on a woodworking piece), so take it for what it's worth.

Scott/Kirk,
(cut 'n pasted from a previous post)

Mesquite is quite a remarkable wood. There was a whole article in AW #87 (6/01) about it so I'll touch on the highlights:

Mesquite is exceptionally stable. Red Oak moves 11.3%, Maple moves 9.9%, Walnut moves 8.7 % and Mahogany moves 4.1%. Mesquite moves 2.6%! (The percent change in size is based on a swing in moisture content from 14% to 6%)

Mesquite doesn't distort or cup easily. If you take a tangential:radial ratio, Mesquite is at 1.2 while Red Oak is at 2.2 and Eastern Whie Pine is at 2.9. Cupping is usually the result of plain-sawn boards shrinking more in the tangential direction than radially. In most American species, this ratio is more than 2:1 but mesquite is closer to 1:1. This means boards tend to stay flat.

Mesquite is hard. The standard test for hardness is to measure the force it takes to drive a 7/16" steel ball half-way into a flatsawn board. In pounds of pressure, we have:

Pine is at ~660
Red Oak is at ~1120
Hard Maple is at ~1580
Mesquite is at ~2300

Mesquite's high silica content, high extractive content and extreme hardness can dull your tools quickly. Be sure to clean your tools (mineral spirits) to remove the buildup often. There isn't much oil in Mesquite so it can be sanded and glued easily. Due to its extreme hardness, it finishes quite nicely with fine sanding grits and buffing so very little finish is needed.

Mesquite truly shines on the lathe and turning green logs is the way to go. Since it dries with little change, go ahead and turn it and finish it green. It'll dry nice and slowly through the finish.

The sapwood of Mesquite, however, is a no-no as insects appear to LOVE it...even after finishing a product you could find bore holes and piles of yellow (the color of the sapwood) sawdust in place of your prized piece! :)

I took portions of the article and summarized some brief points here. After reading this article I was excited to get some Mesquite until I saw the prices for it! Yikes! :(

Magazine articles are great...but sometimes they just don't represent the 'real world'. I've been working primarily with mesquite for the past four years, so I feel somewhat qualified to make the following observations... :)

True: Mesquite is very stable, as far as shrinkage and general movement are concerned. We think nothing of gluing a breadboard end all the way across a 48" tabletop, or wrapping a tabletop in a 'picture frame' border. It just isn't an issue.

True: It finishes up beautifully. Darkens like cherry...more quickly, but not as dramatically over the long haul.

False: It's not prone to cup or warp. Speaking only of Texas mesquite, rare is the piece that will stay flat once planed, or straight once jointed. Close...yes...flat and straight? Not very often, and that seems to hold true of both kiln and air dried material. It can be quite frustrating to find suitable material for a tall door (as on an upper hutch)...and once you find it, you still really don't know what you've got until after final milling. I have noticed a difference in some South American mesquite (Argentina)...it stays flat and straight.

Subjective: It's harder on cutting tools. When I'm not working with mesquite, I'm working cherry...and I think the cherry is far and away harder on tools. I've had a DeWalt planer at home for five or six years, and I've changed the knives twice. When we built the arena tables a couple years back (54 serving tables 12' wide by 20-something wide with six legs each, 175 end/coffe table tops), we'd go a month or more before changing the blades in the Grizzly 20" planer. I just don't see it.

True: It's expensive.

And I have to make a comment about the sapwood. You can usually tell if you've got an insect issue by looking at it. People don't seem to be able to grasp that insects are gonna be found in the sapwood more if for no other reason than that's where they enter the tree. Once it's lumber, the insects can go in anywhere they want.

Excellent...I'm glad to hear some real world experience with it. If it was all those things the article mentioned, it would surely be a lot more popular, I think.

Sam,

I've never dropped a hundred pound anvil. At least not on furniture. I did barely miss a foot once. But... I have a couple of relatives that weigh WELL over that weight who just "flop" down on furniture. You would have to see it. Anyway, I have no doubt that they could have broken the "biscuit cube". I cringe when they "hit" my mortised and tenoned stuff. Just a thought.

David

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Originally Posted by Sam Chambers

IMHO, I think biscuits do add some sthregth, but not as much as, say, a mortise and tenon joint. One of the woodworking magazines recently did a test. They built two boxes - one using bisucits and one using mortise and tenon joints. Then they dropped an anvil on each to assess the comparative strength. The biscuit box broke apart on the first hit. It took two hits to break the other box. I don't know how "real world" the test is (can't really imagine a situation where one would drop an anvil on a woodworking piece), so take it for what it's worth.

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