I’ve been hearing a lot of discussion about the utility of biscuit joints. Many are arguing the relative value compared to tenon joinery. I realize that tenon joinery is much more traditional and likely better structurally but both are likely to be more than adequate given appropriate design and adhesive choice.
I’ve always felt biscuits were a wonderful option given todays technology – what do you guys think???
Replies
I think biscuits are great in the appropriate use. On something like a kitchen cabinet face frame or reinforcing a miter they are capable of the task. You can slip them in where a spline wouldn't fit or be hard to cut. Biscuits are good for limiting lateral movement. Because they vary in fit, the face gluing holding power is a bit suspect. You don't clamp biscuits the flat way, it the biscuit doesn't expand equally and place enough pressure on the glue, you won't get a good bond. Forces that can pull on a biscuit will often cause failure. Biscuits don't project into the opposing piece very deeply and they do it in an arc. Forces that push down on the biscuit, coupled with possible glue joint weakness make them a poor choice in these situations. Biscuits need a water base glue to swell them. You may be able to use a gap filling glue to improve the chance of getting a better glue bond but you still have limited surface area. These joinery issues are better done with a joint like a mortice and tenon.
Biscuits are more for utility type work than fine furniture. They can have uses in furniture like constructing dust baffles, webbing and other non critical joints. When it comes to structure, I think they are just too weak, and just a bit low end. Fine quality joinery will stand on it's own without glue. It's very rare that a glue line will stand up to wood movement over the long run. Of course, everything we build doesn't have to last 200 years.
People tend to get very adamant over this question. I don't know why. It seems to me that with experience everyone figures when to use biscuits and when not. Personally, I find them great for glueing face frames to boxes, more for assuring alignment than concern over strength. But I wouldn't replace the tenons of an table apron-to-leg with anything else. In short, biscuits are a great invention. But they're not the answer to everything.
DR
I think its all personal preferance
Artistry In Carpentry
Pittsburgh Pa
Mike,
I've used biscuits for 5 years now, primarily in butt, mitre and edge joints. I've gone through nearly 3000 of the No 20s and hundreds of the smaller ones. If you have a decent biscuit jointer and use it properly then I believe you'll find biscuit joints to be extremely strong. And easy to make.
You don't really need them for edge jointing but they help align the planks flat in a table top or similar. Also, they help in edge joints when the wood is resistant to glue (eg teak and aformosia); although a spline joint would have the same effect but give an even bigger glue surface. Biscuits are easier to do though.
FWW did some test in Issue 111 page 58 et seq. This demontrated that biscuit joints are as good as mortise and tennon in terms of their strength within a butt joint.
There was some speculation in the article that a M&T joint would be better long term, as the author thought biscuits might fail "because of cycles of expansion and contraction with humidity changes". A subsequent letter pointed out that there is no evidence for this. I know Lamello have since published research stuff to show that biscuit joints actually survive such cycles better than M&T. This is because the grab of a biscuit, following its expansion inside the joint, provides mechanical strength in a way that M&T joints don't.
If you feel that the depth of No 20 biscuits is not as deep as you'd like, try the giant biscuits. Lamello call them S6 I think - there's just a "6" on the biscuit itself.
The giant biscuits are 8.5 cm long and 3cm wide (ie they project 1.5 cm into the slot). I've successfully used pairs of these to joint kitchen and dining table aprons to legs, across the "short" aprons. (I use bedbolts for the long apron-to-leg joint, for knockdown purposes, so haven't tested the big biscuits where the most stress might occur). I only glued the middle 4 cm of these big biscuits, not the ends, to avoid stress due to differential movement between the leg and apron grain.
I believe that the 8.5 cm length helps to prevent racking between the leg and apron better than an M&T. Four years with no sign of failure for the first table so-made, anyway. And this table regularly gets dragged over an uneven tiled floor.
As to biscuits not being used in "proper" furniture - well, I think that if they are invisible but just as strong (maybe stronger) than a traditional joint, the furniture IS "proper". Of course, we need to wait a hundred years or so to find out for sure....
Lataxe
My test for strength and durability of biscuits has been the white oak screen door that I made for the front door of my house. Spring hinges slam the screen door shut, year round, every time I pass through. And it's done so for at least a dozen years without any joints opening even the slightest. This test was uncontrolled and not done in a laboratory. Your results may vary.
Here's a questions that may shed light on my inexperience with woodworking.
What is a spline joint?
Splines:
Matching grooves are cut in the faces of two parts that are to be mated together. Another piece of wood is made that will fill the two grooves when those parts are so mated. The bit of wood in the grooves is the spline.
It's best practice to ensure that the grain of the spline runs across, not along, the grooves thereby forming a simple plywood structure.
Using loose tennons that fit into mortises made on both a rail end and the style it butts onto are one example of a spline joint. The grain of the loose tennon runs with that of the rail, as though it were a normal tennon milled on the end of that rail.
If you groove two edges of a board along their lengths, you can reinforce their joining by inserting a spline. The spline provides more glue surface as well as the reinforcement of an internal "tie". The spline also helps align the edges of course. To ensure some grain runs across the grooves, it's best to use a plywood strip of the same thickness as the grooves. It would be difficult to make a 6ft (say) spline with the grain running across its width rather than down its length.
Think of biscuits as splines, the grooves for which are the slots made by the biscuit joiner. The biscuit has all the virtues of a spline and some additional ones. It aligns the mating parts, it provides increased glue surface and it provides a tie. It also provides mechanical grab via its expnasion in the joint and its cross-hatched surface.
So, biscuits are in fact improved (and easy to apply) splines. In their guise as loose tennons (one variety of spline) they seem to equal or even outperform traditional mortise and tennons joints, despite what purists want to believe.
I now await my kicking from the purists, with the ointment and bandages just handy.
Lataxe
.
Lataxe,
While I agree with you that a biscuit may be an acceptable substitite for a spline, it seems to me that a biscuit can only equal a tenon's strength when its surface area is the same. That, I think is where the biscuit must fall short, in that its football shaped profile cannot match the glue area of even a stubby little square-cornered tenon of equal length. No such thing as a through-biscuit, is there? You can't wedge 'em then, either. Then, I'm thinking that there is twice the opportunity for failure of a biscuit rail-to-stile joint, which is glued into a slot on both sides of the joint. At least a true tenon is part of the rail.
'Course, I'm old school, so those new-fangled things, like biscuits, Euro hinges, mdf and sheet-rock screws are JUST WRONG. :-)
Regards,
Ray Pine
Ray,
In this neck of the wood we feed all fangles, new or a bit past their best, to the cows and sheep. Prisoners who have been exceptionally bad are also given fangle soup with their crust. You might think this appropriate. :-)
There's no doubt that a through tenon with wedges, or one that's pinned, is stronger than biscuits. In fact, you can dispense with the glue altogether then.
As to ordinary tenons having a greater glue area than biscuits, that's true. But I would argue that a bigger glue area is actually a bad thing. The glued faces of a tenon have their grain stuck to that of the mortised piece at right angles. There will be differential shrinkage between tenon and mortise when humidity changes. Something has to give. There are a lot of tenons that have gone slack, in older furniture, for this reason.
When I glue a larger tenon, I just glue the middle bit and not the edges. This allows some expansion of the tenon in the mortise, from the middle outwards. If the joint needs extra strength, I generally pin it with a through dowel or two in a contrasting wood. If 2 dowels are used, 6mm ones are best as one will break before the tenon, if there is major differential movement in the M&T
Biscuits are essentially plywood. They don't expand and contract (after their initial expansion when wetted with the glue) like the real wood of a tenon. This gives them a better chance of not shearing from the mortise (ie the biscuit slot) with humidity changes. Their shallow depth in the wood also helps in this respect.
Biscuit-to-wood interfaces are also very much closer, even, and tighter than most M to T interfaces. You need to be pretty good with the handtools (or have a woodrat) to make piston-fit M&Ts every time.
At bottom, I agree with you that tenons are more satisfying, aesthetically pleasing and a well-tested joint (about 5000 years apparently). But biscuits are very strong and great for speeding up a project. I use both and gulp down my fangle soup with a will.
Lataxe
PS If you want to see a very new fangle, have a look at this Festool thang!
http://www.festool.co.uk/artikel/artikel_weiterleiten.cfm?id=372
Edited 3/20/2006 3:30 pm ET by Lataxe
That is cool. Ive seen the comercial stationary version, but leave it to Festool to make it wood worker freindly.
Mike
I find this system good for joining apron to leg,on big tables I use two coach screws.
http://i3.photobucket.com/albums/y79/cicero1520/table.jpg
I got a pullout section from Popular Woodworking, the pullout is myths and facts (or something like that) about glue, from Titebond. They claim ( what do they know) that biscuits offer NO structural strenght. They only aid in alignement. does not mean that a joint with biscuit is not strong, it just means that it is just as strong as the same joint without biscuits. Those who disagree, dont shoot me, shoot the Titebond people
3B,
I suppose the mechanical grab that any biscuit adds to the chemical grab of the glue depends on the nature and quality of the biscuit. (And the quality of the slot you make, of course).
Some cheap biscuits look like they were molded out of sawdust and will crumble if you dunk them in your tea. Might be good for aligning things but nowt else.
Lee Valley sell a mini biscuit that looks like hardboard, which I've used for the mitres of picture frames and to locate/align thin slats in the end of a mission table; but for nothing that would rely on them having great strength.
Lamello make biscuits from their own beech forest, no less! They claim that the laminating, drying and compressing of the biscuit, along with the grid impressed in its surface, all combine to make a swelled biscuit in the joint grab mechanically as well as via the glue.
I did once see a (printed) test paper of Lamello's that purported to demonstrate all their claims. Can't find it on-line but that may be my poor surfing technique.
Lataxe
I got a pullout section from Popular Woodworking, the pullout is myths and facts (or something like that) about glue, from Titebond. They claim ( what do they know) that biscuits offer NO structural strenght. They only aid in alignement. does not mean that a joint with biscuit is not strong, it just means that it is just as strong as the same joint without biscuits.
They must have been talking about panel glue-ups, which are long-grain-to-long-grain and are plenty strong without the biscuits. I don't even find them very useful for alignment in this case, as there is too much slop in the fit between biscuit and the slot.
The case being talked about here is where mortise and tenon is normally used. Without either mortise and tenon or a biscuit, the joint would be end-grain-to-long-grain, which has essentially zero strength. A biscuit would be immensely stronger than nothing at all.
Biscuits were originally developed for melamine-faced chipboard weren't they? For one offs they work well in that environment, but for production batches I'd use dowels any day as I can machine them faster. I use M & T on trad. work such as doors where they are to an extent "expected" (they also stop joint creep stone dead), but I cut with a square chisel mortiser and a tenoner or spindle moulder. Guess that makes me a traditionalist
Scrit
Before I'd use biscuit joinery to replace mortise and tenon.. I would use pocket hole joinery.
The purist may swear by mortise and tenon.. but pocket hole joints are every bit as strong and durable.. with the added virtue of being easily disassembled for moving and storage.
That said, when I make a table for family members, I use mortise and tenon. It's the love.
V,
Like you, I love a mortise and tennon joint. I like to dress them in their Sunday best, with dowels or wedges. They are so much smarter than them modern biscuit chaps, even if they are a bit "gentlemanly" and behind the times.
Mind, the biscuits are rapid little fellahs.
Lataxe
The main problem that I see with biscuts is that they are for alignment use. They hold no structual uses. Most of the time when they are used your glueing long grain to long grain. That is what is holding your joints together not the biscut. But a Tenon is actually increasing your long grain to long grain surface for glueing giving you a much better and stroger joint than a biscut ever would.
When I first got my $200 biscuit joiner I used it for every thing; carcases, face frames, attatching face frames, miters, holding shelves, jigs around the shop. I argued with the best of yall that biscuits are better that those half a$$ed pocket screws. Fairly quickly I quit using them for various operations and now I literally haven't touched the tool in about a year, probably more. I keep thinking I ought to sell it.......but yall know how that goes!!
Now I use a mix of pocket screws and dado/ rabbit joinery. I generally use dado/ rabbit joinery on carcass work because the assembly time is much faster. There are no alignment issues during glueup, so clamping is a lot easier. I can set up the TS to mill Dados and rabbits in about 30 minutes after that I can process parts as fast as I can move them. The biscuit joiner takes me more time to mark and cut all the slots, then trying to get the biscuits in the oposing holes during clamp time is a PITA. I use pocket screws for face frame assembly and attatchment unless there is some astetic reason not to. Once the parts are assembled it is all very solid and no part will ever fail even if abused. I could say that I keep the biscuit joiner for large panel glueups and simple shop jigs....... but I don't even do that.
I think that it boils down to your experience and the machines in your shop. Setting up the saw for dado and rabbit operations used to take me an hour or more, and it wasn't very easy to do on my old contractor saw. Now on a large cabinet saw it takes me longer to change the blade than to dial in the cut. The same is true with tennon joinery. I use a radial arm saw with a dado blade and I can make a tenon in about 30 minutes, including blade changes, each additional tennon takes 3 minutes or so. I also have a mortiser, so that part is as easy as marking the location of the mortise on the stock. If you dont have these machines and haven't performed the operation a few dozen times it will seem like a lot more trouble than using the biscuit joiner.
As far as strength, the difference in strength between a biscuit, pocket screw, dowel or a mortise and tennon is irrelevent in most of the situations commonly discussed. The times that it matters (leg to apron, chair parts, large doors) tennons have a track record for success and are still the most used option today in high end profesional shops; large dowels and loose tennons are more common in production shops but are just as reliable. Probably every one of you that has a wood entry door in your home has seen the reliability of dowel construction.
Mike
I'am surprized nobody has mentioned a reversable glue joint bit and a router table, Which is my prefered method of glue jointing long grain.. Everybody knows glue is stronger than wood, And with the reversable glue joint you can allmost double the glue surface area, Plus you can throw in the ease of allignment with those nice tapered tongues the reversable glue joint provides. No slipping and creeping while clamping. Not a big fan of biscits, But I do spline 90 degree corners and mortise/tenon most other joints. Bruce, http://WWW.EAGLEAMERICA.COM
This topic is like mold in a basement; every time ya think it's dealt with, it keeps re-appearing.
My opinion on this topic is well known to frequent posters here.
Jack,
When younger, I was often accused (sometimes minutes apart and by the same person) of being far too tenacious at arguing my point but also prone to change my mind. I always felt, arrogant puppy that I was, that these accusations were both true and together represented a virtue.
These days one has aged and learnt to be less vigorous with the arguments in order to cock an ear for other possibilities.
If I have an opinion that lasts longer than a year, I treat it with great suspicion and take it down my cellar for a thorough interrogation. What do you know? The little rascals often turn out to be charlatans and the truth lies elsewhere.
Lataxe.
Yes, I've been open minded to both biscuits and tenons. Each has their place. On this forum biscuits are often condemned as inferior when they're not. Because biscuits are compressed beechwood of diagonal grain, they offer a great compomise in quick effective joinery.
And tenons work great, as everyone knows.
This forum post is now archived. Commenting has been disabled