jo4hn, your brain is working just fine. There are numerous studies of
biscuits and joint strength. You might start, as I just did, by Googleing
with[ "biscuit joinery" joint strength study] in the search criteria[just
use what is in brackets]. You can find more if you massage the search words
I published a note to a fine wood working book at one time. Some other
person posted the FWW Magazine article, and then a follow-up article I
See Practical Design -- Solutions and Strategies
Article Choosing the Strongest Joinery for Doors
Rail & Stile type joint.
Dowel -- 1800 lbs.
Loose Tenon & M&T 2500 Lbs.
Two Biscuits 2700
Three Biscuits 2900
I think the test was redone with slightly different results -- but not
significant for most applications.
I recall Fine Woodworking published a study of various
(non-mechanical) joints. Mortise and tenon was the strongest and when
it failed, it did so slowly. The joint held together by biscuits was
surprisingly strong as a T-joint but when it failed, it did so without
much warning. For an edge-to-edge joint, biscuits add a little
additional strength over a plain butt joint. Biscuits work
exceptionally well with MDF and chipboard where other types of joints
often fall short. A butt joint is the weakest of all as there is less
glue surface area. It is the glue-surface area that effects overall
strength, and since bicuits increases glue area it adds strength,
similar to a spline.
Yes they add stregnth...no they don't add stregnth. What's a poor
newbie to do. I was planning a coffe table with a shelf. As I don't
have the skills to do a good M&T yet and I don't want to waste wood
(makore), I thought I would use biscuits on the stretchers between to
legs. I'm concerned about the amount of shear the joint will be able
to handle with the weight of the wood and 100 of SWMBO's magazines
piled on the shelf (I'm not concerend about the table top as it is
supported by the legs).
I think I've determined that I shouldn't use the biscuits and should
just keep practising my M&T's tell I get it right. Thoughts?
You're going to have to make up your own mind, but consider this. There's a
great deal of woodworkers out there (and here) that own and use biscuit
joiners. I'm one of them. I'd wager that most of owners of these machines
use them for edge joining boards to make wider panels just like the one
you're considering. If these panels were prone to easy failure or extremely
difficult to construct with the aid of a biscuit joiner, there would be a
whole lot of screaming going on. There isn't. Need I say more?
Biscuits do add *some* structural strength, (long grain to long grain but
the total surface area of the biscuits is probably minimal compared to the
surface are of the boards to be glued. However one good reason to use them,
is that they stabilize the joint during glue up. Using modern glues, it's
been demonstrated time and time again that in long grain to long grain glue
joints, it's usually the wood itself that will fail before the glue joint
fails. While the biscuits probably don't add all that much to the strength
of the joint, they will aid dramatically in producing a perfectly flat
surface after gluing, (or one that needs minimal sanding). By always
cutting the biscuit slots from the top of the boards, this will account for
any minor variations in the width of the boards. If the application was so
extreme that wood failure is a possibility, (i.e. grandsons using a coffee
table as a launching platform to jump to the couch <G>) I wouldn't depend on
biscuits or dowels, but would cross brace the wood from the backside, maybe
even to the point of laminating a hardboard on the back. (Either that, or
traumatizing a grandson for life).*
However, I have used biscuits, with a good deal of success in strengthen end
grain to end grain on cabinet faceframes. In this particular instance, even
a single biscuit would almost double the glue surface. And if you're trying
to mount the face frames without nail holes, using biscuits to align the
faceframe to the cabinet carcass while the glue dries is invaluable.
IMHO, biscuits are normally an acceptable substitute for dowels, in most
instances. Most doweling jigs exactly center the holes in the boards and if
the boards aren't exactly the same thickness, this can produce minor, but
extremely annoying variations on the surface. Doweling jigs can be shimmed
to offset the holes, but you're really getting touchy here.
Just chatting. Not preaching.
*Two weeks ago, a grandson of mine was using a lower cabinet door (walnut)
to climb on the kitchen cabinet to get to the cookies. The joints, nearest
the hinges failed, on the door. I was able to fix the door, and I'm
confident the repaired door will hold up just fine.
My grandson has a new found respect for Grandma's cabinets, too. One down.
Three to go.
I don't know if this will help, but it helped me...
when I got my 1st biscuit jointer, I made a sample frame out of 1/3 fir, using a
#10 biscuit in each corner and titebond III...
I let it sit a few days, then put it in the vice and started applying a r9ocking
motion to it like a bookcase or cabinet might get when moved...
It took a while, but I got all 4 corners loose, and every one broke the wood
around the joint, not the biscuit.. those lil' suckers really glue in there!
Please remove splinters before emailing
Consider the biscuits to be first glued into the wood. I.E. suppose
them to be an integral part of the wood, being embedded in it. Now
examine the cross-section [edge-on] of the biscuits and the wood. The
area of wood far exceeds that of the biscuits. When glued, the area
of wood glued edge to edge far exceeds that of the biscuits in the
cross-sectional view. That is, the amount of glue on the wood to wood
area is the major factor in the gluing.
Look at the material used for biscuits. If it was steel, then it
would help strengthen the joint in that it would be less likely to
flex and so break, even at a slight distance from the joint within the
dimensions of the biscuit. The biscuit material is relatively so weak
that it does nothing of any consequence like that.
The biscuits help line up the material. Keep using them for that
purpose. I think they do a great job of that.
Now, this topic has been beaten to death over and over, so let's leave
it be and concentrate on more important topics like Antivirus
software, whether or not you need to learn AutoCad before you learn to
use a hammer, and religion B.S. and the other interesting OT redneck
discussions that fill this forum.
I'm not sure of your table design, and so cannot comment on that,
Dowels are still considered 'old school' at some of the 'fine
woodworking' programs of note. And that is a term of pride, son.
For those without a wizened old mentor, you might look at the Miller
Dowel system, as a means of making a strong, beautiful, repeatable
joint. I haven't tried it yet, but several of my experienced
woodworking friends in analog life have, and speak of it with high
I have used traditional dowels, with good results, and would do so
again. And I built bathroom vanity drawers with rabbets and biscuits in
the project currently on my bench, because my dovetail jig would not
accomodate the thick stock I had already cut to less than jointer
length. And I didn't want to hand cut DTs for 8 drawers in a bathroom.
Just a thought. No financial interest, etc.
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