Thread: stronger join

IMHO biscuits will be as strong, if not stronger than a tongue and groove. Also less stuffing around with just using a few quick bikkies. I don't like seeing the tongue and grove on the end grain.

Tongue & Groove is stronger as it has a mechanical strength without glue.
As the glueline is larger the bond between the pieces of wood is increased

Originally Posted by echnidna

Tongue & Groove is stronger as it has a mechanical strength without glue.
As the glueline is larger the bond between the pieces of wood is increased

Hi Bob, how wide is a tongue? A size #20 biscuit sticks out 12mm.

thanks both joins would be glued

simon

if you work out the total surface area of the join assuming a 6mm tongue the T&G will have a much larger glueline as its continuous, whereas biscuits are spaced around 300mm apart. Biscuits are also cross-grained but with T&G all the grain is in the same plane.

G'day Cyco,
Maybe one of those lock mitre bits we're learning about on the router forum???
They claim 70% increase in gluing area with respect to a normal butt joint.
They also use them to join panels as you want to do.

I agree with matrix - go for the biscuits. With the biscuits and modern glues the joint will be stronger than the timber. The t&g may well be stronger than the biscuits but it's all academic when the timber will fail first.

Cheers
Michael

I got canned in another thread by suggesting the "drastic" option in fixing a cupped table top, so I won't be suggesting the "perfect" solution here.

I've used biscuits and PVA for years for bench and table tops and have never had any problems. In fact, the biscuits have helped in aligning the boards at glue-up which has been handy. All we know, glue time can be a stressful, so anything that helps shouldn't be ignored.

That said, I've never tried splines. Theoretically stronger I guess, but today's glues are said to be stronger than the surrounding wood. A recent article in Fine Wood Working supports that view, particularly for PVA glue.

Probably of more importance that the glue and the method itself is making sure (dry fit) that all the faces and edges are square and the boards straight. That way, you solve 99% of your problems before you begin.

As they say FWIW.

Jefferson

Go with what Mic-D and Jefferson said. Sure, a spline is stronger, but if the joint's done properly, the wood will fail first, so it's all academic. Even the biscuits are really only helping to align the timber.

Then again, if you don't have a biscuit jointer, a spline's fine to align the timber.

FWIW, I've built panels both with and without any spline/biscuits and I can't see any significant difference in strength. As Jefferson said, the critical bit is ensuring they boards are straight and true and the edges square and clean. A panel clamp is almost an essential, though if you use biscuits or a spline you can get away without any, as long as your sash clamps are good and straight (and strong).

I have used the reverse glue joint bit with success on a Tassie Oak top for a display cabinet and some other projects.

You don't loose as much material as with the lock mitre bit for straight joins like this.

Bought mine off McJing, but other router bit suppliers have them as well

I've used dowels for butt jointing boards - usually 150 to 200 mm spacing. They provide enough strength to stop bending along the joint and plenty enough glue area together with the butted faces to keep the joint closed.

Main reason though is alignment. Using a Benchmate dowelling jig with the finished face down very little sanding is required to get a smooth surface. Quite an amount of warp can easily be taken out with the dowels - allows me to use 'seconds' narrower pine boards (quite cheap) to form up bookshelves and even table tops up to 600 mm wide.
PVA glue I find is good enough - as AlexS pointed out the glue is often stronger than the wood.

Bought a reverse glue joint bit a while back but find that the timber has to be quite true to get anywhere near a smooth joint. Quite as strong as the dowel joints, however you do lose a bit of width.

But Jefferson got to the crux of the matter - spending time to ensure the boards match, the edges are square and true and generally checking the dry fit for all joints and identifying them properly is the key. Then go into assembly line mode and joint everything up.

One day I'd like to see someone do a strength test on an edge to edge join with all these various methods like they seem to like doing with right angle joints.

I don't believe you need anything to strengthen this type of joint. It is long grain to long grain, which is as good as it gets. I believe you only need reinforcement with right angle joints - mortice and tenon, dovetails, biscuits, splines etc, etc.

Tongue and groove is used when you don't want to glue edge to edge but want the boards to move independently of each other and not show a gap. It's is not meant to be glued. I believe people use dowels, biscuits etc to help align the edges when gluing and that once the glue has dried they serve no purpose.

This might be a good enough reason for wanting to use them, but it's not as if your joint is going to open up if you don't (or that it wont open up if you do).

Gary Rogokwski In The Complete Illustrated Guide to Joinery:

Some edge joints do have reinforcing, like biscuits, dowels, or even a tongue and groove. But these types of reinforcement are used as much for alignment as for strength. What edge joinery depends on is two good mating edges cut straight and true and bonded together with a good adhesive.

By all means, use biscuits or whatever in edge joins but don't do so in the mistaken belief that it's going to be any stronger than a plain glue joint.

I agree with Gary Rogokwski.