Thread: Show me a better Joint

Yes but we also all know that a dowel is very much weaker than a M/T. Tests have been done to prove that. Dowel - biscuit - M/T in that order. This is, and I may be labouring the point, largely because of the long-grain to long-grain glue surface.

Why is a double tenon stronger than a single tenon? There are two types of double tenon: one where the tenons are side by side and another end to end. The latter is often used on wide tenons to counter problems with wood movement (also known as twin tenon). The former doubles the long-grain contact, so could that be the reason?

So are 4 dowels, with much less long grain contact, stronger than a single M/T with lots of it?

Well I guess Silent that Im largly thinking without glue,and just about leveridge,and it would be harder to move 4 small joints than 1 big joint because of the leveridge.


roddy

Originally Posted by roddy97

Well I guess Silent that Im largly thinking without glue,and just about leveridge,and it would be harder to move 4 small joints than 1 big joint because of the leveridge.


roddy

I don't have any trouble moving small joints, they're cheaper than the big ones.

OK, you're probably talking about friction. The amount of friction between the tenons and the mortices should be a factor of the surface area of the tenons.

Assuming that you have 4 dowels with a diameter of 10mm and spaced by 5mm, or a single 10mm thick tenon taking up the same amount of space on the end of a rail (10 + 5 + 10 + 5 + 10 + 5 + 10) and both are 20mm long:

One round tenon: Pi * 10 = 31mm * 20mm = 620mm2
Total surface area = 4 * 620 = 2480mm2

One rectangular tenon: 10 * 20 * 2 + 55 * 20 * 2 = 2600mm2

(Hope I got that right, someone check for me )
Pretty much the same, favouring the single tenon slightly.

Perhaps the friction generated perpendicular to the direction of movement counts for more, so if you forced the joint in a direction that was inline with the four dowels, there might be a slight advantage.

The tests done by FWW a few years ago compared twin biscuits(which are not usual) to a pair of dowels (which is normal) and not to twin paired dowels so the tests were not on a level playing field. Nor were tests done on twin M&T's

The test was done only on glued joints which is ok to compare new joints.

But it ought have also been done without glue to compare joints in an aged situation after the glue has disintegrated. In such case the biscuits would be totally useless but the dowels would still have a significant strength.

I've seen results of a different set of tests done specifically on doors. Wish I could find them. They tested M/T, dowel and biscuit joints. M/T had a slight edge over biscuits but both were streets ahead of dowels.

The joints were tested until failure. The M/T joint lasted the longest, followed by the biscuit, then the dowel. They made the point that the biscuit joint completely fell apart a soon as the glue failed, whereas the other two held together a bit longer.

This seems logical. There's plenty of dowel jointed furniture around that creaks and groans but holds together. Can we say the same for M/T? Maybe not. I still think it will last longer before glue failure though.

Well arnt you guys glad I brought this up?its realy got your brains awake,

I have enjoyed this debate,and I have also learned alot,I also believe that there is less difference between the two as people make out.

Roddy

OK, well all I can do is to keep reiterating what I think is the difference between them:

It's an established fact that long-grain to long-grain gluing is strong and long-grain to end-grain gluing is weak. Do we all agree?

If you drill a hole in the face of a board, then cut the board in half so that you can see the hole in section, the only long grain you can see is at the extremities of the circle parallel to the grain. The rest is end grain.

When you coat a dowel with glue and insert it in such a hole, the overwhelming majority of glue surface is long-grain to end-grain, which is, as we decided above, weak. You might have a millimetre or so that is actual long-grain to long-grain contact.

With a M/T on the other hand, the majority of the glue surface is long-grain to long-grain.

On that basis, I believe that a M/T joint will be stronger, all other things being equal.

Originally Posted by roddy97

Someone was asking which was the strongest joint.
Let me explain which I believe to be a very strong joint,EG:.


Timber 100mmx20mm ,connecting to timber 70mmx50mm.

Drill with a hole saw,4@x15mm dowels evenly spaced,into the end of the piece,100x20,cut the shoulders on the panel saw,marry the dowels to the stile 70x50,mark and drill your 15mm holes,glue,best joint in the world.

Roddy

Roddy,

I acknowledge that I misunderstood your original post. But this was because you misleadingly referred to round tenons as dowels, and you spoke of drilling into the end of the rail, when you meant forming round tenons on its end. I still do not understand how you would remove the waste from between and around the tenons without a lot of fiddly messing about. It is far easier to cut the shoulders of regular tenons on the table saw, then cut the tenon cheeks with a tenoning jig and rout the mortice with a morticing jig. If you have suitable micro-adjustable jigs it is pretty easy to make an accurately fitting M&T joint in a few minutes.

With your joint, I am still at a loss to understand how you ensure that the round tenons are correctly aligned, if you are forming them with a hole-saw mounted in a hand-held drill. And do the tenons end up with a hole in their centre, further weakening them? And how do you ensure that you drill corresponding holes in the stile in the right place? I suppose you could drill a 6 mm hole in the centre of the tenon by using a 6 mm bit in the centre of your hole saw, and then use dowel centres, but a hole that size in a 15 mm round tenon would seriously weaken it.

Rocker

You cut the dowels on the drill press,cut the shoulders on the table saw,just like conventional tenons,and the bits between the dowel/tenon just fall out"Try it".



Roddy

This is because you overlap the holes cut by the hole saw so that there is no material between the tenons, right?

Yes thats right Silent.

Roddy

I'm lost. I need a drawing or a picture. :confused:

Originally Posted by roddy97

Yes thats right Silent.

Roddy

Wow, it took all these posts to get to this point. Y'know Roddy, you may have been a joiner for 45 years, but I am willing to bet my lefty you've never had to teach the subject!

The concept is interesting though, I may play with it next chance I get in the shop.