Single dominoes are as strong as double dominoes???

[ChuckM]

According to the Fine Woodworking joinery test, the domino joint (1 domino) broke at 597 pounds; dowel joint (3 dowels) at 759 pounds.

But this video at 5:37 shows something different: The domino joint breaking at 600 pounds, even though 2 dominoes were used:
=337

(At 7:20, the video shows the dowel joint (5 dowels) failing at 900 pounds, which seemed to be in line with the FW's test on 3 dowels.)

Comments welcome.

Unfortunately, the video doesn't have a close-up of the joints after the test.

 

[woodferret]

The test setup tries to get as pure of a lateral shear stress as possible.  This puts both dominos in parallel and the lowest breaking domino joint will dictate it.

Another test would have been a bending test with the load at the tip of a long arm.  In that case, the distance of the furthest domino to the pivot point determines the ultimate load with further dominos/dowels experiencing less 'pull' shearing.

 

[Michael Kellough]

Have not looked at the "test" but the illustration posted above shows that the wood failed (not the joint) at the point where the tenons (both dowels and Dominos) stop reinforcing the wood.

Use longer tenons.

 

[Birdhunter]

I don’t think I’ve ever built anything where I had a cantilevered joint as in the test.

 

[ChuckM]

Have not looked at the "test" but the illustration posted above shows that the wood failed (not the joint) at the point where the tenons (both dowels and Dominos) stop reinforcing the wood.

Use longer tenons.

I noticed that too, but the video doesn't cover the breakage in depth with any close-up.

 

[ChuckM]

I don’t think I’ve ever built anything where I had a cantilevered joint as in the test.

I did, using the joint to support a table top -- now 5 years old, used almost daily. I employed double, twin tenons (i.e. 4 dominoes per joint), milled with my DF500.

 

[Birdhunter]

Neat. How did you cut the angles?

 

[ChuckM]

I figured out the tablesaw fence setting, using the kerf thickness of the saw blade (k), the desired width of the arris of the hexagonal post, and a formula (2 * k * 1/Tan 30). Alternatively, lay out a hexagon of the desired size on the end of a square blank and use it as a guide to set the saw fence for the 60* angled cuts.

 

[Crazyraceguy]

From the looks of the samples they have shown, it is 3/4 material or very close to it. If that is indeed the case, they have compromised the joints from the beginning. The caption clearly shows 10mm x 50mm Dominos were used. That is way to big for this situation. The tennon itself is stronger, but the sides of the styles are way too thin. The principle of 3rds would suggest 6mm Dominos in this case. It would probably break in exactly the same place/way, but a different load rating.

 

[Cheese]

From the looks of the samples they have shown, it is 3/4 material or very close to it. If that is indeed the case, they have compromised the joints from the beginning. The caption clearly shows 10mm x 50mm Dominos were used. That is way to big for this situation. The tennon itself is stronger, but the sides of the styles are way too thin. The principle of 3rds would suggest 6mm Dominos in this case. It would probably break in exactly the same place/way, but a different load rating.

Good observation... [smile]...I wonder if the 1/3rd rule also applies when using dowels?

Some more confusion, in the May/June 2001 issue of FW, here's one of the articles, "Double Mortise and Tenon Improves Joint Strength".  The article continues with "One of my favorite versions is the double mortise and tenon. I use it to increase the strength of a joint on relatively small furniture parts."

 

[RobS888]

He makes the Dowelmax tool and has skin in the game, but I assume what he is showing is correct.

Could anyone explain how you could have 600 lbs on one joint like that in real life? Assuming it is furniture how could you get a 600lb load that close to a joint in that direction? If you put 700lbs on a table the top would help distribute the wight to all the supports, a chair tilted back would be a different type of force.

So while it may be accurate, does it matter?

 

[ChuckM]

From the looks of the samples they have shown, it is 3/4 material or very close to it. If that is indeed the case, they have compromised the joints from the beginning. The caption clearly shows 10mm x 50mm Dominos were used. That is way to big for this situation. The tennon itself is stronger, but the sides of the styles are way too thin. The principle of 3rds would suggest 6mm Dominos in this case. It would probably break in exactly the same place/way, but a different load rating.

If both tests used the same 10mm dominoes, they were comparing apples to apples.

It'd indeed be instructive if someone would repeat the tests (single domino vs double dominoes) with 6mm tenons.

 

[Birdhunter]

ChuckS, thanks for the response. Neat.

Also, I frequent a BBQ place in Canton, GA and see people who might put that much strain on a chair. One of the Atlanta hospitals had to go to double wide wheel chairs and reinforced toilets to the e tremendous size of the patients.

 

[ChuckM]

Snip.
Some more confusion, in the May/June 2001 issue of FW, here's one of the articles, "Double Mortise and Tenon Improves Joint Strength".  The article continues with "One of my favorite versions is the double mortise and tenon. I use it to increase the strength of a joint on relatively small furniture parts."

Twin tenons provide more glue surface and are considered stronger than a large one (Woodwork Joints, Charles Haywood). The illustrated twin-tenon joint in that article seems to more or less follow the "one-third" rule.

 

[ChuckM]

Snip.

... see people who might put that much strain on a chair. One of the Atlanta hospitals had to go to double wide wheel chairs and reinforced toilets to the e tremendous size of the patients.

One can't but admire Tage Frid's design brilliance that prevents stresses on the backrest (about 10") of his renowned three-legged stool.

 

[Michael Kellough]

Snip.
Some more confusion, in the May/June 2001 issue of FW, here's one of the articles, "Double Mortise and Tenon Improves Joint Strength".  The article continues with "One of my favorite versions is the double mortise and tenon. I use it to increase the strength of a joint on relatively small furniture parts."

Twin tenons provide more glue surface and are considered stronger than a large one (Woodwork Joints, Charles Haywood). The illustrated twin-tenon joint in that article seems to more or less follow the "one-third" rule.

“Cut the mortises as deep as you can without going all the way through.”

And make the tenons almost as long as the depth of the mortise.

 

[ChuckM]

An educated guess: The Domino Joiner probably takes just 1/5 of the time or even less that's needed to do the twin M&T joint with a mortiser and router/jig.

 

[Birdhunter]

I used to be an engineer eons ago so I tend to think in terms of how things should work.

In the illustrated joint, I see the function of the tenon or the dowels is to keep the two surfaces tightly against each other.

I see the integrity of the joint being in that tightness. I seriously doubt that either the tenon or the dowels could be extracted through cantilevered forces. The wood will fracture first.

 

[Crazyraceguy]

He makes the Dowelmax tool and has skin in the game, but I assume what he is showing is correct.

Could anyone explain how you could have 600 lbs on one joint like that in real life? Assuming it is furniture how could you get a 600lb load that close to a joint in that direction? If you put 700lbs on a table the top would help distribute the wight to all the supports, a chair tilted back would be a different type of force.

So while it may be accurate, does it matter?

Most likely not, at least when used properly. With tables and things of the nature, a little common sense about what you put on it goes a long way. Chairs are a different matter, but even if someone did lean back in one, the load is on two of the legs. There are usually some kind of lower stretchers involved too, this changes the dynamic a lot.
I would hope that a person who is big enough to crush a wooden chair would at bare minimum check it out for other stabilizing factors like that?