Originally Posted by
Steve Rozmiarek
Keegan and Mike, note this part of my post "An appropriate sized dowel that cannot be subjected to twisting force, is just as strong as any other loose tenon. In fact a dowel made of the same wood as a domino would actually have higher shear strength than a domino if you chose size based on equal surface areas."
That's the key, do the math. A dowel of equal surface area as a domino will have more cross sectional area then a somewhat oval shaped domino. A bigger cross sectional area equals more shear strength, as I said all other things being equal.
Mike, you're doing the math wrong. The correlation of the space it takes to fit the dowels and empty space between as to equaling a domino that occupies the same total area is the wrong way to look at it. Like I said, use surface area. To take your analysis to the absurd, why not use 1/8" dowels in the theoretical situation? It'd still justify using the same enormous domino following your logic.
Dominos are fine, so are dowels. Fixed tenons are better. That is the art of good craftsmanship, knowing when and how much.
The analysis that I did was for the real world situation. I didn't use 1/8" dowels because chair manufacturers use 3/8" dowels. And if you want to use dowels that would have approximately the same long-grain-to-long-grain surface area as the loose tenons that I calculated, you'd have to use about 9/16" dowels. The problem with using dowels that large is that it would take up a lot of a 3/4" thick piece of wood. 3/4 (or 12/16) minus 9/16 gives 3/16. That has to be divided in two because you're going to center the dowel. That leaves 3/32" on each side of the dowel which is pretty weak.
No, dowels are not as strong as a good mortise and tenon joint, even if it's a loose tenon. No matter how you look at it - from the mathematics or from experience with failed chair joints - it always comes out with the same answer.
Mike
[And, of course, my analysis is all about surface area.]
[One more thing: A dowel with the same surface area as a tenon does not have greater shear strength. It may have greater torsion (twisting) strength.]
Last edited by Mike Henderson; 07-27-2021 at 1:30 AM.
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