Keegan is correct about loose tenons being stronger than dowels. Like him, I've repaired a lot of chairs and almost every one that had a failed joint (and it's almost always the joint where the seat attaches to the back) it was a joint of two dowels.
Let's look at the mathematics. The numbers used are as follows:
Length of the dowels and the loose tenon = 3"
Diameter of the dowels = 3/8"
Number of dowels = 2
Space between dowels = 1/8"
The two dowels and the space between them will occupy a space 7/8" wide. We'll make the loose tenon fit into that same space. So the loose tenon will be 7/8" wide by 3 inches long.
I'm going to compute the long-grain-to-long grain glue surface for both the dowels and the loose tenon.
The area of a dowel is 2*pi*r*h, where r is the radius and h is the length of the dowel. I'm going to compute the long grain surface area in each side of the joint so h will be 1.5 inches.
The area of the two dowels is 3.534292 square inches in each side of the joint. However, only half of the surface area is long-grain-to-long-grain surface area. The other half is long-grain-to-end-grain. You can visualize this by considering the dowels to be square with the same surface area. Only half will be long-grain-to-long-grain.
The resulting long-grain-to-long-grain surface area is 1.767146 square inches. This is for one side of the joint.
The loose tenon long-grain-to-long-grain surface area can be computed as follows. The loose tenon will be 1.5 inches into the wood and 7/8" wide, giving 1.3125 square inches. Since there are two sides to the loose tenon, the total area is 2.625 square inches. This is for one side of the joint.
The loose tenon has 48.5% more long-grain-to-long-grain surface area than the two dowels.
Additional comments: Most of the wood we work with and most chair parts are about 3/4" thick. It's difficult to put bigger dowels into 3/4" material without compromising the strength. And in the world of commercial chairs, they use 3/8" dowels.
The space between the dowels has been reduced to the minimum in this example. In real chairs, the space between the dowels is much larger. Increasing the space between the dowels allows for a wider loose tenon which provides more surface area to the tenon. The example given here is about as good as it gets for the dowels.
I have a spreadsheet that computes this stuff so if you want me to run other numbers, let me know.
Mike
Last edited by Mike Henderson; 07-26-2021 at 4:31 PM.
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