Q: I received the following information, but I have a feeling that it is not correct. Regarding a dowel pin joint, the glue bond at the bottom of the dowel provides 80 percent of the total holding strength, with a spiral dowel, the sides provide 15 percent of the total strength, and 5 percent comes from the joint between the two core materials. What do you think?

 

A: A dowel pin joint provides several advantages over a joint without the dowels. Let’s consider a few basics in order to understand the benefit better. When joining two pieces of wood, we are generally concerned about two different strength values: the tensile strength of the joint (that is, how hard is it to pull the joint apart) and the shear strength of the joint (that is, how hard would it be to get the two pieces to slide past each other). Let’s consider what a dowel pin does to the joint and its strength values.

 

First, if the core materials being joined are end grain or composite materials of low density or poor gluability, then the joint between these materials without a dowel is not very strong (tensile is very weak and also shear). With a dowel, both strength values are increased substantially. The increase will be related to the surface area of the dowel itself.

 

Second, a dowel pin allows the joint to be very accurately positioned; the dowels are the guides. Sometimes, the movement within a joint without dowels while the adhesive is curing can weaken the joint.

 

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Third, the dowel greatly enhances the shear strength of the joint. Essentially, the dowel pin itself must break or the wood around the pin must crush severely to get a shear failure. That is, shear strength of a doweled joint depends on the compression strength of the pieces holding the dowel, as it is the wood around the hole that fails (lets the dowel pin move). As a dowel pin gets shorter, then there is less wood that has to be crushed in order for the dowel to move. Unless the dowel pin is very weak itself, shear failure will not occur in the pin.

 

Fourth, when two pieces are joined using side grain (not end grain) and the joint is made properly, this joint without dowels will be stronger in tensile strength than the wood itself. In other words, the wood will fail first and not the glue joint. Adding a dowel to this strong joint provides only a little (1 or 5 percent) extra tensile strength to the joint. Shortening the dowel's length in half would reduce the tensile strength provided by the dowel by about one-half. As the tensile strength provided by the dowel when gluing side grain is just a few percent, the loss will be quite small.

 

Fifth, a dowel should not be used in lieu of forming a strong joint between the two mating surfaces. If a production joint is weak, find out why and fix the basic problem; don’t add a dowel and hope for the best.

 

Now to your specific question. I think that the end of a dowel provides essentially no strength to the dowel. Most commonly used wood adhesives do not have any strength when they bridge a gap over 0.006 inches; the end of a dowel would seldom be that close to the bottom of a hole. So, the 80 percent number you quote is zero in my way of thinking.

 

Second, considering the gluing of side grain, such as edge gluing of staves in a panel, the sides of the dowel provide all of the tensile strength for the dowel. However, the area of the dowel sides compared to the area of the joint between the two pieces is the important ratio. Seldom would dowel area be over 5 percent of the total joint area, so the dowel provides about 5 percent to the total joint tensile strength. I could envision that the dowel will proved 25 percent or more additional shear strength.

 

Therefore, the inherent strength of the joint between the two mating surfaces that are being doweled together is typically 75 percent to 95 percent of the total doweled joint strength.

 

When gluing two materials together that do not glue well (end grain, low density composites, etc.), then the strength from the dowel sides can contribute substantially to the overall joint strength. But if the two mating materials do not glue well to each other, what will make them glue well to a dowel? The dowel may actually contribute very little if there are gluing problems. In this case, it is time to look for a different fastening system.

 

Incidentally, the dowel hole should not be much deeper than the dowel pin’s length. That is, do not over-bore the hole. At most, the hole should be 1/16 inch deeper than the dowel’s length. Also, I believe that a fluted dowel is the best design. A spiral design is my second choice and a smooth dowel is unacceptable.

 

One special hint when using dowels is to keep them in a very dry location before use. (Some people use a small heater, which is set to 90 F will provide very dry air.) This will allow the dowels to shrink just a bit so they will fit into the hole easily and leave just a small bit of room for the glue to travel up the dowel. Then, as the dowel absorbs moisture from the glue, the dowel will swell and make a very tight fit, which also means a very good glue bond. It is not a good procedure to have dowels that are too large for the hole and have to be pounded into the hole. There is not enough glue on the sides to provide a strong joint.