Gluing dovetails and finger joints

      Which adhesive is best for these joints? May 15, 2002

What type of glue is best for dovetails and finger joints? I have been using Titebond and Titebond Extended but am having difficulty getting the joints to draw completely together after the glue has been applied. I think the water base glue is swelling the wood, making it almost impossible to get a tight joint even with a lot of clamping pressure.

Forum Responses
Most people who use these kind of joints use PVAs (similar to those you mentioned). Assuming your joints are cut correctly, perhaps it's a result of the species you are gluing or the moisture content of the wood. If the moisture content was too low (say 5-6%), it could absorb abnormal amounts of moisture. You could try using a reactive polyurethane such as Gorilla, Excel, RPA or Franklin. It might solve your problem but it can be difficult to work with and give you other problems you didn't have to begin with.

PVAs tack up very quickly, especially on warm summer days. This can be an advantage in many cases but on complex assemblies it can be a disaster, as the glue will grab before the joints can be properly pulled up. Try plastic resin glue. It will give you more assembly time. Itís cheap, strong and almost waterproof (type 2) and it wonít clog sandpaper.

Gluing dovetails and finger joints are really two different things based on the geometry of the joints. Both are mechanical joints involving the end grain of wood, but that is as far as it goes.

Finger joints are an end to end press fit that creates a continuous relationship of the grain between two pieces. A dovetail is an interlocking fit with a perpendicular (cross grain) relationship.

In years of both hand cutting and machine cutting dovetails, I have never had a problem with PVA glue unless I was using an odd species of oily wood. Early on when I cut dovetails too tightly, most of the glue would be squeezed out when assembling the joint. This is why most people tell you to have a paper's thickness of space in the joint.

Finger joints are another matter. There are many adhesives that work well in finger joints. The main issues are the viscosity of the adhesive, the fit of the joint, the way the material is pressed together and the species of the wood.

Unfortunately, unless you are in production and can spend a lot on finger joint tooling and/or a finger jointer, it is nearly impossible to get tooling that allows you to easily adjust the fit. Because of this we need to look at the other issues.

In this discussion, viscosity may be the main culprit. An adhesive that is too viscous (thick) will create a hydraulic action when the wood is pressed. This will force the joint back open. Most manufacturers can provide you the right adhesive if you tell your rep what you are doing. We have many customers using Franklin's Multibond 2000. It has the appropriate viscosity for many finger joint applications. There are of course equivalents produced by other manufacturers, but Franklin tends to be a very accessible company for support.

Species issues make this even more interesting. Softwood tends to be more forgiving because the wood will crush slightly in the pressing application. Thus, less chance to hydraulic.

A finger joint is essentially a mechanical joint. If cut with no tolerance for adhesive and pressed, it is actually very strong if used within certain orientations. However, this is incredibly impractical. Pressing gets the joint as tight as it can be. It creates the frictional heat that starts the adhesive curing. Without a good press it is hard to get a successful finger joint.

Another note about adhesive. Be careful. It is important to know what the use of the material will be in the final application. Water-resistant does not mean waterproof and different adhesives are needed for different uses.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contribtor A:
Being a mechanical joint, a dovetail does not rely on glue for strength in drawer box construction. The glue is only needed to keep the joint from sliding apart sideways. To avoid hydraulic problems, I just apply a thin coat of glue to the diagonals of the joint and leave the rest of the joint dry.

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