A popular misconception is that finger joints are the only means to join solid wood parts, end-grain-to-end-gain, regardless of what the product will be used for. Too often there is little consideration given as to whether the joined-together parts will be used as trim-stock moulding, where strength properties are not a needed element, or as a structural member part in a larger assembly, as in a solid wood table, chair, construction beam or similar item.
In the simplest terms, a glue joint can be nearly any shape of mated surfaces where the primary intent is to enlarge the area of the surfaces to be joined together with the use of an adhesive.
There are as many glue joint profiles as one might devise to bring two mated surfaces together. We join solid wood members together, end-to-end, as a simple butt joint, we can assemble longer individual parts edge-to-edge, such as those to be used as flat panels in cabinet or tabletop construction.
There is an almost endless variety of shaped profiles - grooves, mirror-imaged rounded shapes - all having the purpose of simply making the contact surfaces larger than they would be if otherwise butted together.
Tongue-and-groove profiles, mortise-and-tenon, lock-miters, dowels and other similar shapes are also essentially glue joints, but add the strength of their mechanical shape. In the case of dowels, another part is introduced into the joint. The mechanical advantage of these joints is their ability to transmit any stress placed on them across the joint, in two or all three planes of involvement.
If a moulding used to trim a cover, a window, door opening or similar corner is made up of several short pieces to form the body of the part itself, there is no need for great structural strength. In the building trades, such commercially produced trim stock made up of several short pieces is called paint-grade moulding, meaning that the material is intended to be finished as a painted surface. This covers the end-to-end grain arrangement that would be unsightly if finished only with a clear or transparent stain.
Glue joints generally fall into four broad categories:
1. Those that provide a simple, clean, open cell surface that rely on the strength of the glue film alone. This includes simple end-to-end butt joints and edge-glued joints.
2. Joints that make the effort to greatly expand the contact surface area by shaping the mated surfaces. These include several varieties of lap joints and the angular or rounded glue-joint cutting profiles. These differ from the following group by not adding mechanical strength to the joint assembly.
3. The mechanical glue joints are shaped in such a manner that the wood material itself adds strength to the joint well beyond its glue-film strength. Mortise-and-tenon, tongue-and-groove, lock miters and dowel joints are all mechanical glue joints.
4. Fingerjointing, even though it is a mechanical joint, is in a class of its own due to the precision and attention to detail needed to be completely successful. Some of the unusual characteristics of cutting and assembling finger joints are:
- The finger-profiles are precise, requiring tolerances more stringent than nearly any other wood machining processes.
- The condition of the wood materials involved also demand close monitoring throughout the entire operation. Oxidation and dimensional changes begin to take place within 90 seconds after being machined and depending on the condition of the wood material prior to being cut. There may only be several minutes available to successfully complete the joint.
- Finger joints commonly require from 300 to 1,200 psi clamping force to be completely closed. The density and strength of the involved wood species causes the wide range in required pressure.
For all of our glue-joint assembly processes the critical factors are:
- The cutting tools must be kept in the best possible sharpened condition.
- The wood that we are joining must be clean, fault-free and dimensionally stable.
- The machined surfaces must be smooth and free of any imbedded debris or dust.
- The clamping methods and pressures must be appropriate for the type of joint involved.
- The glue-jointed part must be appropriate for the intended use.
The most common and repeated failures of glue joint construction have only three common denominators:
1. The mated surfaces are not completely wetted.
2. The adhesive was allowed to deterioriate, either dry out or lose viscosity.
3. The fit of the joint was incorrect.
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