Mark Stavig, David Zamora, Tommy Guess, and John Emerson
Sandia National Laboratories, Albuquerque, NM 87185-0958
In the architectural market, wood structure joints have traditionally been constructed using mortise and tenon joinery methods. Within the last ten years cabinetmakers have adopted plate or 'biscuit"reinforcement for joinery,resulting in large savings on working space, tooling, materials, waste disposal, and time. Is this new technology strong enough for building heavier architectural woodworking, e.g. residential doors?
The authors built and tested a number of samples in two different configurations to compare the strength and stiffness of joints constructed with large Lamello S6) plates (biscuits) to the properties of similar joints utilizing traditional mortise and tenon joinery methods. Authors compare the two kinds of joints in force-versus-deformation response, tested to failure. Data from a limited number of experiments indicates that biscuit reinforced joints nearly equal the strength of the mortise and tenon.Results suggest methods of increasing strength of both types of wood joinery.
Key Words: biscuit joinery, plate Joinery, urea adhesives
Until the second half of this century craftsmen lacked adhesives which could make wood-to-wood joints as strong as the wood itself. They relied on variations of interlocking wood joinery especially the mortise and tenon.(Figure 1) Making this joint is very labor intensive for small shop builders of custom wood products. In both small-shop and industrial settings traditional joinery consumes more materials and makes more waste than modern alternatives.
Modern adhesives can glue long"or 'Side"grain areas of wood together making a joint stronger than the wood itself. Gluing the endgrain of wood to another section of endgrain or to sidegrain of another board results in a weak joint. The difference occurs because of the disproportionate expansion and contraction across the grain of the wood as well as the nature of wood itself. This pre-hominid composite relies on overlapping strands of cellulose bound by lignin. Without overlapping fibers, un-reinforced endgain makes weak glue joints.
To avoid the expense and waste of traditional joinery, European cabinetmakers of post WW II invented a method of substituting an inserted wafer in place of a tenon. The football- shaped wafers fit into two slots cut into the edges of the material to be joined. A woodworker cuts the horizontal slots in each of the mating surfaces with a motorized tool popularly called a 'biscuit joiner. The biscuits become the reinforcing overlapping fibers. (Figure 2) Within the last 10 years North American woodworkers have widely adapted plate (or biscuit) joinery for much of interior and light-duty architectural woodworking.
The slots measure 4 mm in height (the width of the blade) and have a 50 mm concave radius. Preset stops determined depth and width of the cuts, chosen to match the selected size of commercially available "biscuits" (figure 2), 4 mm thick, football-shaped beech wafers colloquially named for the quilted pattern compressed into the flat surfaces. In the presence of water, the compressed wood fibers expand to firmly contact the sides of the slots.When used with non-water-based adhesives, the quilt-like pattern provides additional surface area and improved mechanical bonding.
Despite the great efficiencies of plate joinery, fabricators have not widely adapted the technique for exterior and structural uses. Test data regarding the strength of joinery comparing different joints and adhesives could inform potential users of the relative suitability of plate joinery for exterior architectural woodwork. In this paper we focus on its initial strength in several configurations approximating the joints of a residential entry door.