CHAPTER 3, LESSON 2 of 3
GOAL: To understand the purpose, procedure and possible limitations of adding splines or biscuits to your butt joints.
An edge to edge joint, bonded only with glue, may not seem like a suitably strong connection between two workpieces. So you might consider adding splines or biscuits in the hopes of reinforcing your butt joints. Do these options really help? In this lesson, you'll learn the answer as well as the proper technique for bridging two edges with a spline or biscuits.
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| A continuous wood spline, used here to attach a breadboard end to its base board.
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| Procedure for Installing Splines or Biscuits |
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| Deposit glue in the biscuit or spline slots, wet the slot sides with a paddle and remove excess glue (left). Then roll glue on the two edges to coat the mating surfaces (right).
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| Assemble the two boards, aligning them with the chevron mark that records their composition (left). Repeat the procedure to include the third board, if applicable. Position the clamps alternately above and below the assembly (right). This will counteract the tendency for the boards to buckle as you apply pressure.
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You may be familiar with two methods that are believed to make a butt joint stronger, the most common being a spline. Splines are strips of solid wood, preferably the same material as the board, inserted into a groove cut along each joined edge. Now come the questions: How wide and deep should the groove be? What material should the spline be? How tightly should it fit the groove? The answers are overshadowed by two other considerations. First, cutting the spline grooves will remove a significant percentage of the area where the glue’s specific adhesion can take place, thereby weakening the joint. Second, even if the interfaces of the joint are correctly made and it is tested to destruction, it will fail at the bottom of the groove where you have reduced the board’s thickness.
In light of these two points, if you still believe that a spline truly helps, keep it thin: a 1/8" saw kerf is enough. Make the spline a snug push fit and its width about three times its thickness in each board.
Another option is to use biscuits, which improve upon the spline method in several ways. For one, the groove is intermittent rather than continuous, which preserves more area for the glue’s specific adhesion to occur. Second, because the grain of the biscuit runs on the bias, it is angled across the joint line, resulting in greater strength. Third, the biscuit is compressed at the same time it is stamped out: a biscuit snugly fits in its 4mm kerf when dry, but when wetted by the glue, it swells and creates strong resistance to being pulled apart.
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