The techniques of jointing sections of wood together to form rigid self-supporting and permanent structures have been evolved through many centuries to accommodate the natural movement of wood, and can rarely be improved upon. Only in the case of prefabricated boards (plywoods, particle boards, etc.). where the total movement is relatively small. have any worthwhile developments taken place (see Chapter 31 Knock-down fittings), but in the main the machine has merely adapted pure hand-techniques, simplifying them wherever possible and making greater use of the immensely strong adhesives now available, but still relying on the same fundamental principles. The best illustration of this is the dovetail joint, one of the most intricate to cut but still the best method of jointing two pieces of wood together at right angles to each other, where the machine, instead of evolving a simple worthwhile substitute—if that were possible—has accepted the joint as such, and only altered it sufficiently to allow for the rotary movements of the cutting-tool.
There are very many different varieties of joints to cover every possible contingency, but before describing them in detail some reference should be made to the function of the joints, what they have to do and how they have to do it. Glue is in itself a wonderful jointing medium, but wood imposes its own conditions. Thus two pieces of wood can be glued side by side with their grain direction parallel and will then, if properly jointed, be as strong as a single piece (131A) , but the same two pieces cannot be glued end to end satisfactorily (A-B), for the cut pores of the end grain will merely suck up the glue like a sponge, preventing any permanent bond. It is true that end grain can be repeatedly sized with weak glue until the open pores are choked, and this will undoubtedly strengthen the bond, but the joint itself is still not good enough, and will always fail under any sudden impact because the natural elasticity of the long fibres lying side by side (A-A) is not there to cushion the shock; moreover, the interaction of a good glue with a wood surface is more subtle than a mere bonding skin. In the same way two pieces of wood with the grain or fibres opposed to each other (B-C) will shrink across their width in opposite directions, and this natural shrinkage is so powerful that nothing can permanently oppose it. Even if the junction is reinforced then the shrinkage will travel until it finds the weakest link, which will ultimately fail (131D).
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