Other joint types

Among other commonly encountered joint types the mitre and the dowel joint will be briefly mentioned here. Parallel side-grain to side-grain mitres make a very efficient corner joint. Mitre joints where the grain meets at right-angles, for example in picture frames, are attractive but present serious technical problems because of the difference in dimensional change along and across the grain. They are apt to open up if the moisture content change is large or the joint is wide (Figure 2.26a,b). Gluing is relatively ineffective because of the large end-grain component in the joint. Joint strength can be improved by introducing dowels or splines that increase side-grain to side-grain gluing surfaces (Figure 2.26c,d).

Dowels are widely used in joints for a variety of different purposes, as tenons, as pins and as gluing accessories to hold parts in alignment. Where dowels are used to modify end-grain to side-grain joints, a mortise and tenon joint is effectively created (Figure 2.27). In a joint with two dowels that is subjected to racking, one of the dowels carries a critical share of the load in tension and the remaining load is transferred as

surface compression. Increasing the height of the end-grain member (e.g. a seat rail) allows the dowels to be spaced more widely, reducing the stress on the joint for a given load (Hoadley, 1980). Dowels need to be large enough to carry the tensile stress and deep enough to resist pulling out. They should also be able to carry and transfer the shear load parallel to the side-grain member of the joint.

Great care is needed when using dowels or splines as gluing accessories to help locate the components of a joint to avoid both loss of gluing area and restraint of normal movement. Dowels can be used to help align boards that are to be edged-joined (Figure 2.23b) provided that the dowels themselves are kept reasonably short and are not glued into their sockets. If they are glued in then the restraint to normal movement of the boards may cause the wood to fail at or near the glue joint. Loss of surface gluing area when using dowels for alignment is likely to be small. In contrast, when using traditional short crossgrained tongues (Figure 2.24d) the loss of gluing area may be substantial if the splines are used dry, whilst if they are glued in position and restrain movement of the timber, splits may result.

Figure 2.27 Dowel joint. Although dowel joints date from the nineteenth century, pegs may be found in earlier furniture, for example in the eighteenth century spiral fluted urn illustrated in Figure 10.6a

Figure 2.26 Corner framing joints: (a) picture frame mitre; (b) picture frame mitre after shrinkage; (c) mitre reinforced with dowels; (d) mitre reinforced with a loose tongue or spline

Figure 2.27 Dowel joint. Although dowel joints date from the nineteenth century, pegs may be found in earlier furniture, for example in the eighteenth century spiral fluted urn illustrated in Figure 10.6a

Figure 2.28 Examples of framed construction: (a) gateleg table underframe: (i) mortise and tenon; (ii) bridle joint; (iii) cross-halved joint; (b) Sheraton style armchair

Figure 2.28 Examples of framed construction: (a) gateleg table underframe: (i) mortise and tenon; (ii) bridle joint; (iii) cross-halved joint; (b) Sheraton style armchair

Figure 2.29 Frame and panel construction: (a) panel sits in groove; (b) panel sits in rebate; (c) frame and panel flush and veneered; (d) mason's mitre (mitre carved onto mortised rail whilst the shoulder of the tenoned stile is square), pegged; (e) true mitre (both stile and rail are mitred), pegged; (f) framing and panelling used to construct a mid-sixteenth century English box chair

Figure 2.29 Frame and panel construction: (a) panel sits in groove; (b) panel sits in rebate; (c) frame and panel flush and veneered; (d) mason's mitre (mitre carved onto mortised rail whilst the shoulder of the tenoned stile is square), pegged; (e) true mitre (both stile and rail are mitred), pegged; (f) framing and panelling used to construct a mid-sixteenth century English box chair

Figure 2.30 Dovetails and associated joints used in an early nineteenth century mahogany bureau

The topic of joining wood is more exhaustively discussed by Hoadley (1980). Use of the joints described above in the construction of furniture is illustrated in Figures 2.28-2.30.

Wood Working for Amateur Craftsman

Wood Working for Amateur Craftsman

THIS book is one of the series of Handbooks on industrial subjects being published by the Popular Mechanics Company. Like Popular Mechanics Magazine, and like the other books in this series, it is written so you can understand it. The purpose of Popular Mechanics Handbooks is to supply a growing demand for high-class, up-to-date and accurate text-books, suitable for home study as well as for class use, on all mechanical subjects. The textand illustrations, in each instance, have been prepared expressly for this series by well known experts, and revised by the editor of Popular Mechanics.

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