The next joint is not frequently used. It is a further variation of the shoulder butt, and could be defined as a "double-shouldered butt." (Fig. 611). The ends of both pieces to be joined are notched and are butted together with the notches interlocked. This joint has more strength than a normal shoulder butt since the pieces being joined bear against each other on three surfaces. However, it is not as popular as it might be, possibly because the notches in both pieces must be very accurate to form a good tight joint. Most constructors with skill and equipment adequate to handle
this joint can also handle mitres, and mitres do make a cleaner-looking job.
The mitre joint involves cutting the ends of both pieces to be joined at a 45° angle and then fitting the angled surfaces to form a corner. In good-quality furniture, variations of this joint are standard procedure (Figs. 612-a,-b,-c). The mitre joint can be strengthened or reinforced in several ways. The least effective is to add only glue blocks on the inside (Fig. 612-b).
Far superior to this is the "mitre and spline" variation. A spline of wood or metal is inserted to run down the length of the joint and at right angles to it. The spline is fitted in grooves cut in each piece before joining (Fig. 612-c).
A common variation of the mitre and spline is the use of clamp nails. These actually constitute partial splines, being inserted at each end of the joint and at right angles to it like a spline, the difference being that they run in only about 3 inches from each end.
All of the joints we have seen thus far can be made on either a table or a swing saw. However, for mitres a shaper (Fig. 613)
is preferred. A shaper consists of a steel table with a spindle sticking up in the middle of it. Various types of cutting knives can be attached to this spindle, and guides are provided for passing the work by the knives. The shaper leaves a much smoother surface where it has cut than does a saw; therefore, the mitres will be cleaner and tighter.
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