In order to keep a tenon tightly fixed, wedges are driven in, as shown in Fig. 196, between the tenon and the sides of the mortise.
The mortise should be slightly dovetail-shaped in plan, being wider on the side from which the wedges are inserted, in order to allow room for them to be driven in alongside of the tenon.
Wedges are used in pairs for tightening up joints (see p. 61), being driven inwards so as to take up more room, and thus to force the parts of the joint together. When they are so used, great care must be taken not to drive them too hard, so as to leave the joint with a violent strain upon it.
When a tenon is to be fastened into a mortise in a rail already fixed against a wall, or in any such position that the end of the tenon cannot be seen, it is secured by "fox wedges," thus -
A wedge is inserted in a saw-cut in the end of the tenon, as shown in Fig. 197. The mortise is made slightly wider at the back, and when the tenon is driven home, the wedge entering it splits and spreads out the wood, and makes it fill up the mortise. With a single wedge there is some chance of splitting the tenon beyond the shoulder. This is thus avoided: - Four or more very thin wedges are inserted, as shown in Fig. 198, the two outer ones being longer than the inner ones. As the tenon is driven home, these in succession split off thin pieces, which easily bend, and therefore the splits do not extend too far. The wedges in the figure are rather short, but they should not be very long, as they would then be apt to be broken off in driving.
Fig. 190. Wedged Tenon Joints.
Fig. 197. Single Fox Wedge.
Fig. 198. Fox Wedging.
The enlargement of the back of the mortise should be a little less than the total thickness of the wedges.
Keys (Figs. 151, 157) are wedges of hard wood and curled grain inserted in a joint, and driven gently home, so as to force the parts into the position they will eventually occupy, before inserting bolts, etc. Without this precaution, there would often be a permanent and injurious strain on the latter.
In some cases keys also assist the joint in its resistance to the strain brought upon it (see Fig. 157).
They should be slightly dovetail-shaped in plan, and carefully driven, so as not to injure the fibres of the beam in which they are inserted.
The keys in scarfs are usually made 1/3 the depth of the timber.