Wedges are in constant use for lifting or separating heavy bodies, as doubtless you know, and the principle of the wedge comes in in using the axe, hatchet, chisel, knife, and the other edge-tools (see page 25). Besides this use of the wedge you will often find it valuable to tighten or clamp objects of various kinds, or to hold them firmly in place.

If you wish to split objects or tear them apart, use a single wedge, for the increasing thickness of the wedge applied at one point tears or splits the wood apart. But if you merely wish to squeeze, or press, or hold firmly, or move, without damaging the shape of the wood, use double wedges, - that is, two wedges having the same inclination or taper and pointing opposite ways. You will see that the sides of the double wedge (that is, the outsides of the wedges) will be parallel no matter how hard you drive the separate wedges, so that the pressure will be exerted without injuring or jamming the surfaces against which the wedge bears (see Fig. 333)- Short, flaring wedges do the work more quickly, but require harder blows to drive, and are more liable to slip. Long, tapering wedges work more slowly, more easily, and are not liable to slip. You will also use wedging to secure tenons and dowels (see Mortising, etc.).

Wedges 715

Fig. 687.