[Fig. 1765] represents the auger bit, which is provided with a conical screw s which pulls it forward into the wood. Its two wings w have cutting edges at d, d, which, being in advance of the cutting edges a, b, sever the fibre of the wood, which is afterwards cut off in layers whose thickness is equal to the pitch of the thread upon its cone s. The sides of the wings w obviously steady the auger in the hole, as do also the tops t of the twist. This tool is more suitable for boring across the grain than lengthways of it, because when boring lengthways the wings w obviously wedge themselves between the fibres of the wood.
Fig. 1766.
This is obviated in Cook’s auger bit, shown in [Fig. 1766], in which the cutting edge is curved, so that whether used either across or with the grain the cutting edge produces a dished seat and cuts the fibre endways while removing the material in a spiral layer. The curve of the cutting edge is such that near the corners it lies more nearly parallel to the stem of the auger than at any other part, which tends to smooth the walls of the hole. This tool while very serviceable for cross grain is especially advantageous for the end grain of the wood.
