Fig. 1771.
Fig. 1772.
Furthermore, after the thread has once stripped it is quite difficult to force the auger to start its cut again. To obviate these difficulties, the screw is fluted as shown. It is obvious also that this flute by imparting a certain amount of cutting action, and thereby lessening the wedging action of the screw, enables it to bore, without splitting it, thinner work than the ordinary auger. But it will split very thin work nevertheless; hence for such work as well as for holes in any kind of wood, when the hole does not require to be more than about twice as deep as that diameter, the centre bit shown in [Figs. 1771] and [1772] is employed, being an excellent tool either for boring with the grain or across it. The centre b is triangular and therefore cuts its way into the work, and the spur or wing a extends lower than the cutting edge c, which on account of its angle cuts very keenly.
Fig. 1773.
[Fig. 1773] represents the twist drill which is used by the wood-worker for drilling iron, its end being squared to fit the carpenter’s brace.