Half-round Bit or Pod Auger.—For drilling or enlarging holes of great depth (in which case it is difficult to drill straight holes with ordinary drills), the half-round bit—[Figs. 1076] and [1077]—is an excellent tool. Its diameter d is made that of the required hole, the cutting being done at the end only from a to b, from b to c being ground at a slight angle to permit the edge from a to b to enter the cut. When a half-round bit is to be used on iron or steel, and not upon brass, it may be made to cut more freely by giving the front face rake as at e f, [Fig. 1078].
Fig. 1079.
To enable a bit of this kind to be adjusted to take up the wear, it may be formed as in [Fig. 1079], in which a quarter of the circumference is cut away at a, and a cutter c is bolted in position projecting into a recess at b to secure the cutter in addition to the bolts. Pieces of paper may be inserted at b to set out the cutter.
Fig. 1080.
