Figure 5-14.--Parts of an auger bit.
Figure 5-15.--Types of screw points.
Figure 5-13.--Drill bits.
The spurs function to score the outer edge of the
chip as the cutting lips chisel or cut the waste material
loose. The spurs and cutting lips must be sharp to
WOOD- AND METAL-BORING BITS
produce a smooth hole.
The twist of the auger bit is responsible for remov-
The wood-boring bits and drills usually found in the
ing waste material after being cut by the spurs and
HT shop are shown in figure 5-13--included are the
cutting lips. It is slightly smaller in diameter than the
auger bit, expansion bit, twist bit, machine spur bit, and
head. The twist comes in three styles--single twist with
solid center, single twist, and double twist.
The single twist with solid center clears chips
more rapidly, is stronger, and is more common
The auger bit (fig. 5-13, view A) is the most com-
than the single or double twist.
mon of the wood-boring bits. It should be used with a
hand brace. It consists of three parts: the head, twist, and
The single twist has less tendency to bind in
shank, as shown in figure 5-14.
certain materials but is more fragile than the
The screw point functions to center the bit and to
single twist with solid center.
pull the bit through the stock. The three types of screw
points are coarse, medium, and fine (fig. 5-15). The
The double twist produces a clean, smooth,
coarse bit is drawn through the stock faster than a fine
accurate hole and bores slower than other
one, but there is more roughness because of the faster
twist bits. It is the most suitable of the three to
bore holes for wooden dowels.