Fig. 1187.
[Fig. 1186] represents a half-round countersink, in which the cutting edge is produced by cutting away the coned point slightly below the dotted axial line. This secures two advantages: first, it gives the cutting edge clearance without requiring the grinding or filing such clearance; and, secondly, the cone being the same angle as the lathe centres, filing away more than half of it causes it to give the lathe centre at first a bearing at the small end of the countersink, as in [Fig. 1184], and this secures the advantage mentioned with reference to that figure. It is obvious that such a reamer, however, does not produce strictly a cone countersink, as is shown in [Fig. 1187], where the cutting away of the cone is carried to excess simply to explain the principle, and the cone becomes an hyperbolic curve.
The small amount, however, that it is necessary to carry the face below the line of centres, practically serves to make the cone somewhat less acute, and is not therefore undesirable.
Fig. 1188.
Fig. 1189.