Fig. 1054. Sectional View.
Fig. 1055. Top View.
Fig. 1056. Sectional View.
In [Fig. 1053] we have a top, and in [Fig. 1054] a sectional, view of a conical recess cut by a drill, with a cylinder r lying in the same. p represents in both views the outer arc or circle which would be described by the outer corner, [Fig. 1045], of the drill, and q the path or arc described or moved through by the point at f, [Fig. 1045], of the drill. At v and w are sectional views of the cylinder r, showing that the clearance is greater at v than at w. The cylinder obviously represents the end of a drill as usually ground. In [Figs. 1055] and [1056] we have two views of a cone lying in a recess cut by a drill, the arcs and circles p and q corresponding to those shown in [Fig. 1055], and it is seen that in this case the amount of clearance between v and p and between w and q are equal, v representing a cross-section of the cone at its largest end, and w a cross-section at the point where the cone meets the circle q. It follows, therefore, that drills ground upon this principle may be given an equal degree of clearance throughout the full length of each cutting edge, or may have the clearance increase or diminished towards the point at will, according to the angle of the line b in [Fig. 1052].
In order that the greatest possible amount of duty may be obtained from a twist drill, it is essential that it be ground perfectly true, so that the point of the drill shall be central to the drill and in line with the axis on which it revolves. The cutting edges must be of exactly equal length and at an equal degree of angle from the drill axis. To obtain truth in these respects it is necessary to grind the drill in a grinding machine, as the eye will not form a sufficiently accurate guide if a maximum of duty is to be obtained. The cutting speeds and rates of feed recommended by the Morse Twist Drill and Machine Company are given in the following table.
The following table shows the revolutions per minute for drills from 1⁄16 in. to 2 in. diameter, as usually applied:—