The drill is flattened out and tapered thinnest at the point c. The side edges that form the diameter of the drill are for rough work given clearance, but for finer work are made nearly cylindrical, as in the figure.
The flattening serves two purposes: first, it reduces the point of the drill down to its proper thinness, enabling it to enter the metal of the work easily, and secondly, it enables the cuttings to pass upward and find egress at the top of the hole being drilled.
The cutting edges are formed by grinding the end facets at an angle as shown, and this angle varies from 5° for drilling hard metal, such as steel, to 20° for soft metal, such as brass or copper.
Fig. 1717.
The angle of one cutting edge to the other varies from 45° for steel to about 35° or 40° for soft metals. The object of these two variations of angles is that in hard metal the strain and abrasion is greatest and the cutting edge is stronger with the lesser degree of angle, while in drilling the softer metals the strain being less the cutting edge need not be so strong and the angles may be made more acute, which enables the drill to enter the metal more easily. The most imperfect cutting edge in a drill is that running diagonally across the point, as denoted by a in [Fig. 1717], because it is less acute than the other cutting edges, but this becomes more acute and, therefore, more effective, as the angles of the facets forming it are increased as denoted by the dotted lines in the figure. It is obvious, however, that the more acute these angles the weaker the cutting edge, hence an angle of about 5° is that usually employed.
It is an advantage to make the cutting edge at a, [Fig. 1717], as short as possible, which may be done by keeping the drill point thin; but if too thin it will be apt either to break or to operate in jumps (especially upon brass), drilling a hole that is a polygon instead of a true circle.
The cutting edges should not only stand at an equal degree of angle to the axial line of the drill, but should be of equal lengths, so that the point of the drill will be in line with the axial line of the drill. If the drill runs true the point will then be in the axial line of rotation, and the diameter of hole drilled will be equal to the diameter of the drill.
If, however, one cutting edge is longer than the other the hole drilled will be larger than is due to the diameter of the drill.