Drilling is the term used by shop men to denote hole production by means of a rotating tool which is provided with cutting edges located at its point. The drill, therefore, is an end cutting tool as distinguished from the ordinary reamer which usually cuts on its sides.
Drills are of two general classes, the flat and the twist. A flat drill of a common type is shown in Fig. 63. The angle between the two cutting edges should be about 110 degrees. These drills are usually made from round tool steel drawn out wide and thin, as shown, the undressed end being used for holding. The flat drill is usually made in the shop where it is to be used. Its low first cost is the principal reason for its existence.
Flat drills made from thin flat stock are used in connection with a slotted rest to start and enlarge previously cored holes in lathe chuck work. They are called chucking drills. The end of the shank of the drill is provided with a center hole to receive the dead center of the machine. The drill and rest are shown in Fig. 64.
The simplest form of twist drill is cylindrical throughout its entire length, as shown in Fig. 65, and has two spiral flutes which at the end serve to form the cutting lips, and which also serve to carry the chips from the hole. The included angle of the lips is 118 degrees. The twist drill will work more accurately than the flat drill, as the cylindrical portion serves as a guide to keep the cutting lips in their proper position. The edges, being somewhat hooking, removes the metal by a cutting instead of a scraping action as in the flat drill. This form of drill not only can be fed faster but can be forced into the work with less power, as it has a tendency, especially noticeable in soft metals, to feed itself into the work. Straight shank twist drills are made from .0135 inch to 2 1/2 inches in diameter; the smaller sizes are sold in sets designated by the numbers 1 to 80; by the letters A to Z; or by the fractional sizes 1/2 inch to 1/16 inch.
Fig. 63. Blacksmith's Drills.
The taper-shank twist drill is shown in Fig. 66. It consists of a body A, which is fluted and does the actual work, and a taper shank B, by which it is held. This taper fits accurately into the spindle or chuck of the drill press. At the end there is a tongue C, which slips into the keyway in the spindle or chuck. As this surface is flat, it serves as a bearing by which the drill is driven. This relieves the tapered portion from the stress of driving by fractional resistance alone. For small drills this fractional resistance is sufficient, but for larger sizes it will not do at all. If for any reason the tongue should become broken, no dependence should be placed upon the frictional resistance of the taper shank to drive the drill.
Fig. 64. Chucking Drill Rest.
Fig. 65. Typical Twist Drill Courtesy of Union Twist Drill Company, Athol, Massachusetts.
The drill will slip and wear the socket, which will become enlarged and make a misfit for other drills.
The standard taper for drill shanks, known as the Morse, is f inch to the foot. There is another standard taper, known as the Jarno, which has a taper of 6/10 inch to the foot. No attempt should be made to run the drills of one taper in the sockets of the other.
Fig. 66. Taper Shank Twist Drill Courtesy of Morse Twist Drill Company, New Bedford, Massachusetts.
A flat taper key, or drift, introduced into the keyway, engages the end of the tongue and serves to remove the drill from the spindle.
Drills of cylindrical form are also made with straight flutes as shown in Fig. 67. They are used for drilling soft metals, such as brass, especially when the drill passes entirely through the piece. As it breaks through the metal, a drill with spiral flutes tends to draw itself through rapidly, as if it were a screw working in a nut. This may break the drill or move the work from positior. Straight flutes give the same cutting action as a flat drill and avoid the tendency to draw.
Fig. 67. Straightway or Straight Fluted Drill Courtesy of Union Twist Drill Company, Athol, Massachusetts.