To understand the principle of drilling efficiently, it is necessary to study the mechanism of a drill point. Drills are used to separate small particles of metal by scraping or cutting and to do this there must be a central or leading point about which the cutting edges turn. Figure 135 shows the cutting edges of a flat drill. The mechanical principle of the cutting edge of the drill is that of the wedge. It is seen that the left lip A B is ground at an angle sloping in the opposite direction from the right-hand lip CD. The angle of these slopes, called the clearance angle, is shown in the side view. The line bd in the plan view represents the intersection of these sloping lip faces and is called the drill point.
Clearance with a drill is practically the same as with a cold chisel. It is very important that the clearance angle for the metal to be cut should be ground correctly. Giving the lip of a drill clearance is nothing more than cutting back or "backing off" the face of the lip, so that its cutting edge will cut clean and will not scrape or rub on the bottom of the hole which is being drilled.
The two-fluted twist drill (Fig. 136) and the counterbore (Fig. 137) are among the most extensively used types. Twist drills work more accurately than flat drills. The cylindrical shape fills the hole, keeps the drill properly ground, and also serves as a channel through which the chips may pass out of the hole, their spiral form wedging or forcing them out as the drill rotates. Twist drills are sometimes made with three flutes. These are used for enlarging cored or punched holes, but they will not drill the initial hole. Some twist drills are made with a small groove cut around the outside, which contains a tube for carrying oil to the drill point.
Fig. 135. - Cutting Edge of Flat Drill.
Fig. 136. Two-Fluted Twist Drill.
Fig. 137. Counter-bore.