COMMON HAND-TOOLS 



273 



FIG. 135. Cutting 

 Edge of Flat Drill. 



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 AB is 

 ground at an angle sloping in the opposite 

 direction from the right-hand lip CD. 

 The angle of these slopes, called the clear- 

 ance 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 cor- 

 rectly. 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 



18 



FIG. 136. 

 Two- 

 Fluted 

 Twist 

 Drill. 



FIG. 137. 

 Counter- 

 bore. 



