Figure 12-23. - The web of the drill bit and how the drill point is relieved by grinding.

be thinned to minimize the pressures required to make the drill bit penetrate the material. The thinning must be done equally to both sides of the web, and care must be taken to ensure that the web is centered.

The DRILL POINT GAUGE (fig. 12-24) is the tool most frequently used to check the drill point during the sharpening operation.

Use a coarse wheel for roughing out the drill point if much metal must be ground away. Complete the operation on a fine wheel.

Many hand sharpening techniques have been developed. The following are recommended:

1. Grasp the drill shank with the right hand and the rest of the drill bit with the left hand.

2. Place the fingers of the left hand that are supporting the drill bit on the grinder tool rest. The tool rest should be slightly below center (about 1 inch on a 7-inch wheel).

Figure 12-24. - Using a drill point gauge.

3. Stand so the centerline of the drill bit will be at a 59-degree angle with relation to the centerline of the wheel (fig. 12-25), and lightly touch the drill lip to the wheel in approximately a horizontal position.

4.Use the left hand as a pivot point and slowly lower the shank with the right hand. Increase the pressure as the heel is reached to ensure proper clearance.

5. Repeat the operation on each lip until the drill bit is sharpened. DO NOT QUENCH HIGH-SPEED STEEL DRILLS IN WATER TO COOL. LET THEM COOL IN CALM AIR.

6. Check the drill tip frequently with the drill point gauge to assue a correctly sharpened drill bit.

Secure a drill bit that is properly sharpened and run through the motions of sharpening it. When you have acquired sufficient skill, sharpen a dull drill bit. To test, drill a hole in soft metal and observe the chip formation. When proper] y sharpened, the chips will come out of the flutes in curled spirals of equal length. The tightness of the chip spiral is governed by the RAKE ANGLE (fig. 12-26).

Figure 12-25. - The correct position of the drill bit at the start of the grinding operation. View is looking down on the grinder.

Figure 12-26. - Rake angle of drill bit for ordinary work.