DRILL BITS

Common drill bits are known as TWIST DRILLS because most of them are made by forging or milling rough flutes and then twisting them to a spiral configuration. After twisting, the drill bits are milled to the desired size and heat-treated.

The general-purpose twist drill is made of highspeed steel. Figure 12-17 shows a typical plastic-cutting drill bit and a typical metal-cutting drill bit. Notice the smaller angle on the drill bit used for drilling plastics.

Drill bit sizes are indicated in three ways: by inches, by letter, and by number. The nominal inch sizes run from 1/16 inch to 4 inches or larger. The letter sizes run from "A" to "Z" (0.234 inch to 0.413 inch).

Figure 12-17.-Comparison of a twist drill for plastic and a twist drill for metal.

The number sizes run from No. 80 to No. 1 (0.0135 inch to 0.228 inch).

Before putting a drill bit away, wipe it clean and then give it a light coating of oil. Do not leave drill bits in a place where they maybe dropped or where heavy objects may fall on them. Do not place drill bits where they will rub against each other.

A drill bit should be reground at the first sign of dullness. The increased load that dullness imposes on the cutting edges may cause a drill bit to break.

Cutting Fluids

When drilling steel and wrought iron, use a cutting oil. Cast iron, aluminum brass, and other metals may be drilled dry; therefore, at high-drilling speeds it is advisable to use some medium for cooling these metals to lessen the chances of overheating the drill bit with the resultant loss of the cutting edge. Compressed air may be used for cast iron; kerosene for aluminum; oleic acid for cooper; sulphurized mineral oil for Monel metal; and water, lard, or soluble oil and soda water for ferrous metals. (Soda water reduces heat, overcomes rust, and improves the finish.)

Sharpening Drill Bits

A drill bit becomes dull with use and must be resharpened. Continued use of a dull drill bit may cause it to break or bum up as it is forced into the metal. Improper sharpening will cause the same difficulties.

Remove the entire point if it is badly worn or if the margins are burned or worn off near the point. If, by accident, the drill bit becomes overheated during grinding, do NOT plunge it into the water to cool. Allow it to cool in still air. The shock of sudden cooling may cause it to crack.

Three factors must be considered when repainting a drill bit:

1. LIP CLEARANCE (fig. 12-18). The two cutting edges or lips are comparable to chisels. To cut effectively, you must relieve the heel or that part of the point back to the cutting edge. Wh.bout this clearance, it would be impossible for the lips to cut. If there is too much clearance, the cutting edges are weakened. Too little clearance results in the drill point merely rubbing without penetration. Gradually increase lip clearance toward the center until the line across dead center stands at an angle of 120 to 135 degrees with the cutting edge (fig. 12-19).

2. LENGTH AND ANGLE OF LIPS. The material to be drilled determines the proper point angle. The angles, in relation to the axis, must be the same. Fifty-nine degrees has been found satisfactory for most metals. If the angles are unequal, only one lip will cut and the hole will be oversize (fig. 12-20).

3. THE PROPER LOCATION OF THE DEAD CENTER (fig. 12-21). Equal angles but lips of different lengths results in oversize holes and the resulting "wobble" places tremendous pressures on the drill press spindle and bearings.

Figure 12-18.-Lip clearance.

Figure 12-19.-Angle of the dead center.

Figure 12-20.-Unequal drill point angles.

Figure 12-21.-Drill point off center.

A combination of both faults can result in a broken drill bit, and if the drill bit is very large, permanent damage to the drilling machine. The hole produced (fig. 12-22) will be oversize and often out-of-round.

The web of the drill bit increases in thickness toward the shank (fig. 12-23). When the drill bit has been shortened by repeated grindings, the web must

Figure 12-22.-Drill point with unequal point angles and with the drill point sharpened off center.

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.

An attachment for conventional tool grinders is shown in figure 12-27. In a shop where a high degree of hole accuracy is required and a large amount of sharpening is to be done, a machine or attachment is a must.

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