4-3 TOOLS AND EQUIPMENT

TWIST DRILLS

Twist drills are the most common cutting tools used with drilling machines. Twist drills are designed to make round holes quickly and accurately in all materials. They are called twist drills mainly because of the helical flutes or grooves that wind around the body from the point to the neck of the drill and appear to be twisted (Figure 4-7). Twist drills are simply constructed but designed very tough to withstand the high torque of turning, the downward pressure on the drill, and the high heat generated by friction.


Figure 4-7.  Twist drill nomenclature.

There are two common types of twist drills, high-speed steel drills, and carbide-tipped drills. The most common type used for field and maintenance shop work is the high-speed steel twist drill because of its low cost. Carbide-tipped metal drills are used in production work where the drill must remain sharp for extended periods, such as in a numerically controlled drilling machine. Other types of drills available are: carbide tipped masonry drills, solid carbide drills, TiN coated drills, parabolic drills and split point drills. Twist drills are classified as straight shank or tapered shank (Figure 4-7). Straight shank twist drills are usually l/2-inch or smaller and tit into geared drill chucks, while tapered shank drills are usually for the larger drills that need more strength which is provided by the taper socket chucks.

Common twist drill sizes range from 0.0135 (wire gage size No. 80) to 3.500 inches in diameter. Larger holes are cut by special drills that are not considered as twist drills. The standard sizes used in the United States are the wire gage numbered drills, letter drills, fractional drills, and metric drills (See Table 4-1). Twist drills can also be classified by the diameter and length of the shank and by the length of the fluted portion of the twist drill.

Table 4
Drill Designation Size (in) Size (mm) Drill Designation Size (in) Size (mm)
80 0.014 0.343 27 0.144 3.658
79 0.015 0.368 26 0.147 3.734
78 0.016 0.406 25 0.149 3.797
77 0.018 0.457 24 0.152 3.861
76 0.020 0.508 23 0.154 3.912
75 0.021 0.533 22 0.157 3.988
74 0.023 0.572 21 0.159 4.039
73 0.024 0.610 20 0.161 4.089
72 0.025 0.635 19 0.166 4.216
71 0.026 0.660 18 0.169 4.305
70 0.028 0.711 17 0.173 4.394
69 0.029 0.742 16 0.177 4.496
68 0.031 0.787 15 0.180 4.572
67 0.032 0.813 14 0.182 4.623
66 0.033 0.838 13 0.185 4.699
65 0.035 0.889 12 0.189 4.801
64 0.036 0.914 11 0.191 4.851
63 0.037 0.940 10 0.194 4.915
62 0.038 0.965 9 0.196 4.978
61 0.039 0.991 8 0.199 5.055
60 0.040 1.016 7 0.201 5.105
59 0.041 1.041 6 0.204 5.182
58 0.042 1.067 5 0.206 5.220
57 0.043 1.092 4 0.209 5.309
56 0.046 1.181 3 0.213 5.410
55 0.052 1.321 2 0.221 5.613
54 0.055 1.397 1 0.228 5.791
53 0.059 1.511 A 0.234 5.944
52 0.064 1.613 B 0.238 6.045
51 0.067 1.702 C 0.242 6.147
50 0.070 1.778 D 0.246 6.248
49 0.073 1.854 E 0.250 6.350
48 0.076 1.930 F 0.257 6.528
47 0.079 1.994 G 0.261 6.629
46 0.081 2.057 H 0.266 6.756
45 0.082 2.083 I 0.272 6.909
44 0.086 2.184 J 0.277 7.036
43 0.089 2.261 K 0.281 7.137
42 0.094 2.375 L 0.290 7.366
41 0.096 2.438 M 0.295 7.493
40 0.098 2.489 N 0.302 7.671
39 0.099 2.527 0 0.316 8.026
38 0.101 2.578 P 0.323 8.204
37 0.104 2.642 Q 0.332 8.433
36 0.106 2.705 R 0.339 8.611
35 0.110 2.794 S 0.348 8.839
34 0.111 2.819 T 0.358 9.093
33 0.113 2.870 U 0.368 9.347
32 0.116 2.946 V 0.377 9.576
31 0.120 3.048 W 0.386 9.804
30 0.129 3.264 X 0.397 10.08
29 0.136 3.454 Y 0.404 10.26
28 0.141 3.569 Z 0.413 10.49

Wire gage twist drills and letter twist drills are generally used where other than standard fractional sizes are required, such as drilling holes for tapping. In this case, the drilled hole forms the minor diameter of the thread to be cut, and the major diameter which is cut by tapping corresponds to the common fractional size of the screw. Wire gage twist drills range from the smallest to the largest size; from No 80 (0.01 35 inch) to No 1 (0.2280 inch). The larger the number, the smaller the diameter of the drill. Letter size twist drills range from A (0.234 inch) to Z (0.413 inch). As the letters progress, the diameters become larger.

Fractional drills range from 1/64 to 1 3/4 inches in l/64-inch units; from 1/32 to 2 1/4 inches in 1/32-inch units, and from 1/1 6 to 3 1/2 inches in 1/16-inch units.

Metric twist drills are ranged in three ways: miniature set, straight shank, and taper shank. Miniature metric drill sets range from 0.04 mm to 0.99 mm in units of 0.01 mm. Straight shank metric drills range from 0.05 mm to 20.0 mm in units from 0.02 mm to 0.05 mm depending on the size of the drill. Taper shank: drills range in size from 8 mm to 80 mm in units from 0.01 mm to 0.05 mm depending on the size of the drill.

The drill gage (Figure 4-8) is used to check the diameter size of a twist drill. The gage consists of a plate having a series of holes. These holes can be numbered, lettered, fractional, or metric-sized twist drills. The cutting end of the drill is placed into the hole to check the size. A micrometer can also be used to check the size of a twist drill by measuring over the margins of the drill (Figure 4-9). The smaller sizes of drills are not usually marked with the drill size or worn drills may have the drill size rubbed off, thus a drill gage or micrometer must be used to check the size.


Figure 4-8. Drill gage.

 


Figure 4-9.  Measuring a drill with a micrometer.

It is important to know the parts of the twist drill for proper identification and sharpening (Figure 4-7).

SPECIAL DRILLS

Special drills are needed for some applications that a normal general purpose drill cannot accomplish quickly or accurately. Special drills can be twist drill type, straight fluted type, or special fluted. Special drills can be known by the job that they are designed for, such as aircraft length drills, which have an extended shank. Special drills are usually used in high-speed industrial operations. Other types of special drills are: left hand drill, Silver and Deming, spotting, slow spiral, fast spiral, half round, die, flat, and core drills. The general purpose high-speed drill, which is the common twist drill used for most field and maintenance shops, can be reground and adapted for most special drilling needs.