Machinery's Handbook, 31st Edition
Twist Drills 961 Table 17. British Standard Stub Drills — Metric Sizes BS 328: Part 1:1959
0.50 0.80 1.00 1.20 1.50 1.80 2.00 2.20 2.50 2.80 3.00 3.20 3.50 3.80 4.00 4.20 4.50
3 5 6 8 9
20 24 26 30 32 36 38 40 43 46 49 52
5.00 5.20 5.50 5.80 6.00 6.20 6.50 6.80 7.00 7.20 7.50 7.80 8.00 8.20 8.50 8.80 9.00
9.50 9.80 10.00 10.20 10.50 10.80 11.00 11.20 11.50 11.80 12.00 12.20 12.50 12.80 13.00 13.20 13.50
40
84
14.00 14.50 15.00 15.50 16.00 16.50 17.00 17.50 18.00 18.50 19.00 19.50 20.00 21.00 22.00 23.00 24.00
54 107 56 111
26
62
43
89
28
66
58 115
11 12 13 14 16 18 20
31
70
60 119
47
95
62 123
34
74
64 127
66 131 68 136 70 141 72 146 75 151
51 102
37
79
22
55
24 26
58 62
40
84
54 107
4.80 25.00 All dimensions are given in millimeters. Tolerances on diameters are given in Table 14. 9.20 13.80 Steels for Twist Drills.— Twist drill steels need good toughness, abrasion resistance, and ability to resist softening due to heat generated by cutting. The amount of heat generated indicates the type of steel that should be used. Carbon Tool Steel may be used where little heat is generated during drilling. High-Speed Steel is preferred because of its combination of red hardness and wear resis tance, which permits higher operating speeds and increased productivity. Optimum properties can be obtained by selection of alloy analysis and heat treatment. Cobalt High-Speed Steel alloys have higher red hardness than standard high-speed steels, permitting drilling of materials such as heat-resistant alloys and materials with hardness greater than 38 RC (Rockwell C scale). These high-speed drills can withstand cutting speeds beyond the range of conventional high-speed steel drills and have superior resistance to abrasion but are not equal to tungsten carbide-tipped tools. Accuracy of Drilled Holes.— Normally, the diameter of drilled holes is not given a toler ance; the size of the hole is expected to be as close to the drill size as can be obtained. The accuracy of holes drilled with a two-fluted twist drill is influenced by many factors, including accuracy of the drill point; size of the drill; length and shape of the chisel edge; whether or not a bushing is used to guide the drill; work material; length of the drill; runout of the spindle and the chuck; rigidity of the machine tool, workpiece, and the setup; and cutting fluid used, if any. The diameter of the drilled holes will be oversize in most materials. The table Oversize Diameters in Drilling on page 962 provides the results of tests reported by The United States Cutting Tool Institute in which the diameters of over 2800 holes drilled in steel and cast iron were measured. The values in this table indicate what might be expected under average shop conditions; however, when the drill point is accurately ground and the other machining conditions are correct, the resulting hole size is more likely to be between the mean and average minimum values given in this table. If the drill is ground and used incorrectly, holes that are even larger than the average maximum values can result.
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