Machinery's Handbook, 31st Edition
2172 Diameter of Tap Drill In selecting drills to produce holes within the limits given in Table 6 it should be recog nized that drills have a tendency to cut oversize. The material on page 962 may be used as a guide to the expected amount of oversize. British Standard Tapping Drill Sizes for Screw and Pipe Threads.— British Standard BS 1157:1975 (2004) provides recommendations for tapping drill sizes for use with fluted taps for various ISO metric, Unified, British Standard fine, British Association, and Brit - ish Standard Whitworth screw threads as well as British Standard parallel and taper pipe threads. Table 7. British Standard Tapping Drill Sizes for ISO Metric Coarse Pitch Series Threads BS 1157:1975 (2004)
Standard Drill Sizes a
Standard Drill Sizes a
Recommended
Alternative
Recommended
Alternative
Theoretical Radial Engagement with Ext. Thread (Percent)
Theoretical Radial Engagement with Ext. Thread (Percent)
Theoretical Radial Engagement with Ext. Thread (Percent)
Theoretical Radial Engagement with Ext. Thread (Percent)
Nom. Size and Thread Diam.
Nom. Size and Thread Diam.
Size 0.75
Size 0.78 0.88 0.98 1.15 1.30 1.50 1.65 1.80 2.10 2.55 2.95 3.40 3.80 4.30 5.10 6.10 6.90 7.90 8.60 9.60
Size
Size 10.40 12.20 14.25 15.75 17.75 19.75 21.25 24.25 26.75 29.75
74.5 b 73.4 b 71.3 c 73.4 c 73.4 c 73.4 c 74.7 b 74.7 b 75.7 b 75.7 b
M 1
81.5 81.5 81.5 81.5 81.5 81.5 81.5 81.5 81.5 81.5 81.5 81.5 86.8 81.5 81.5 81.5 78.5 78.5 81.5 81.5
71.7 71.7 71.7 67.9 69.9 69.9 71.3 72.5 72.5 73.4 74.7 69.9 b 76.1 71.3 b 73.4 73.4 71.7 b 71.7 b 76.1 76.1
M 12 10.20 M 14 12.00 M 16 14.00 M 18 15.50 M 20 17.50 M 22 19.50 M 24 21.00 M 27 24.00 M 30 26.50 M 33 29.50 M 36 32.00 M 39 35.00 M 42 37.50 M 45 40.50 M 48 43.00 M 52 47.00 M 56 50.50 M 60 54.50 M 64 58.00 M 68 62.00
83.7 81.5 81.5 81.5 81.5 81.5 81.5 81.5 81.5 81.5 81.5 81.5 81.5 81.5 81.5 81.5 81.5 81.5 81.5 81.5
M 1.1 0.85 M 1.2 0.95 M 1.4 1.10 M 1.6 1.25 M 1.8 1.45 M 2.2 1.75 M 2.5 2.05 M 2 1.60
M 3
2.50
M 3.5 2.90
… … … … … … … … … … … … … … … … … … … …
M 4
3.30
M 4.5 3.70
M 5 M 6 M 7 M 8 M 9
4.20 5.00 6.00 6.80 7.80
M 10 8.50 M 11 9.50
a These tapping drill sizes are for fluted taps only. b For tolerance class 6H and 7H threads only. c For tolerance class 7H threads only. Drill sizes are given in millimeters.
In the accompanying Table 7 , recommended and alternative drill sizes are given for pro ducing holes for ISO metric coarse pitch series threads. These coarse pitch threads are suit able for the large majority of general-purpose applications, and the limits and tolerances for internal coarse threads are given in the table starting on page 2053. It should be noted that Table 7 is for fluted taps only since a fluteless tap will require for the same screw thread a different size of twist drill than will a fluted tap. When tapped, holes produced with drills of the recommended sizes provide for a theoretical radial engagement with the external thread of about 81 percent in most cases. Holes produced with drills of the alternative sizes provide for a theoretical radial engagement with the external thread of about 70 to 75 percent. In some cases, as indicated in Table 7, the alternative drill sizes are suitable only for medium (6H) or for free (7H) thread tolerance classes. When relatively soft material is being tapped, there is a tendency for the metal to be squeezed down towards the root of the tap thread, and in such instances, the minor diame ter of the tapped hole may become smaller than the diameter of the drill employed. Users
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