Machinery's Handbook, 31st Edition
1848 SELF-THREADING SCREWS Table 17. Approximate Hole Sizes for Type U Hardened Steel Metallic Drive Screws In Ferrous and Nonferrous Castings, Sheet Metals, Plastics, Plywood (Resin-Impregnated) and Fiber Screw Size Hole Size Drill Size Screw Size Drill Size Screw Size Drill Size
Hole Size .120 .136 .144 .161
Hole Size .191 .221 .295 .358
00
.052 .067 .086 .104
55 51 44 37
6 7 8
31 29 27 20
12 14 5 ∕ 16
11 M 2
0 2 4
10 T All dimensions are in inches except whole number screw and drill sizes and letter drill sizes. Table 18. ANSI Standard Torsional Strength Requirements for Tapping Screws ANSI/ASME B18.6.3-2013 3 ∕ 8
Types C, D, F, G, and T
Types C, D, F, G, and T
Types AB, B, BF, BP, and BT
Nom. Screw Size
Types AB, B, BF, BP, and BT
Nom Screw Size
Type A
Coarse Thread
Fine Thread
Type A
Coarse Thread
Fine Thread
… 142
140
179
2 3 4 5 6 7 8
4 9
4 9
5 9
6
1 ∕ 4 16 18 5 ∕ 16 20
10 15 20 27 47 74
152 196 250 492
… … … … … … … … … … … … 306 370
12 18 24 30 39 48 83
13 18 24 30 39 56 88
13 18 23 42 56 93
… 290
… … 24
… 590 … 620 … 1020
560 700
710 820
3 ∕ 8
10 12 14
7 ∕ 16
108
1075
1285
1 ∕ 2
125
… … … … … … … …
Torsional strength data are in pound-inches. Self-Tapping Thread Inserts.— Self-tapping screw thread inserts are essentially hard bushings with internal and external threads. The internal threads conform to Unified and American standard classes 2B and 3B, depending on the type of insert used. The external thread has cutting edges on the end that provide the self-tapping feature. These inserts may be used in magnesium, aluminum, cast iron, zinc, plastics, and other materials. Self- tapping inserts are made of case-hardened carbon steel, stainless steel, and brass, the brass type being designed specifically for installation in wood. Screw Thread Inserts.— Screw thread inserts are helically formed coils of diamond- shaped stainless steel or phosphor bronze wire that screw into a threaded hole to form a mating internal thread for a screw or stud. These inserts provide a convenient means of repairing stripped-out threads and are also used to provide stronger threads in soft materi als such as aluminum, zinc die castings, wood, magnesium, etc. than can be obtained by direct tapping of the base metal involved. According to the Heli-Coil Corp., conventional design practice in specifying boss diameters or edge distances can usually be applied since the major diameter of a hole tapped to receive a thread insert is not much larger than the major diameter of thread the insert provides. Screw thread inserts are available in thread sizes from 4-40 to 1 1 ∕ 2 -6 inch National and Unified Coarse Thread Series and in 6-40 to 1 1 ∕ 2 -12 sizes in the fine-thread series. When used in conjunction with appropriate taps and gages, screw thread inserts will meet requirements of 2, 2B, 3, and 3B thread classes. ANSI Standard Metric Thread Forming and Thread Cutting Tapping Screws.— Table 1 shows the various types of metric thread forming and thread cutting screw threads covered by the standard ANSI/ASME B18.6.5M-2000 (Withdrawn). The designations of the American National Standards Institute are shown.
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