SELF-THREADING SCREWS Machinery's Handbook, 31st Edition
1833
SELF-THREADING SCREWS ANSI Standard Sheet Metal, Self-Tapping, and Metallic Drive Screws.— Table 1 shows the various types of “self-tapping” screw threads covered by the ANSI/ASME B18.6.3- 2013 standard. (Metric thread forming and thread cutting tapping screws are discussed beginning on page 1848 ). ANSI designations are also shown. Types A, AB, B, BP and C when turned into a hole of proper size form a thread by a displacing action. Types D, F, G, T, BF and BT when turned into a hole of proper size form a thread by a cutting action. Type U when driven into a hole of proper size forms a series of multiple threads by a displacing action. These screws have the following descriptions and applications: Type A: Spaced-thread screw with gimlet point primarily for use in light sheet metal, resin-impregnated plywood, and asbestos compositions. This type is no longer recom mended. Use Type AB in new designs and whenever possible substitute for Type A in existing designs. Type AB: Spaced-thread screw with same pitches as Type B but with gimlet point, pri marily for similar uses as for Type A. Type B: Spaced-thread screw with a blunt point with pitches generally somewhat finer than Type A. Used for thin metal, nonferrous castings, plastics, resin-impregnated ply wood, and asbestos compositions. Type BP: Spaced-thread screw, the same as Type B but having a conical point extending beyond incomplete entering threads. Used for piercing fabrics or in assemblies where holes are misaligned. Type C: Screws having machine screw diameter-pitch combinations with threads approximately Unified Form and with blunt tapered points. Used where a machine screw thread is preferable to the spaced-thread types of thread forming screws. Also useful when chips from machine screw thread-cutting screws are objectionable. In view of the declining use of Type C screws, which in general require high driving torques, in favor of more efficient designs of thread tapping screws, they are not recommended for new designs. Types D, F, G, and T: Thread-cutting screws with threads approximating machine screw threads, with blunt point, and with tapered entering threads having one or more cutting edges and chip cavities. The tapered threads of the Type F may be complete or incomplete at the producer’s option; all other types have incomplete tapered threads. These screws can be used in materials such as aluminum, zinc, and lead die castings; steel sheets and shapes; cast iron; brass; and plastics. Types BF and BT: Thread-cutting screws with spaced threads as in Type B, with blunt points, and one or more cutting grooves. Used in plastics, asbestos, and other similar compositions. Type U: Multiple-threaded drive screw with large helix angle, having a pilot point, for use in metal and plastics. This screw is forced into the work by pressure and is intended for making permanent fastenings. ANSI Standard Head Types for Tapping and Metallic Drive Screws: Many of the head types used with “self-tapping” screw threads are similar to the head types of American National Standard machine screws shown in the section with that heading. Round Head: The round head has a semi-elliptical top surface and a flat bearing surface. Because of the superior slot driving characteristics of pan head screws over round head screws, and the overlap in dimensions of cross recessed pan heads and round heads, it is recommended that pan head screws be used in new designs and wherever possible substi tuted in existing designs. Undercut Flat and Oval Countersunk Heads: For short lengths, 82-degree and oval countersunk head tapping screws have heads undercut to 70 percent of normal side height to afford greater length of thread on the screws.
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