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100 THE DISTRIBUTOR’S

100 THE DISTRIBUTOR’S LINK BRUNO MARBACHER THREAD FORMING SCREWS FOR COST-EFFICIENT FASTENING IN METAL AND PLASTIC from page 14 Tapping Screws Features Self-tapping screws have a wide range of point and thread shapes and are available with a broad variety of head styles. The tapping thread covers the whole length, point to head. The thread is case hardened, hard enough to form the thread in the mating material. For hard materials such as light metal castings or hard plastics, the screw thread is furnished with a cutting flute and cutting edges similar to a tap, thus it can cut a thread into those hard materials, some limits apply. For materials such as wood or soft plastics, a selftapping screw with a gimlet point (tapered point) can form a mating thread sometimes even though there is no pre-drilled hole. it may need a shallow indentation so the screw point can catch, a hard wood or plastic is likely to crack. Standards Covering Tapping Screws Tapping screws are covered by standards. The inch dimensions tapping screws are specified by ANSI/ASME B18.6.4 The metric tapping screws, which are essentially a soft conversion from inch tapping screws (ST 6.3 = 1/4”) are specified by DIN 7971 to 7973… DIN 7981 to 7983, they are also covered by ISO 1481 to 1483…ISO 7049 to 7051.The thread diameter d1 is considered the nominal diameter: ST 4.2: Nominal diameter of thread = 4.2 mm (ST indicates „Self Tapping “) Like regular threads, the flank angle is 60°. The thread pitch (P)is roughly twice as big as in the regular thread, depending on whether it is a fine or coarse thread. There is a cylindrical core area between the threads. JIS (Japanese industrial standard) has introduced metric diameters (ST 3, ST 5 etc.) for sheet metal screws nationally, they are not widely used elsewhere. Metric and inch tapping screw threads dimension are the same, however for metric they are expressed in millimeters. The head and drive dimensions differ. Example of thread sizes call-out ¤ For metric: ST 4.9 x 32 (4.9 x 32) ¤ For inch: #8 x 1 1/2 (3/8 x 2) Materials, Mechanical Properties Low carbon steels with cold heading quality are typically utilized for tapping screws. The mechanical properties are covered by: ISO 2702 specifying mechanical properties for metric tapping screws SAE j933 specifying mechanical properties for inch tapping screws. They are case hardened to: ¤ Core hardness: 270 HV to 390 HV (~28 – 38 HRC) ¤ Surface hardness: 450 HV (~45 HRC) Tapping screws can also be made using stainless steel. ISO 3506-4 (no US equivalent standard) covers thread forming screws made of stainless steel. Tapping screws made of a strain-hardened austenitic chromenickel steel A2 (AISI 304) or A4 (AISI 316). They normally can be driven into aluminum sheets. They are too soft for steel sheets including stainless steel. Screws made of martensite hardened chromium-steel (400 Series) are harder and also work in soft steel plates. However, they are not commonly applied. And being made from 400 series stainless steels, they are not highly corrosion resistant. Common Tapping Screws Thread Points Per ISO the gimlet point has the designation “C” and designation “AB” per ANSI. Then, the blunt point has the designation “F” per ISO and designation “B” per ANSI. The rounded point designation R per ISO as well as point type BP are not commonly applied. CONTINUED ON PAGE 148