Machinery's Handbook, 31st Edition
BRITISH Machine Screws 1799 intermediate change to ISO inch threads. Fasteners covered by BS 57, BS 450 and BS 2827:1957(obsolescent)eventuallywouldbesupersededandreplacedbyfastenersspecified by BS 4183. British Standard Whitworth (BSW) and Fine (BSF) Machine Screws.— British Stan dard BS 450:1958 (obsolescent) covers machine screws and nuts with British Standard Whitworth and British Standard Fine threads. All the various heads in common use in both slotted and recessed forms are covered. Head shapes are shown on page 1809 and dimensions on page 1811 . It is intended that this standard will eventually be superseded by BS 4183, “Machine Screws and Machine Screw Nuts, Metric Series.” British Standard Machine Screws and Machine Screw Nuts, Metric Series.— British Standard BS 4183:1967 (obsolescent) gives dimensions and tolerances for: countersunk head, raised countersunk head, and cheese head slotted head screws in a diameter range from M1 (1 mm) to M20 (20 mm); pan head slotted head screws in a diameter range from M2.5 (2.5 mm) to M10 (10 mm); countersunk head and raised countersunk head recessed head screws in a diameter range from M2.5 (2.5 mm) to M12 (12 mm); pan head recessed head screws in a diameter range from M2.5 (2.5 mm) to M10 (10 mm); and square and hexagon machine screw nuts in a diameter range from M1.6 (1.6 mm) to M10 (10 mm). Mechanical properties are also specified for steel, brass and aluminum alloy machine screws and machine screw nuts in this standard. Material: The materials from which the screws and nuts are manufactured have a tensile strength not less than the following: steel, 40 kgf/mm 2 (392 N/mm 2 ); brass, 32 kgf/mm 2 (314 N/mm 2 ); and aluminum alloy, 32 kgf/mm 2 (314 N/mm 2 ). The unit, kgf/mm 2 is in accordance with ISO DR 911 and the unit in parentheses has the relationship, 1 kgf = 9.80665 newtons. These minimum strengths are applicable to the finished products. Steel machine screws conform to the requirements for strength grade designation 4.8. The strength grade designation system for machine screws consists of two figures, the first is 1 ∕ 10 of the minimum tensile strength in kgf/mm 2 , the second is 1 ∕ 10 of the ratio between the yield stress and the minimum tensile strength expressed as a percentage: 1 ∕ 10 minimum tensile strength of 40 kgf/mm 2 gives the symbol “4”; 1 ∕ 10 ratio % minimum tensile strength yield stress = 1 ∕ 10 × 32 ∕ 40 × 100 ∕ 1 = “8”; giving the strength grade designation “4.8.” Multiplication of these two figures gives the minimum yield stress in kgf/mm 2 . Coating of Screws and Nuts: It is recommended that the coating comply with the appro priate part of BS 3382. “Electroplated Coatings on Threaded Components.” Screw Threads: Screw threads are ISO metric coarse pitch series threads in accordance with BS 3643. “ISO Metric Screw Threads,” Part 1, “Thread Data and Standard Thread Series.” The external threads used for screws conform to tolerance Class 6g limits (medium fit) as given in BS 3643, “ISO Metric Screw Threads,” Part 2, “Limits and Tol erances for Coarse Pitch Series Threads.” The internal threads used for nuts conform to tolerance Class 6H limits (medium fit) as given in BS 3643: Part 2. Nominal Lengths of Screws: For countersunk head screws the nominal length is the dis tance from the upper surface of the head to the extreme end of the shank, including any chamfer, radius, or cone point. For raised countersunk head screws the nominal length is the distance from the upper surface of the head (excluding the raised portion) to the extreme end of the shank, including any chamfer, radius, or cone point. For pan and cheese head screws the nominal length is the distance from the underside of the head to the extreme end of the shank, including any chamfer, radius, or cone point. Standard nominal lengths and tolerances are given in Table 5.
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