Machinery's Handbook, 31st Edition
1800 BRITISH Machine Screws Lengths of Thread on Screws: The length of thread is the distance from the end of the screw (including any chamfer, radius, or cone point) to the leading face of a nut without countersink which has been screwed as far as possible onto the screw by hand. The mini mum thread length is shown in the following table:
Nominal Thread Dia., d a Thread Length b (Min.) Nominal Thread Dia. , d a Thread Length b (Min.)
M1 M1.2 (M1.4) M1.6 M2 (M2.2) M2.5 M3 (M3.5) M4
15 16 17 18 19 20 22
b
b
b
(M4.5) M5 M6 M8 M10 M12 (M14) M16 (M18) M20
24 25 28 34 40 46 52 58 64 70
a Items shown in parentheses are non-preferred. b Threaded up to the head. All dimensions are in millimeters.
Screws of nominal thread diameter M1, M1.2 and M1.4 and screws of larger diameters that are too short for the above thread lengths are threaded as far as possible up to the head. In these screws the length of unthreaded shank under the head does not exceed 1 1 ⁄ 2 pitches for lengths up to twice the diameter and 2 pitches for longer lengths, and is defined as the distance from the leading face of a nut that has been screwed as far as possible onto the screw by hand to: 1) the junction of the basic major diameter and the countersunk por - tion of the head on countersunk and raised countersunk heads; and 2) the underside of the head on other types of heads. Diameter of Unthreaded Shank on Screws: The diameter of the unthreaded portion of the shank on screws is not greater than the basic major diameter of the screw thread and not less than the minimum effective diameter of the screw thread. The diameter of the unthreaded portion of shank is closely associated with the method of manufacture; it will generally be nearer the major diameter of the thread for turned screws and nearer the effective diameter for those produced by cold heading. Radius Under the Head of Screws: The radius under the head of pan and cheese head screws runs smoothly into the face of the head and shank without any step or discontinuity. A true radius is not essential providing that the curve is smooth and lies wholly within the maximum radius. Any radius under the head of countersunk head screws runs smoothly into the conical bearing surface of the head and the shank without any step or discontinuity. The radius values given in Table 1 and Table 2 are regarded as the maximum where the shank diameter is equal to the major diameter of the thread and minimum where the shank diameter is approximately equal to the effective diameter of the thread.
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