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Technical Information Thread Forms Components & Tap Limits

Thread Systems

The ISO standard is the international standard intended to be adopted throughout the world to unify and rationalise screw threads at an international level. The ISO standard recognises two groups of screw threads,( a) ISO metric, a complete thread system in metric units and( b) ISO inch Unified which is covered by British Standard BS 1580 and American Standard ANSI – B1-1 – Unified screw thread systems. The Whitworth and BA screw threads are obsolete but still widely used during the period of transition. All measurements must have a controlling point or base from which to start. In the case of a screw thread, this control point is called BASIC or theoretically correct size, which is calculated on the basis of a full thread form. Thus, on a given screw thread, we have the Basic Major Diameter, the Basic Pitch Diameter, and the Basic Minor Diameter. The Basic Profile is the profile to which the deviations, which define the limits of the external and internal threads, are applied. While it is impossible in practice to form screw threads to their precise theoretical or BASIC sizes, it is possible and practical to establish limits to which the deviation must not exceed. These are called the“ Maximum” and“ Minimum” Limits. If the product is no smaller than the“ Minimum Limit” and no larger than the“ Maximum Limit”, then it is within the size limits required. This difference between the Maximum and Minimum Limits is the TOLERANCE. In actual practice, the Basic size is not necessarily between Maximum and Minimum Limits. In most cases, the Basic Size is one of the Limits. In general, tolerances for internal threads will be above Basic and for external threads, below Basic.

Basic Profile for ISO Inch( Unified) and ISO Metric

The basic form is derived from an equilateral triangle which is truncated 1 / 8 of the height at the major diameter and 1 / 4 of the height at the minor diameter. The corresponding flats have a width of P / 8 and P / 4 respectively. Figure 1. In practice major diameter clearance is provided by the tap beyond the P / 8 flat on internal threads and beyond the P / 4 flat on external threads. These clearances are usually rounded.

ISO Metric Tolerance Positions

Three tolerance positions are standardised for bolts and two for nuts. These are designated e, g and h for bolts and G and H for nuts. As in the ISO System for limits and fits, small letters are used to designate tolerance positions for bolts and capital letters are used for nut tolerance positions. Also the letters h and H are used for tolerance positions having the maximum metal limit coincided with the basic size, i. e., with a fundamental deviation of zero. Figure 2.

ISO Metric Tolerance Grades

A series of tolerance grades designated 4, 5, 6, 7 and 8 for nut pitch diameters. An extended series of tolerance grades, designated 3, 4, 5, 6, 7,8 and 9, for bolt pitch diameters. An important factor here is that for the same tolerance grade the nut pitch diameter tolerance is 1.32 x the corresponding bolt pitch diameter tolerance. Size and recommendations of fits can be obtained from the Australian Standards AS 1275 or AS 1721.

Figure 1 Figure 2

H = 0,86603P— H = 0,21651P 4

— H = 0,10825P 8

3— 8

H = 0,32476P

5— 8

H = 0,54127P

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