Machinery's Handbook, 31st Edition
2046 British Standard ISO Metric Screw Threads Tolerance Grades.— Tolerance grades specified in the Standard for each of the four main screw thread diameters are as follows: Minor diameter of nut threads ( D 1 ): tolerance grades 4, 5, 6, 7, and 8. Major diameter of bolt threads ( d ): tolerance grades 4, 6, and 8. Pitch diameter of nut threads ( D 2 ): tolerance grades 4, 5, 6, 7, and 8. Pitch diameter of bolt threads ( d 2 ): tolerance grades 3, 4, 5, 6, 7, 8, and 9. Tolerance Positions.— Tolerance positions are G and H for nut threads and e, f, g, and h for bolt threads. The relationship of these tolerance position identifying letters to the amount of fundamental deviation is shown in Table 2.
H /8
P
P /8
3/8 H
D = maj. diam. of internal thread d = maj. diam. of external thread D 2 = pitch diam. of internal thread d 2 = pitch diam. of internal thread D 1 = minor diam. of internal thread d 1 = minor diam. of external thread P = Pitch H = height of fundamental angle
60°
5/8 H
30°
H
D / d
P /2
D 2 / d 2
H /4
P /4
90°
D 1 / d 1
Axis of Screw Thread
Fig. 1. Basic Profile of ISO Metric Thread
Tolerance Classes.— To reduce the number of gages and tools, the Standard specifies that the tolerance positions and classes shall be chosen from those listed in Table 3 for short, normal, and long lengths of thread engagement. The following rules apply for the choice of tolerance quality: Fine: for precision threads when little variation of fit character is needed; Medium: for general use; and Coarse: for cases where manufacturing difficul - ties can arise as, for example, when threading hot-rolled bars and long blind holes. If the actual length of thread engagement is unknown, as in the manufacturing of standard bolts, normal is recommended. Table 3. Tolerance Classes for Nuts and Bolts a ,b, c Tolerance Classes for Nuts Tolerance Quality Tolerance Position G Tolerance Position H Short Normal Long Short Normal Long Fine … … … 4H b 5H b 6H b Medium 5G a 6G c 7G c 5H a 6H a , d 7H a Coarse … 7G c 8G c … 7H b 8H b Tolerance Classes for Bolts Tolelance Quality Tolerance Position e Tolerance Position f Tolerance Position g Tolerance Position h Short Normal Long Short Normal Long Short Normal Long Short Normal Long Fine ………………………3h4h c 4h a 5h4h c Medium … 6e a 7e6e c … 6f a … 5g6g c 6g a , d 7g6g c 5h6h c 6h b 7h6h c Coarse … … … … … … … 8g b 9g8g c … … … a First choice. b Second choice. Note: See Table 4 for short, normal, and long categories. Any of the recommended tolerance classes for nuts can be combined with any of the recommended tolerance classes for bolts with the exception of sizes M1.4 and smaller for which the combination 5H/6h or finer shall be chosen. However, to guarantee a sufficient overlap, the finished components should preferably be made to form the fits H/g, H/h, or G/h. c Third choice; these are to be avoided. d For commercial nut and bolt threads.
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