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Machinery's Handbook 27th Edition 1816 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. D=maj. diam. of internal thread; d=maj. diam. of external th 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; 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 difficulties 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 Classesa,b,c for Nuts and Bolts Tolerance Classes for Nuts Tolerance Position G Tolerance Position H Tolerance Quality Short Normal Long Short Normal Long Fine … … … 4Hb 5Hb 6Hb Medium 5Ga 6Gc 7Gc 5Ha 6Ha,d 7Ha Coarse … 7Gc 8Gc … 7Hb 8Hb Tolerance Classes for Bolts Tolelance Tolerance Position e Tolerance Position f Tolerance Position g Tolerance Position h Quality Short Normal Long Short Normal Long Short Normal Long Short Normal Long Fine … … … … … … … … … 3h4hc 4ha 5h4hc Medium … 6ea 7e6ec … 6f a … 5g6gc 6ga,d 7g6gc 5h6hc 6hb 7h6hc Coarse … … … … … … … 8gb 9g8gc … … … a First choice. b Second choice. c Third choice; these are to be avoided. d For commercial nut and bolt threads. 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. Copyright 2004, Industrial Press, Inc., New York, NY
BRITISH STANDARD ISO METRIC SCREW THREADS 1817 Table 4. Lengths of Thread Engagements for Short, Normal, and Long Categories Basic Major Diameter d Up to and Incl. Pitch P Machinery's Handbook 27th Edition Short Normal Length of Thread Engagement Long Up to and Incl. Over Up to and Incl. Over Basic Major Diameter d Fig. 2. Tolerance Positions with Respect to Zero Line (Basic Size) Design Profiles.—The design profiles for ISO metric internal and external screw threads are shown in Fig. 3. These represent the profiles of the threads at their maximum metal condition. It may be noted that the root of each thread is deepened so as to clear the basic flat crest of the other thread. The contact between the thread is thus confined to their sloping flanks. However, for nut threads as well as bolt threads, the actual root contours shall not at any point violate the basic profile. Designation.—Screw threads complying with the requirements of the Standard shall be designated by the letter M followed by values of the nominal diameter and of the pitch, expressed in millimeters, and separated by the sign ×. Example: M6 × 0.75. The absence of the indication of pitch means that a coarse pitch is specified. The complete designation of a screw thread consists of a designation for the thread system and size, and a designation for the crest diameter tolerance. Each class designation consists of: a figure indicating the tolerance grade; and a letter indicating the tolerance Up to and Incl. Pitch P Short Normal Length of Thread Engagement Long Up to and Incl. Over Up to and Incl. Over Over Over 0.2 0.5 0.5 1.4 1.4 1 4 4 12 12 0.99 1.4 0.25 0.6 0.6 1.7 1.7 1.5 6.3 6.3 19 19 0.3 0.7 0.7 2 2 2 8.5 8.5 25 25 0.2 0.5 0.5 1.5 1.5 22.4 45 3 12 12 36 36 0.25 0.6 0.6 1.9 1.9 3.5 15 15 45 45 1.4 2.8 0.35 0.8 0.8 2.6 2.6 4 18 18 53 53 0.4 1 1 3 3 4.5 21 21 63 63 0.45 1.3 1.3 3.8 3.8 1.5 7.5 7.5 22 22 0.35 1 1 3 3 2 9.5 9.5 28 28 0.5 1.5 1.5 4.5 4.5 3 15 15 45 45 2.8 5.6 0.6 0.7 1.7 2 1.7 2 5 6 5 6 45 90 4 5 19 24 19 24 56 71 56 71 0.75 2.2 2.2 6.7 6.7 5.5 28 28 85 85 0.8 2.5 2.5 7.5 7.5 6 32 32 95 95 0.75 2.4 2.4 7.1 7.1 2 12 12 36 36 5.6 11.2 1 1.25 3 4 3 4 9 12 9 12 90 180 3 4 18 24 18 24 53 71 53 71 1.5 5 5 15 15 6 36 36 106 106 1 3.8 3.8 11 11 3 20 20 60 60 1.25 4.5 4.5 13 13 180 300 4 26 26 80 80 11.2 22.4 1.5 1.75 5.6 6 5.6 6 16 18 16 18 6 40 40 118 118 2 8 8 24 24 2.5 10 10 30 30 All dimensions are given in millimeters Copyright 2004, Industrial Press, Inc., New York, NY
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