Общий каталог OSG 2018 - 2019 - страница 664

ISO Screw Threads Metric ISO TOLERANCE SYSTEM The ISO Metric Screw Thread Tolerance System provides for tolerance grades and tolerance positions (allowances) for the pitch diameter and crest diameter. TOLERANCE GRADES SELECTION OF TOLERANCE CLASSES A series of numbers, 3 through 9, were established as symbols, to Two factors determine the selection of a suitable tolerance class: reflect the size of the tolerance; the higher the number the larger the tolerance. 1. Length of thread engagement (short, normal or long) 2. Quality requirement (fine, medium or coarse) See table below for FOR EXTERNAL THREADS - (LOWER CASE LETTER SYMBOLS) preferred tolerance classes. • Tolerance Position “e” has a large allowance. The upper limit is below basic by a large amount. TOLERANCE POSITIONS • Tolerance Position “g” has a small allowance. The upper limit is They define the maximum-material limits of the pitch and crest below basic by a small amount. diameters and indicate their relationship to the basic profile. For • Tolerance Position “h” has no allowance and the upper limit is plating requirements and ease of assembly, a series of tolerance basic. positions were established. FOR INTERNAL THREADS - (CAPITAL LETTER SYMBOLS) • Tolerance Position “G” has a small allowance. The lower limit is above basic by a small amount. • Tolerance Position “H” has no allowance and the lower limit is basic. PREFERRED TOLERANCE CLASSES External Thread (Bolts) Internal Thread (Nuts) Tolerance Position e Tolerance Position g Tolerance Position h Tolerance Position G Tolerance Position H Quality (Large Allowance) (Small Allowance) (No Allowance) (Small Allowance) (No Allowance) Requirement Length of Thread Length of Thread Length of Thread Length of Thread Length of Thread Engagement Engagement Engagement Engagement Engagement Short Normal Long Short Normal Long Short Normal Long Short Normal Long Short FINE Close Fit 3h-4h 4h 5h-4h 4H Applications MEDIUM General Purpose 6e 7e-6e 5g-6g 6g 7g-6g 5h-6h 6h 7h-6h 5G 6G 7G 5H Applications COARSE DifficultManufacturing 8g 9g-8g 7G 8G Applications Tolerance Position “e” is not to be applied to pitches finer than 0.5 mm. Tolerance classes 6g and 6H are for commercial screw, bolt and nut threads. THREAD DESIGNATIONS Basic Designations: The letter “M” and the nominal size (basic major When the pitch and crest diameters have different tolerance classes, the pitch diameter in millimeters) followed by “X” and the pitch in millimeters, diameter symbol is followed by the crest diameter symbol. designates metric screw threads. For coarse series thread, the “X” and Example: M6 x 0.75—5g—6g pitch may be omitted. To indicate a specified thread fit between mating parts, the internal thread tolerance Example: Coarse series threads; M6 class symbol is followed by that of the external thread, other threads; M8 x 1 separated by a slash. Example: M20 x 2—6H/5g—6g A complete designation comprises, in addition to the basic designation, the tolerance When rounded root external threads are to be specified, the minimum root radius class symbol separated by a dash. When the pitch and crest value shall be added to the tolerance class designation. diameter tolerance classes are identical, the symbol need only be given once. Example: M6—5g —6g 0.100R Example: M20 x 2—6H 664 Threading Technical WWW.OSGTOOL.COM