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 A series of numbers, 3 through 9, were established as symbols, to reflect the size of the tolerance; the higher the number the larger the tolerance. FOR EXTERNAL THREADS - (LOWER CASE LETTER SYMBOLS) • Tolerance Position “e” has a large allowance. The upper limit is below basic by a large amount. • Tolerance Position “g” has a small allowance. The upper limit is below basic by a small amount. • Tolerance Position “h” has no allowance and the upper limit is basic. 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.
Selection of Tolerance Classes Two factors determine the selection of a suitable tolerance class: 1. Length of thread engagement (short, normal or long) 2. Quality requirement (fine, medium or coarse) See table below for preferred tolerance classes. Tolerance Positions They define the maximum-material limits of the pitch and crest diameters and indicate their relationship to the basic profile. For plating requirements and ease of assembly, a series of tolerance positions were established.
Preferred Tolerance Classes
External Thread (Bolts)
Internal Thread (Nuts)
Tolerance Position e (Large Allowance) Length of Thread Engagement
Tolerance Position g (Small Allowance) Length of Thread Engagement
Tolerance Position h (No Allowance) Length of Thread Engagement
Tolerance Position G (Small Allowance) Length of Thread Engagement
Tolerance Position H (No Allowance)
Quality Requirement
Length of Thread Engagement Short Normal Long ShortNormal Long Short Normal Long Short Normal Long Short Normal Long
FINE (Close Fit Applications)
3h-4h 4h 5h-4h
4H
MEDIUM (General Purpose Applications) COARSE (Difficult Manufacturing Applications)
6e 7e-6e 5g-6g 6g 7g-6g 5h-6h 6h 7h-6h 5G 6G 7G 5H
8g 9g-8g
7G 8G
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 diameter in millimeters) followed by “X” and the pitch in millimeters, designates metric screw threads. For coarse series thread, the “X” and pitch may be omitted. Example: Coarse series threads; M6 other threads; M8 x 1 A complete designation comprises, in addition to the basic designation, the tolerance class symbol separated by a dash. When the pitch and crest diameter tolerance classes are identical, the symbol need only be given once. Example: M20 x 2—6H
When the pitch and crest diameters have different tolerance classes, the pitch diameter symbol is followed by the crest diameter symbol. Example: M6 x 0.75—5g—6g To indicate a specified thread fit between mating parts, the internal thread tolerance class symbol is followed by that of the external thread, separated by a slash. Example: M20 x 2—6H/5g—6g When rounded root external threads are to be specified, the minimum root radius value shall be added to the tolerance class designation. Example: M6—5g —6g 0.100R
Threading Technical
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