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Threads and Threading - Sportpilot.info

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Machinery's H<strong>and</strong>book 27th Edition<br />

1790 METRIC SCREW THREADS M PROFILE<br />

Fig. 5. Metric Tolerance System for Screw <strong>Threads</strong><br />

Tolerance: The tolerance is defined by a series of numerical grades. Each grade provides<br />

numerical values for the various nominal sizes corresponding to the st<strong>and</strong>ard tolerance for<br />

that grade.<br />

In the schematic diagram the tolerance for the external thread is shown as negative. Thus<br />

the tolerance plus the fit define the lower deviation (ei). The tolerance for the mating internal<br />

thread is shown as positive. Thus the tolerance plus the fit defines the upper deviation<br />

(ES).<br />

Tolerance Grade: This is indicated by a number. The system provides for a series of tolerance<br />

grades for each of the four screw thread parameters: minor diameter, internal<br />

thread, D 1 ; major diameter, external thread, d; pitch diameter, internal thread, D 2 ; <strong>and</strong> pitch<br />

diameter, external thread, d 2. The tolerance grades for this St<strong>and</strong>ard ANSI B1.13M were<br />

selected from those given in ISO 965/1.<br />

Dimension Tolerance Grades Table<br />

D 1 4, 5, 6, 7, 8 Table 8<br />

d 4, 6, 8 Table 9<br />

D 2 4, 5, 6, 7, 8 Table 10<br />

d 2 3, 4, 5, 6, 7, 8, 9 Table 11<br />

Note: The underlined tolerance grades are used with normal length of thread engagement.<br />

Tolerance Position: This position is the allowance (fundamental deviation) <strong>and</strong> is indicated<br />

by a letter. A capital letter is used for internal threads <strong>and</strong> a lower case letter for external<br />

threads. The system provides a series of tolerance positions for internal <strong>and</strong> external<br />

threads. The underlined letters are used in this St<strong>and</strong>ard:<br />

Internal threads G, H Table 6<br />

External threads e, f, g, h Table 6<br />

Designations of Tolerance Grade, Tolerance Position, <strong>and</strong> Tolerance Class: The tolerance<br />

grade is given first followed by the tolerance position, thus: 4g or 5H. To designate the<br />

tolerance class the grade <strong>and</strong> position of the pitch diameter is shown first followed by that<br />

for the major diameter in the case of the external thread or that for the minor diameter in the<br />

case of the internal thread, thus 4g6g for an external thread <strong>and</strong> 5H6H for an internal<br />

thread. If the two grades <strong>and</strong> positions are identical, it is not necessary to repeat the symbols,<br />

thus 4g, alone, st<strong>and</strong>s for 4g4g <strong>and</strong> 5H, alone, st<strong>and</strong>s for 5H5H.<br />

Copyright 2004, Industrial Press, Inc., New York, NY

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