Page 16 - Threading_Handbook

1.4 Thread Accuracy
INTRODUCTION In thread manufacturing, permissible limits are defined for the actual thread profile’s

deviations from the theoretical thread profile. To ensure correct threading operations, the
thread profile cannot cross the theoretical profile, and therefore, this permissible limit is set
by tolerances. The external and internal thread should only contact the sides of the threaded
profile, therefore the main parameter that influences thread tolerance is the pitch diameter.
Tolerances on major and minor diameters are set in such a way as to exclude the possibility
of collision on the tops and bottoms of the thread. Each thread standard contains a different
tolerance method.

ISCAR provides tools for thread production at all standards and in all accuracy classes

Tolerance System ISO Standard
In accordance with the ISO standard, threading accuracy is determined by the combination
of tolerance grade and tolerance position.
Tolerance grades are classified by numbers according to major, pitch and minor diameters
and are different for internal and external threads.

Internal Threads External Threads
Dimension Tolerance grade Dimension Tolerance grade
Minor diameter 4, 5, 6, 7, 8 Major diameter 4, 6, 8
Pitch diameter 4, 5, 6, 7, 8 Pitch diameter 3, 4, 5, 6, 7, 8, 9

Tolerance positions are classified by letters and are different for external and internal threads.

Gender Tolerance Position
Internal Threads G, H
External Threads e, f, g, h

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