Page 19 - Threading

Thread Accuracy

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 INTRODUCTION
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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