Page 20 - Threading_Handbook

INTRODUCTION Tolerance System BSW Standard • Medium Class (Medium tolerance)
The BSW standard defines 3 classes of accuracy for external threads and 2 classes of
accuracy for internal threads:

External Threading
Internal Threading
• Close Class (Tight tolerance)

• Normal Class (Loose tolerance)
• Medium Class (Medium tolerance)
• Free Class (Loose tolerance)

The value and range of the tolerance according to the accuracy class are described by
tables, formulas, and charts in the BSW standard.

Recommended combinations:

• Close Class for external threading is intended for use with Medium Class
for internal threading

• Medium Class for external threading is intended for use with Normal Class
for internal threading
• Free Class for external threading is intended for use with Normal Class
for internal threading

There are two options for thread description according to the BSW standard:

Major Thread per Bolt (for External Threads)
Diameter - Inch (TPI) B.S.W (Accuracy Class) Nut (for Internal Threads)

Major Thread per Bolt (for External Threads)
W - (Accuracy Class)
Diameter Inch (TPI) Nut (for Internal Threads)

Examples for thread description according to the BSW standard with accuracy class:

External Threading

• 5/16 -18 B.S.W. (close) bolt or W 5/16 -18 (close) bolt
• 1/2 -12 B.S.W. (medium) bolt or W 1/2 -12 (medium) bolt
• 11/8-7 B.S.W. (free) bolt or W 11/8-7 (free) bolt

Internal Threading
• 5/16 -18 B.S.W. (medium) nut or W 5/16 (medium) nut
• 11/8 -7 B.S.W. (normal) nut or W 11/8 (normal) nut

Useful formulas:
1 inch = 25.4 mm

25.4
T.P.I = Pitch (mm)

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