Page 17 - THREADING CATALOG

INTRODUCTION

Tolerance System UN/ UNC/ UNJ/ UNR/ UNS/ UNRS/ UNF/ UNEF Standards

The UN/ UNJ/ UNR/ UNS/ UNRS/ UNF/ UNEF major thread per thread accuracy
standards define 3 classes of accuracy for diameter inch (TPI) standard - class INTRODUCTION
external threads and internal threads:
External Threading Internal Threading
External Threading Internal Threading
• 3A (tight tolerance) • 3B (tight tolerance) 3/8 16 UNC – 3A 3/8 16 UNC – 3B
• 2A (medium tolerance) • 2B (medium tolerance) 1/2 20 UNF – 2A 1/2 20 UNF – 2B
• 1A (loose tolerance) • 1B (loose tolerance) 9/16 24 UNEF – 1A 9/16 24 UNEF – 1B
** bold - most common
Thread Unified National Standards Compatibility
The value and range of the tolerance according to External UN* UNJ UNR
accuracy class are described by tables, formulas, and Internal
charts in the UN standard. Possible combinations are UN*  - 
described in the UN standard.
Not all combinations exist in all three tolerance classes. UNJ   
Examples for thread description according
to UN/ UNC/ UNJ/ UNR/ UNS/ UNRS/ UNF/ UNR  - 
UNEF standards with accuracy class: UN* insert can produce thread standars UNC, UNF; UNEF

Tolerance System BSW Standard

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) • medium class (medium tolerance)
• medium class (medium tolerance) • normal class (loose 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:
bolt (for external threads)
major diameter - thread per inch (TPI) B.S.W (accuracy class) nut (for internal threads)
bolt (for external threads)
W major diameter - thread per inch (TPI) (accuracy class)
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 = Pitch (mm)
T.P.I.

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