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Chapter: 5 Miscellaneous engineering data
    Section: 5.1 Screw threads

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Updated: 12 October 2012
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5.1 Screw threads

The International Organization for Standardization (ISO) has published recommendations for parallel screw threads which recognize the ISO metric thread and the ISO inch thread. The ISO inch thread is the same as the Unified screw thread previously standardized by the USA, Britain and Canada. The ISO metric thread has the same form of thread as that of the Unified thread, namely, a symmetrical vee-form having a 60° included angle between the flanks.

Prior to the issue of the ISO Recommendations it was customary for bolts and machine screws made in Britain to have a thread of Whitworth form (55° included angle) for diameters inch and above, and for sizes less than inch diameter the British Association (BA) thread (47° included angle) was most commonly used. The Unified thread was also extensively used in the motor vehicle and aeronautical industries. BA threads and threads of Whitworth form will eventually become obsolete, but it may be years before this occurs. It is likely also that Unified screw threads will continue to be used for many years in certain industries.

The major diameter of a screw thread is the diameter over the crests of an external thread or to the roots of an internal thread. The pitch is the distance between adjoining crests (say) of the same thread, measured parallel to the axis of the screw. It is specified by the reciprocal of the number of turns per inch (t.p.i.) for Whitworth and Unified screws.

For any one thread form various combinations of major diameter and pitch have been standardized to form screw-thread series; in the BA system major diameters and associated pitches are designated by means of numbers. The more common standard series are given in the tables below for sizes up to about 25 mm (1 in).

ISO inch (Unified) threads

ISO metric threads

Unified Coarse(U N C)

Unified Fine (U N F)

Coarse

Fine

Major
diameter
/in

t.p.i

Major
diameter/
in

t.p.i

Major
diameter
/mm

Pitch/mm

Major
diameter
/mm

Pitch/mm

 

 

 

 

 

 

 

 

 —

 —

(No. 0)    0.0600

80

1.6

0.35

1.6

0.20

(No. 1)    0.0730

64

(No. 1)    0.0730

72

1.8

0.35

1.8

0.20

(No. 2)    0.0860

56

(No. 2)    0.0860

64

2    

0.40

2    

0.25

(No. 3)    0.0990

48

(No. 3)    0.0990

56

2.2

0.45

2.2

0.25

(No. 4)    0.1120

40

(No. 4)    0.1120

48

2.5

0.45

2.5

0.35

(No. 5)    0.1250

40

(No. 5)    0.1250

44

3    

0.50

3    

0.35

(No. 6)    0.1380

32

(No. 6)    0.1380

40

3.5

0.60

3.5

0.35

(No. 8)    0.1640

32

(No. 8)    0.1640

36

4    

0.70

4    

0.50

(No. 10)  0.1900

24

    (No. 10)  0.1900    

32

4.5

0.75

4.5

0.50

(No. 12)  0.2160

24

    (No. 12)  0.2160    

28

5    

0.80

5    

0.50

1

4

20

1

4

28

6    

1.00

6    

0.75

5

16

18

5

16

24

7    

1.00

7    

0.75

3

8

16

3

8

24

8    

1.25

8    

1.00

7

16

14

7

16

20

9    

1.25

10      

1.25

1

2

13

1

2

20

10      

1.50

12      

1.25

9

16

12

9

16

18

11      

1.50

14      

1.50

5

8

11

5

8

18

12      

1.75

16      

1.50

3

4

10

3

4

16

14      

2.00

18      

1.50

7

8

  9

7

8

14

16      

2.00

20      

1.50

1

  8

1

12

18      

2.50

22      

1.50

 

 

 

 

20      

2.50

24      

2.00

 

 

 

 

22      

2.50

 

 

 

 

 

 

24      

3.00

 

 

 

 

 

 

 

 

 

 


Whitworth form threads

BA threads

British Standard Whitworth
(BSW)

British Standard Fine
(BSF)

BA

Major
diameter
/in

t.p.i

Major
diameter/
in

t.p.i

No.

Major
diameter
/mm

Pitch/mm

 

 

 

 

 

 

 

1

4

20

1

4

26

0

6.0

1.00

9

32

26

1

5.3

0.90

5

16

18

5

16

22

2

4.7

0.81

3

8

16

3

8

20

3

4.1

0.73

7

16

14

7

16

18

4

3.6

0.66

1

2

12

1

2

16

5

3.2

0.59

9

16

12

9

16

16

6

2.8

0.53

5

8

11

5

8

14

7

2.5

0.48

11

16

11

11

16

14

8

2.2

0.43

3

4

10

3

4

12

9

1.9

0.39

7

8

  9

7

8

11

10

1.7

0.35

1

  8

1

10

 

 

 

 

 

 

 

 

 

 

References

  • BS 1580 - Unified screw threads - Part 1: Screw threads with diameters ¼ in and larger. Requirements (2007).
  • BS 1580 - Unified screw threads - Part 3: Screw threads with diameters below ¼ in. Requirements (2007).
  • BS 3643 - ISO metric screw threads - Part 1: Principles and basic data (2007).
  • BS 84 - Parallel screw threads of Whitworth form. Requirements (2007).
  • ISO 228 - Pipe threads where pressure-tight joints are not made on the threads - Part 1: Dimensions, tolerances and designationn (2000).
  • BS 93 - British Association (B.A.) screw threads. Requirements (2008).
  • BS EN 10226 - Part 1: Pipe threads where pressure tight joints are made on the threads. Taper external threads and parallel internal threads. Dimensions, tolerances and designation (2004).
  • BS EN 10226 - Part 2: Pipe threads where pressure tight joints are made on the threads. Taper external threads and taper internal threads. Dimensions, tolerances and designation (2005).

O.C.Jones

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