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Thread Measuring Wires (60-degree threads) - Calibration
Thread Measuring Wires (60-degree threads) - Calibration

Thread Measuring Wires (60-degree threads) - Calibration


Calibrating external thread pitch diameter with three thread wires is a well-proven method and the most common one in use. Although any size wire between the minimum wire size and the maximum wire size may be used, common practice is to use the best wire size.  This is the size that contacts the thread at the pitch diameter thus eliminating thread angle variation errors.  For 60-degree threads these cardinal wire sizes may be calculated as follows:

1.      Best wire size = 0.57735 / TPI
2.      Minimum wire size = 0.505182 / TPI
3.      Maximum wire size = 1.010363 / TPI

For maximum accuracy, thread wire tolerances are very tight.  The governing authority is the American National Standard Institute/American Society of Mechanical Engineers standard ANSI/ASME B1.2 (English) and ANSI/ASME B1.16M (Metric), appendix B in each. Note that appendices in ANSI/ASME standards are “…included for informational purposes only”.  In other words information therein is not mandatory but has been widely adopted.  Each thread wire is hardened steel and held to these tolerances:

1.      Less than or equal to 20 micro-inches of the best wire size and
2.      The largest of the three wires does not exceed the smallest by more than 10 micro-inches and
3.      The maximum taper over the central inch of wire length does not exceed 10 micro-inches.

Calibration of thread wires is performed at the same force that would be used to measure threads based on that TPI (see chart). The wire is placed perpendicular against a lapped cylinder of a specified size (see chart) and these are then placed between two parallel flat anvils of a high accuracy measuring instrument.

 

Calibrating Thread Wires for 60-Degree Threads

Threads-per-inch

Best Wire Size

Calibration Force

Lapped Cylinder

(TPI)

(inch)

(oz. ± 10%)

(inch)

80

0.00722

8

0.125

72

0.00802

8

0.125

64

0.00902

8

0.125

56

0.01031

8

0.125

50

0.01155

8

0.125

48

0.01203

8

0.125

44

0.01312

8

0.125

40

0.01443

16

0.750

36

0.01604

16

0.750

32

0.01804

16

0.750

30

0.01925

16

0.750

28

0.02062

16

0.750

27

0.02138

16

0.750

26

0.02221

16

0.750

24

0.02406

16

0.750

22

0.02624

16

0.750

20

0.02887

40

0.750

18

0.03208

40

0.750

16

0.03608

40

0.750

14

0.04124

40

0.750

13

0.04441

40

0.750

12

0.04811

40

0.750

11.5

0.05020

40

0.750

11

0.05249

40

0.750

10

0.05774

40

0.750

9

0.06415

40

0.750

8

0.07217

40

0.750

7.5

0.07698

40

0.750

7

0.08248

40

0.750

6

0.09623

40

0.750

5.5

0.10497

40

0.750

5

0.11547

40

0.750

4.5

0.12830

40

0.750

4

0.14434

40

0.750


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Pratt & Whitney offers several machines suitable for use in calibrating thread wires:
1. Labmaster Universal (Model 175, Model 1000A, Model 1000M)
2. Laser Measuring Machine
3. Labmicrometer (Model 900, Model 1600)
4. Universal Supermicrometer (Model 501, Model 504)

ISO 17025 Accredited