MEASURING SCREW THREADS Machinery's Handbook, 31st Edition
2135
Wire Sizes for Three-Wire Measurement of Acme Threads with Lead Angles of Less than 5 Degrees
Threads per Inch
Best Size
Threads per Inch
Best Size
Max.
Min.
Max.
Min.
1
0.51645 0.38734 0.34430 0.25822 0.20658 0.17215 0.12911
0.65001 0.48751 0.43334 0.32501 0.26001 0.21667 0.16250
0.48726 0.36545 0.32484 0.24363 0.19491 0.16242 0.12182
5 6 8
0.10329 0.08608 0.06456 0.05164 0.04304 0.03689 0.03228
0.13000 0.10834 0.08125 0.06500 0.05417 0.04643 0.04063
0.09745 0.08121 0.06091 0.04873 0.04061 0.03480
1 1 ∕ 3 1 1 ∕ 2
2
10 12 14 16
2 1 ∕ 2
3 4
0.03045 Wire sizes are based upon zero helix angle. Best size = 0.51645 × pitch; maximum size = 0.650013 × pitch; minimum size = 0.487263 × pitch. Example: An Acme General Purpose thread, Class 2G, has a 5-inch basic major diameter, 0.5-inch pitch, and 1-inch lead (double thread). Assume the wire size is 0.258 inch. Deter mine measurement M for a thread thickness T at the basic pitch line of 0.2454 inch. ( T is the maximum thickness at the basic pitch line and equals 0.5 P , the basic thickness, −0.259 × allowance from Table 4, page 2061.) . . . . . . . . . M 5 025862 0 258 1 29152 0 48407 0 06701 0 2454 1 12931 0 5 5 056inches 2 # # # = + + + − = ^ h 6 @ Testing Angle of Thread by Three-Wire Method.— The error in the angle of a thread may be determined by using sets of wires of two diameters, the measurement over the two sets of wires being followed by calculations to determine the amount of error, assuming that the angle cannot be tested by comparison with a standard plug gage, known to be cor rect. The diameter of the small wires for the American Standard thread is usually about 0.6 times the pitch and the diameter of the large wires, about 0.9 times the pitch. The total difference between the measurements over the large and small sets of wires is first deter mined. If the thread is an American Standard or any other form having an included angle of 60 degrees, the difference between the two measurements should equal three times the difference between the diameters of the wires used. Thus, if the wires are 0.116 and 0.076 inch in diameter, respectively, the difference equals 0.116 − 0.076 = 0.040 inch. Therefore, the difference between the micrometer readings for a standard angle of 60 degrees equals 3 × 0.040 = 0.120 inch for this example. If the angle is incorrect, the amount of error may be determined by the following formula, which applies to any thread regardless of angle: sin a B A A = − where A = difference in diameters of the large and small wires used B = total difference between the measurements over the large and small wires a = one-half the included thread angle Example: The diameter of the large wires used for testing the angle of a thread is 0.116 inch and of the small wires 0.076 inch. The measurement over the two sets of wires shows a total difference of 0.122 inch instead of the correct difference, 0.120 inch, for a standard angle of 60 degrees when using the sizes of wires mentioned. The amount of error is determined as follows: . . . . . 0082 0040 04878 . sin a 0122 0040 0040 − = = A table of sines shows that this value (0.4878) is the sine of 29 degrees 12 minutes, approximately. Therefore, the angle of the thread is 58 degrees 24 minutes or 1 degree 36 minutes less than the standard angle. =
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