Machinery's Handbook, 31st Edition
2134 MEASURING SCREW THREADS Formulas (3) and (4) are applied to a 29-degree thread, the difference in measurements M or the error resulting from the use of Formulas (3) will be larger. For example, with an Acme thread having a lead angle of about 34 degrees, the difference in values of M ob- tained by the two formulas equals 0.0008 inch. Three-Wire Measurement of Acme and Stub Acme Thread Pitch Diameter.— For single- and multiple-start Acme and Stub Acme threads having lead angles of less than 5 degrees, the approximate three-wire formula given on page 2126 and the best wire size taken from the table on page 2135 may be used. Multiple-start Acme and Stub Acme threads commonly have a lead angle of greater than 5 degrees. For these, a direct determination of the actual pitch diameter is obtained by using the formula: E = M − ( C + c ) in conjunction with the table on page 2136. To enter the table, the lead angle B of the thread to be measured must be known. It is found by the formula: tan B = L ÷ 3.1416 E 1 where L is the lead of the thread and E 1 is the nominal pitch diameter. The best wire size is now found by taking the value of w 1 as given in the table for lead angle B , with interpolation, and dividing it by the number of threads per inch. The value of ( C + c ) 1 given in the table for lead angle B is also divided by the number of threads per inch to get ( C + c ). Using the best size wires, the actual measurement over wires M is made and the actual pitch diameter E found by using the formula: E = M − ( C + c ). Example: For a 5 tpi, 4-start Acme thread with a 13.952 ° lead angle, using three 0.10024- inch wires, M = 1.1498 inches, hence E = 1.1498 − 0.1248 = 1.0250 inches. Under certain conditions, a wire may contact one thread flank at two points, and it is then advisable to substitute balls of the same diameter as the wires. Checking Thickness of Acme Screw Threads.— In some instances it may be preferable to check the thread thickness instead of the pitch diameter, especially if there is a thread thickness tolerance. A direct method, applicable to the larger pitches, is to use a vernier gear-tooth caliper for measuring the thickness in the normal plane of the thread. This measurement, for an American Standard General Purpose Acme thread, should be made at a distance below the basic outside diameter equal to p /4. The thickness at this basic pitch-line depth and in the axial plane should be p /2 − 0.259 × the pitch diameter allowance from the table on page 2056 with a tolerance of minus 0.259 × the pitch diameter tolerance from the table on page 2061 . The thickness in the normal plane or plane of measurement is equal to the thickness in the axial plane multiplied by the cosine of the helix angle. The helix angle may be determined from the formula: tangent of helix angle = lead of thread ÷ (3.1416 × pitch diameter) Three-Wire Method for Checking Thickness of Acme Threads.— The application of the 3-wire method of checking the thickness of an Acme screw thread is included in the Report of the National Screw Thread Commission. In applying the 3-wire method for checking thread thickness, the procedure is the same as in checking pitch diameter (see Three-Wire Measurement of Acme and Stub Acme Thread Pitch Diameter ), although a dif ferent formula is required. Assume that D = basic major diameter of screw; M = measure ment over wires; W = diameter of wires; S = tangent of helix angle at pitch line; P = pitch; T = thread thickness at depth equal to 0.25 P . . . . . T P M D W S 112931 025862 1 29152 0 48407 2 # # # = + − − + ^ ^ h h This formula transposed to show the correct measurement M equivalent to a given required thread thickness is as follows: . . . . M D W S T P 025862 1 29152 0 48407 112931 2 × × = + + + − ^ h
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online