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Three Wire System It is essential in using this method that the micrometers used be accurate and the measuring faces flat and parallel; that the wires used be hardened. The surfaces must be properly finished and the set of wires the same diameter within .00003 inches if measurement within .0001 inch is desired. In the table of thread elements below is a column headed “Diameter of Best-Size Wires.” This column lists the size of the wire, of a given pitch, that touches the thread exactly on the pitch diameter when placed in between two threads. Adjacent is a diagram showing the method of applying the wires for the measurement of a thread. Below will be found the best size wires to use for the various pitches and the constant to subtract from the micrometer reading to obtain the pitch diameter. The wire sizes and constants given are for use with a thread which has an included angle of 60°. To calculate the “over wire measurement” when the “best-size” wires are available, refer to the table of thread elements. To the basic pitch diameter, add the constant for the “best-size” wire. Example: The basic pitch diameter of a 3/8"-16 thread = .3344 The constant for the “best-size” wire = .0541 The measurement over wires = .3885 To calculate the “over wire measurement” when the “best-size” wires are not available, refer to the table of thread elements. Subtract from the pitch diameter, the single height V-thread and to the result add three times the diameter of the available wire.		P = Pitch Diameter D = Wire Diameter W = Measurement Over Wires Example: The pitch diameter of a 3/8"-16 screw = .3344 Minus the single height V-thread = .0541 .2803 Plus three times .040" (available wire) = .1200 The measurement over wires = .4003
Table of Thread Elements Threads Per Inch Pitch Diameter of "Best Size" Wires Constant for "Best Size" Wires Single* Height Symmetrical Thread Form 0.64952P Width of Flat on Crest and Root NC and NF Single Height V -Thread 140 120 100 96 90 .007143 .008333 .010000 .010417 .011111 .0041239 .0048112 .0057735 .0060141 .0064150 .006186 .007217 .008660 .009021 .009623 .004640 .005412 .006495 .006766 .007217 .0009 .0010 .0013 .0013 .0014 .006186 .007217 .008660 .009021 .009623 80 72 64 60 56 .012500 .013888 .015625 .016667 .017857 .0072168 .0080182 .0090210 .0096225 .0103097 .010825 .012027 .013531 .014434 .015464 .008119 .009021 .010149 .010826 .011598 .0016 .0017 .0020 .0021 .0022 .010825 .012027 .013531 .014434 .015464 50 48 44 40 36 .020000 .020833 .022727 .025000 .027777 .0115470 .0120279 .0131214 .0144337 .0160370 .017320 .018041 .019682 .021650 .024055 .012990 .013531 .014762 .016238 .018042 .0025 .0026 .0028 .0031 .0035 .017320 .018041 .019682 .021650 .024055 32 30 28 27 26 .031250 .033333 .035714 .037037 .038462 .0180421 .0192448 .0206194 .0213833 .0222057 .027063 .028867 .030929 .032074 .033308 .020297 .021650 .023197 .024056 .024982 .0039 .0042 .0045 .0046 .0048 .027063 .028867 .030929 .032074 .033308 24 22 20 18 16 .041666 .045454 .050000 .055555 .062500 .0240558 .0262428 .0288675 .0320746 .0360843 .036083 .039364 .043301 .048112 .054126 .027063 .029523 .032476 .036084 .040595 .0052 .0057 .0062 .0069 .0078 .036083 .039364 .043301 .048112 .054126 14 13 12 11 1/2 11 .071428 .076923 .083333 .086956 .090909 .0412389 .0444114 .0481123 .0502040 .0524863 .061858 .066617 .072168 .075306 .078729 .046394 .049963 .054126 .056480 .059047 .0089 .0096 .0104 .0108 .0114 .061858 .066617 .072168 .075306 .078729 10 9 8 7 1/2 7 .100000 .111111 .125000 .133333 .142857 .0577350 .0641499 .0721687 .0769800 .0824784 .086602 .096224 .108253 .115467 .123717 .064952 .072168 .081190 .086602 .092788 .0125 .0139 .0156 .0167 .0179 .086602 .096224 .108253 .115467 .123717 6 5 1/2 5 4 1/2 4 .166666 .181818 .200000 .222222 .250000 .0962246 .1049726 .1154700 .1282998 .1443375 .144336 .157458 .173205 .192449 .216506 .108253 .118094 .129904 .144338 .162380 .0208 .0227 .0250 .0278 .0312 .144336 .157458 .173205 .192449 .216506
*To calculate the pitch diameter of Unified and American National Form Threads, subtracts the figures found in this column from the Basic Outside Diameter. Use the “Three Wire Method” to convert pitch diameter calculations into Over Wire Measurements. NOTE: Symmetrical thread height equivalent to the basic height, h, of the original American National Form.