Machinery's Handbook, 31st Edition
MEASURING SCREW THREADS 2125 particularly in measuring 60-degree threads. If lead angles are large enough to greatly affect the result, as with most multiple threads (especially Acme or 29-degree worm threads), a formula should be used that compensates for the lead angle sufficiently to obtain the necessary accuracy. The formulas that follow include 1) a very simple type in which the effect of the lead angle on measurement M is entirely ignored. This simple formula usually is applicable to the measurement of 60-degree single-thread screws, except possibly when gage-making accuracy is required; 2) formulas that do include the effect of the lead angle but, neverthe less, are approximations and not always suitable for the higher lead angles when extreme accuracy is required; and 3) formulas for the higher lead angles and the most precise classes of work. Where approximate formulas are applied consistently in the measurement of both thread plug gages and the thread “setting plugs” for ring gages, interchangeability might be secured, assuming that such approximate formulas were universally employed. Wire Sizes for Checking Pitch Diameters of Screw Threads.— In checking screw threads by the 3-wire method, the general practice is to use measuring wires of the so-called “best size.” The “best-size” wire is one that contacts at the pitch line or midslope of the thread because then the measurement of the pitch diameter is least affected by an error in the thread angle. In the following formula for determining approximately the “best-size” wire or the diameter for pitch-line contact, A = one-half included angle of thread in the axial plane. . . cos sec A A 05 05 Best-size wire pitch pitch # # = = For 60-degree threads, this formula reduces to 0.57735 Best-size wire pitch # = Diameters of Wires for Measuring American Standard and British Standard Whitworth Screw Threads Pitch, Inch Wire Diameters for American Standard Threads Wire Diameters for Whitworth Standard Threads Max. Min. Pitch-Line Contact Max. Min. Pitch-Line Contact 4 0.2500 0.2250 0.1400 0.1443 0.1900 0.1350 0.1409 4 1 ∕ 2 0.2222 0.2000 0.1244 0.1283 0.1689 0.1200 0.1253 5 0.2000 0.1800 0.1120 0.1155 0.1520 0.1080 0.1127 5 1 ∕ 2 0.1818 0.1636 0.1018 0.1050 0.1382 0.0982 0.1025 6 0.1667 0.1500 0.0933 0.0962 0.1267 0.0900 0.0939 7 0.1428 0.1283 0.0800 0.0825 0.1086 0.0771 0.0805 8 0.1250 0.1125 0.0700 0.0722 0.0950 0.0675 0.0705 9 0.1111 0.1000 0.0622 0.0641 0.0844 0.0600 0.0626 10 0.1000 0.0900 0.0560 0.0577 0.0760 0.0540 0.0564 11 0.0909 0.0818 0.0509 0.0525 0.0691 0.0491 0.0512 12 0.0833 0.0750 0.0467 0.0481 0.0633 0.0450 0.0470 13 0.0769 0.0692 0.0431 0.0444 0.0585 0.0415 0.0434 14 0.0714 0.0643 0.0400 0.0412 0.0543 0.0386 0.0403 16 0.0625 0.0562 0.0350 0.0361 0.0475 0.0337 0.0352 18 0.0555 0.0500 0.0311 0.0321 0.0422 0.0300 0.0313 20 0.0500 0.0450 0.0280 0.0289 0.0380 0.0270 0.0282 22 0.0454 0.0409 0.0254 0.0262 0.0345 0.0245 0.0256 24 0.0417 0.0375 0.0233 0.0240 0.0317 0.0225 0.0235 28 0.0357 0.0321 0.0200 0.0206 0.0271 0.0193 0.0201 32 0.0312 0.0281 0.0175 0.0180 0.0237 0.0169 0.0176 36 0.0278 0.0250 0.0156 0.0160 0.0211 0.0150 0.0156 40 0.0250 0.0225 0.0140 0.0144 0.0190 0.0135 0.0141 Threads per Inch
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