Machinery's Handbook, 31st Edition
2140 MEASURING SCREW THREADS 50-Degree Buttress Thread with Front-face Inclination of 5 Degrees: This buttress thread form is illustrated by the diagram at the right. Measurement M equivalent to the correct pitch diameter E may be determined by Formula (4): (4) Wire diameter W for pitch-line contact at back of thread = 0.606 × pitch. If the width of flat at crest and root = 1 ∕ 8 × pitch, depth = 0.69 × pitch. American National Standard Buttress Threads ANSI/ASME B1.9-1973 (R2017): This buttress screw thread has an included thread angle of 52 degrees and a front face inclination of 7 degrees. Measurements M equivalent to a pitch diameter E may be determined by Formula (5): (5) The wire angle correction factor c is less than 0.0004 inch for recommended combina - tions of thread diameters and pitches and may be neglected. Use of wire diameter W = 0.54147 P is recommended. Measurement of Pitch Diameter of Thread Ring Gages.— The application of direct meth - ods of measurement to determine the pitch diameter of thread ring gages presents serious difficulties, particularly in securing proper contact pressure when a high degree of preci - sion is required. The usual practice is to fit the ring gage to a master setting plug. When the thread ring gage is of correct lead, angle, and thread form, within close limits, this method is quite satisfactory and represents standard American practice. It is the only method available for small sizes of threads. For the larger sizes, various more or less satisfactory methods have been devised, but none of these have found wide application. Screw Thread Gage Classification.— Screw thread gages are classified by their degree of accuracy, that is, by the amount of tolerance afforded the gage manufacturer and the wear allowance, if any. There are also three classifications according to use: 1) Working gages for controlling production; 2) inspection gages for rejection or acceptance of the finished product; and 3) reference gages for determining the accuracy of the working and inspection gages. Gaging Unified Screw Threads.— ANSI/ASME B1.2-1983 (R2017) covers gaging methods for conformance of Unified Screw threads and provides the essential specifica - tions for applicable gages required for unified inch screw threads. The standard includes the following gages for Product Internal Thread: GO Working Thread Plug Gage for inspecting the maximum-material GO functional limit. NOT GO (HI) Thread Plug Gage for inspecting the NOT GO (HI) functional diameter limit. Thread Snap Gage—GO Segments or Rolls for inspecting the maximum-material GO functional limit. Thread Snap Gage—NOT GO (HI) Segments or Rolls for inspecting the NOT GO (HI) functional diameter limit. Thread Snap Gages—Minimum Material: Pitch Diameter Cone Type and Vee and Thread Groove Diameter Type for inspecting the minimum-material limit pitch diameter. Thread-Setting Solid Ring Gage for setting internal thread indicating and snap gages. Plain Plug, Snap, and Indicating Gages for checking the minor diameter of internal threads. Snap and Indicating Gages for checking the major diameter of internal threads. Functional Indicating Thread Gage for inspecting the maximum-material GO functional limit and size and the NOT GO (HI) functional diameter limit and size. . 091955 . 32235 M E P = − ^ W # # + h ^ h . P W c 0 89064 3 15689 = − + + . M E
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