This Standard provides for two general applications of Acme threads: namely, general purpose and centralizing. The limits and tolerances in this Standard relate to single-start Acme threads and may be used, if considered suitable, for multiple-start Acme threads. The latter threads are used to provide relatively fast traversing motion when necessary. The three classes (2G, 3G, and 4G) of general purpose threads have clearances on all diameters for free movement. This thread relies on the thread flanks to maintain concentric operation. The three classes of centralizing threads have a limited clearance at the major diameters of the external and internal threads so that a bearing at the major diameter maintains approximate alignment of the thread axis and prevents wedging on the flanks of the threads. For any combination of the three classes of threads covered in this Standard, some end play or backlash will result. This is unavoidable for interchangeable products. When backlash or end play is objectionable, some mechanical means should be provided to eliminate the condition. In any case, sufficient end play must be left to provide a close running fit. In addition to limiting dimensions for the standard series of diameters and pitches of ACME single-start threads, tables of tolerances, in terms of pitch and diameter, provide for a wide choice of diameters for a given standard pitch. By using the formulas for diameter and pitch increments, the pitch diameter tolerances for special diameters and pitches can be determined for each class. Formulas and data are also provided for allowances on external threads and major and minor diameter allowances and tolerances. The Appendices provide text and dimensions on the following: (a) Alternate Centralizing ACME Threads; (b) Multiple-Start ACME Threads; (c) General Purpose ACME Threads, Class 5G; (d) Centralizing ACME Threads, Classes 5C and 6C; (e) Three-Wire Method of Measurement of Pitch Diameter of 29 deg External ACME Screw Threads; (f) Ball Methods for Internal Pitch Diameter Measurement of 29 deg ACME Screw Threads; (g) Go Gage Compensation, Calculation of Flank Angle, Limit Gaging of Setting Rings, and Gaging Problem Areas; (h) Tolerances for ACME Screw Thread Gages over 5 in.; (i) Determining Limits of Size for Special Diameter/Pitch Combinations.
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