Machinery's Handbook, 31st Edition
2170 Diameter of Tap Drill the minor diameter tolerance may be reduced without causing tapping difficulties. In other applications the length of engagement of mating threads may be long because of design considerations or the combination of materials used for mating threads. As the threads engaged increase in number, a shallower depth of engagement may be permitted and still develop stripping strength greater than the external thread breaking strength. Under these conditions the maximum tolerance given in the Standard should be increased to reduce the possibility of tapping difficulties. The following paragraphs indicate how the aforementioned considerations were taken into account in determining the minor diameter limits for various lengths of engagement given in Table 3. Recommended Hole Sizes before Tapping.— Recommended hole size limits before thread - ing to provide for optimum strength of fastenings and tapping conditions are shown in Table 3 for classes 1B, 2B, and 3B. The hole size limit before threading, and the toler ances between them, are derived from the minimum and maximum minor diameters of the internal thread given in the dimensional tables for Unified threads in the screw thread section using the following rules: 1) For lengths of engagement in the range to and including 1 ∕ 3 D , where D equals nominal diameter, the minimum hole size will be equal to the minimum minor diameter of the internal thread and the maximum hole size will be larger by one-half the minor diameter tolerance. 2) For the range from 1 ∕ 3 D to 2 ∕ 3 D , the minimum and maximum hole sizes will each be one quarter of the minor diameter tolerance larger than the corresponding limits for the length of engagement to and including 1 ∕ 3 D . 3) For the range from 2 ∕ 3 D to 1 1 ∕ 2 D the minimum hole size will be larger than the minimum minor diameter of the internal thread by one-half the minor diameter tolerance and the maximum hole size will be equal to the maximum minor diameter. 2) For the range from 1 1 ∕ 2 D to 3 D the minimum and maximum hole sizes will each be one-quarter of the minor diameter tolerance of the internal thread larger than the corresponding limits for the 2 ∕ 3 D to 1 1 ∕ 2 D length of engagement. From the foregoing it will be seen that the difference between limits in each range is the same and equal to one-half of the minor diameter tolerance given in the Unified screw thread dimensional tables. This is a general rule, except that the minimum differences for sizes below 1 ∕ 4 inch are equal to the minor diameter tolerances calculated on the basis of lengths of engagement to and including 1 ∕ 3 D . Also, for lengths of engagement greater than 1 ∕ 3 D and for sizes 1 ∕ 4 inch and larger the values are adjusted so that the difference between limits is never less than 0.004 inch. For diameter-pitch combinations other than those given in Table 3 , the foregoing rules should be applied to the tolerances given in the dimensional tables in the screw thread section or the tolerances derived from the formulas given in the Standard to determine the hole size limits. Selection of Tap Drills: In selecting standard drills to produce holes within the limits given in Table 3 it should be recognized that drills have a tendency to cut oversize. The material on pages 961 and 962 may be used as a guide to the expected amount of oversize.
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