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TAP DRILL SIZES AND MINOR DIAMETER CONSTANTS, Continued . . . For example, it is entirely practical to produce a 1/4"-20 UNC thread of 80% in 1/8" thick sheet metal; however, it would not be practical to produce this percentage of thread in a hole tapped to a 1/2-inch depth. Very fine pitch threads are difficult to tap with a high percentage even in the case of shallow holes. For the most economical tapping, the percentage of threads produced should always be as low as permissible, which means that the tap drill used should be the largest diameter that the specifications will permit. Minor diameters for various percentages of American National or Unified form of thread can be obtained by using the constants shown below - It must be remembered, however, that the values obtained are minor diameters and not drill sizes. |
