![]() Example (inch, fine): For 7⁄ 16-20 (same diameter as the previous example, but this time with 20 threads per inch, which is considered fine), 0.437 in × 0.90 = 0.393 in (i.e., if the threads are to be fine, then a slightly larger diameter drill bit should be used before tapping the hole for the screw).Therefore, a size 7⁄ 16 screw ( 7⁄ 16 ≈ 0.437) with 14 threads per inch (coarse) needs a tap drill with a diameter of about 0.371 inches. Example (inch, coarse): For size 7⁄ 16 (this is the diameter of the intended screw in fraction form)-14 (this is the number of threads per inch 14 is considered coarse), 0.437 in × 0.85 = 0.371 in.Some sizes outside that range have different ratios.īelow, these guidelines are explored with examples. The 85% and 90% rules works best in the range of 1⁄ 4–1 in (6.4–25.4 mm), the sizes most important on many shop floors.For both of these rules of thumb (85%/90% and major minus pitch), the tap drill size yielded is not necessarily the only possible one, but it is a good one for general use.Your result will only land near a tap drill size (not directly on one). For example, a screw with a pitch of 1/20 in (20 threads per inch) has a pitch of 0.050 in and a 1⁄ 13 in pitch (13 threads per inch) has a pitch of 0.077 in. The major minus pitch technique also works for inch-based threads, but you must first calculate the pitch by converting the fraction of threads-per-inch (TPI) into a decimal. For metric V threads, the concept of major minus pitch (i.e., the major or widest diameter of the intended screw in millimeters minus the pitch of the threads of that screw in millimeters per thread) yields a good tap drill diameter. ![]()
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