• Example (inch, coarse): For size (this is the diameter of the intended screw in fraction form)-14 (this is the number of threads per inch;
14 is considered
coarse), 0.437in × 0.85 = 0.371in. Therefore, a size screw ( ≈ 0.437) with 14 threads per inch (coarse) needs a tap drill with a diameter of about 0.371 inches. • The drill sizes that are near this are letter U (0.368in; 84.2%), 9.5 mm (0.374in; 85.6%), and in (0.375in; 85.8%); any of these will work well. • Example (inch, fine): For -20 (same diameter as the previous example, but this time with 20 threads per inch, which is considered
fine), 0.437in × 0.90 = 0.393in (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). • The drill sizes that are near this are (0.391in; 89.4%), 10 mm (0.393in; 90%), and letter X (0.397in; 90.8%); any of these will work well. • Example (metric, coarse): For M7.0×1.0 (an intended screw with a diameter of 7.0 mm and a pitch of 1 mm between each thread, which is considered
coarse), 7.0mm × 0.85 = 5.95mm. • The drill sizes that are near this are in (85%), 6.0 mm (85.7%), and 6.1 mm (87.1%); any of these will work well. • Example (metric, fine): For M7.0×0.5, 7.0mm × 0.90 = 6.30. • The drill sizes that are near this are 6.3 mm (90%), in (90.7%), 6.4 mm (91.4%), and 6.5 mm (92.9%); any of these will work well. • For
metric V threads,
major minus pitch yields a good tap drill diameter. • Example (metric, coarse): For M7.0 × 1.0, 7.0 − 1.0 = 6.0 • Example (metric, fine): For M7.0 × 0.5, 7.0 − 0.5 = 6.5 • (The 85% coarse, 90% fine guideline, within its effective range, matches this in net effect) • The
major minus pitch also works for inch-based threads, but you must first determine the pitch by looking at the number of treads per inch (TPI; for example, = 0.050 and ≈ 0.077), and your result will only land
near a tap drill size (not directly on one). • Example (inch coarse): For -14, 1in ÷ 14 = 0.071in; 0.437in − 0.071in = 0.366in; • The drill sizes that are near this are 9.3 mm (0.366in) and letter U (0.368in). In addition, 9.5 mm (0.374in), and in (0.375in) will work well, although
major minus pitch by itself does not tell you this; but the 85% ± 2
pp guideline supports it. • Example (inch fine): For -20, 1in ÷ 20 = 0.050in; 0.437in − 0.050in = 0.387in; • The drill sizes that are near this are letter W (0.386in) and in (0.391in). In addition, 10 mm (0.393in), and letter X (0.397in) will work well, although
major minus pitch by itself does not tell you this; but the 90% ± 2
pp guideline supports it. • 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. • Example (metric coarse): For M7.0×1.0, major minus pitch yields 6.0, but 6.1 also works well. • Example (metric fine): For M7.0×0.5, major minus pitch yields 6.5, which at 92.9% happens to be an example that pushes over the outer bound of the 90% ± 2
pp, but
major minus pitch is still valid, although smaller drills (6.3 mm, , 6.4 mm) will work well. ==Chart==