A good tap drill is 85% (± 2 pp) of major diameter for coarse threads, andĪ good tap drill is 90% (± 2 pp) of major diameter for fine threads.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.A good tap drill is 85% (± 2 pp) of major diameter for coarse threads, and 90% (± 2 pp) of major diameter for fine threads.Regarding the proportion of tap drill to thread major diameter, for standard V threads ( ISO V thread and UTS V thread), there are several rules of thumb with strong predictive power: People frequently use a chart such as this to determine the proper tap drill for a certain thread size or the proper tap for an existing hole. Usually thread depths of 60% to 75% are desired. Many thread sizes have several possible tap drills, because they yield threads of varying thread depth between 50% and 100%. Such an appropriately sized drill is called a tap drill for that size of thread, because it is a correct drill to be followed by the tap. Each standard size of female screw thread has one or several corresponding drill bit sizes that are within the range of appropriate size-slightly larger than the minor diameter of the mating male thread, but smaller than its pitch and major diameters. One of the most common ways to produce such threaded holes is to drill a hole of appropriate size with a drill bit and then tap it with a tap. In manufactured parts, holes with female screw threads are often needed they accept male screws to facilitate the building and fastening of a finished assembly. Learn more about Tapping.Below is a comprehensive drill and tap size chart for all drills and taps, imperial and metric, up to 36.5 millimetres (1.44 in) in diameter. This size drill bit should be used for drilling the initial hole that will then be tapped. Lastly, the recommended tap drill size is provided for each standard tap size. For each thread count, the equivalent thread pitch is provided and for metric taps, the approximate thread count is shown based on the pitch. The metric system uses the thread pitch, which is the distance between threads, measured in millimeters. In the fractional and screw size systems, the thread count is used, measured in threads per inch. The thread spacing, which may be coarse or fine, is listed after the diameter. A screw size number corresponds to a diameter which is larger for a higher screw size. Fractional sizes are listed in inches, while metric sizes are listed in millimeters following the letter "M". The decimal equivalents of the diameters are shown in both English and Metric units. The tap size chart provides a list of standard size taps, specifying the diameter and thread spacing, for fractional, metric, and screw sizes.
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