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.
BSP TAP DRILL CHART ISO
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. 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.
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