Hole Diameter = Nominal Major Diameter - 5/8 Pitch x Cosine 30 degrees I'll use a #10-32 Unified screwthread for a worked example: they both have a 60 degree flank angle, a 1/8 Pitch flat at the Major Diameter, and a 1/4 Pitch flat at the Minor Diameter.) (Remember that the basic forms of the Unified and ISO Metric screwthreads are IDENTICAL. ![]() ![]() Either metric or imperial units may be used, but the same unit must be used for both Major Diameter and Pitch.Ī corresponding rule of thumb to calculate the diameter of a hole to be single-start threaded with a Unified or ISO Metric threadform forming tap is "Nominal Major Diameter minus the Single Depth of Thread", which is itself calculated from the basic threadform as "5/8 Pitch times Cosine 30 degrees". This evening's sermon is given from high atop one of my very favorite soapboxes - that understanding the fundamental geometry of the two most common threadforms, the Unified and ISO Metric threadforms, enables we of the proletariat to calculate thread-chart values for ourselves, in the absence sufficient reference material, without resort to memorized "magic numbers":Īs Forrest Addy describes, the rule of thumb to calculate the diameter of a hole to be threaded by material-removing methods (a cutting tap, single-pointing, or milling) for single-start US Standard, Unified, and ISO Metric threadforms is "Nominal Major Diameter minus Pitch".
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