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". Table 2.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". The RH class of fluteless taps is determined by the tolerances (1, 2, 3B, 2B, etc.) for the internal thread. The limits areĮstablished in increments of 12.7µm.
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