The minor diameter of an internal thread is the bored hole size when using a cutting tap. To calculate tapping drill size for metric threads subtract the pitch from the major diameter, Select the next larger standard drill size. Bored hole diameters and thread engagement. As you mentioned, thread forming has the advantage of adding some compressive stress to the joint so increasing it’s tensile capacity, but on this scale other advantages of Increased root stress concentrations don’t really impact because of the plastic nature, the most important thing is getting the right tap drill diameter. eg for an M5x0.75 thread the tapping drill is 5-0.75mm = 4.25mm.įor Imperial (inch) threads the tapping drill size is calculated in the same way, diameter minus pitch. Note that the tap drill size differs from that used for a cutting tap as shown in most tap drill tables. As a rule of thumb, for Metric threads, the tapping drill can be calculated by subtracting the pitch from the diameter of the thread. Formed threads are typically stronger than cut threads.
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