US Number and Letter Gauge Drill Bit Sizes
Number drill bit gauge sizes are analogous to, but different from, American wire gauge. (See the conversion table below).
Number gauge is routinely used from size 80 (the smallest) to size 1 (the largest) followed by letter gauge size A (the smallest) to size Z (the largest). Number gauge is actually defined at least down to size 97, but these smaller sizes are rarely encountered. It happens that as the technology for making small drill bits and drilling small holes has become more available, metric measurements have become the norm.
Number and letter gauge drill bits are almost always twist drill bits. There is no particular reason why the gauge cannot be used to measure bits of other types, but the gauge covers a size range across which the twist drill bit is the most commonly used.
The gauge-to-diameter conversion does not follow a set formula, but rather was defined as a useful and practical measure. The graph shows how gauge diameters change with gauge. Each step along the horizontal axis is one gauge size. The step size between adjacent gauges is smaller for smaller gauges. This is appropriate, because the tolerance of the diameter of drilled holes is closer for smaller drill bits. The increment from one gauge to the next for a number 92 drill bit at 0.2 mm diameter is just 5%, compared to 10% for standard metric sizes. Number and letter gauge drill bits are still in common use in the U.S. In the past, they were popular elsewhere, but now have been largely discarded in favor of metric sizes.
The US number and letter size drills are sized as needed to provide proper clearance holes for screws and bolts according to ASME B18.2.8. There are three fit classes for clearance holes: close, normal, and loose. Some of the clearances required are not in increments of 1/64 or 1/32. This necessitates ranges of drill sizes in between the fractional sizes, especially in the smaller diameter numbered screw sizes.
Read more about this topic: Drill Bit Sizes
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