#LEFT HAND DRILL BITS WIRE GAGE SERIES#
Other countries that formerly used the number series have for the most part also abandoned these in favour of metric sizes. and to a lesser extent the UK, where they have largely been supplanted by metric sizes. Number and letter gauge drill bits are still in common use in the U.S. Each step along the horizontal axis is one gauge size. The accompanying graph illustrates the change in diameter with change in gauge, as well as the reduction in step size as the gauge size decreases. The gauge-to-diameter ratio is not defined by a formula it is based on-but is not identical to-the Stubs Steel Wire Gauge, which originated in Britain during the 19th century. Number and letter sizes are commonly used for twist drill bits rather than other drill forms, as the range encompasses the sizes for which twist drill bits are most often used. Although the ASME B94.11M twist drill standard, for example, lists sizes as small as size 97, sizes smaller than 80 are rarely encountered in practice. Number drill bit gauge sizes range from size 80 (the smallest) to size 1 (the largest) followed by letter gauge size A (the smallest) to size Z (the largest). Machinists generally truncate the decimals after thousandths for example, a 27/64" drill bit may be referred to in shop-floor speech as a "four-twenty-one drill".) 75 are shown to thousandths, which is how machinists usually think about them. So, instead of 78/64 inch, or 1 14/64 inch, the size is noted as 1 7/32 inch.īelow is a chart providing the decimal-fraction equivalents that are most relevant to fractional-inch drill bit sizes (that is, 0 to 1 by 64ths). One aspect of this method of sizing is that the size increment between drill bits becomes larger as bit sizes get smaller: 100% for the step from 1/64 to 1/32, but a much smaller percentage between 1 47/64 and 1 3/4.ĭrill bit sizes are written as irreducible fractions. For Morse taper-shank drill bits, the standard continues in 1/64 inch increments up to 1¾ inch, then 1/32 inch increments up to 2¼ inch, 1/16 inch increments up to 3 inches, 1/8 inch increments up to 3¼ inches, and a single 1/4 inch increment to 3½ inches. market.ĪNSI B94.11M-1979 sets size standards for jobber-length straight-shank twist drill bits from 1/64 inch through 1 inch in 1/64 inch increments. For example, a set of Forstner bits may contain 10, 15, 20, 25 and 30 mm diameter cutters.įractional-inch drill bit sizes are still in common use in the United States and in any factory (around the globe) that makes inch-sized products for the U.S. Metric dimensioning is routinely used for drill bits of all types, although the details of BS 328 apply only to twist drill bits. There are also Renard series sequences of preferred metric drill bits: This subsetting of standard sizes is in contrast to general practice with number gauge drill bits, where it is rare to find a set on the market which does not contain every gauge. Drill bits of the less routinely used sizes, such as 2.55 mm, would have to be ordered from a specialist drill bit supplier. Sets are not commonly available in smaller size increments, except for drill bits below 1 mm diameter. In 0.1 mm increments, any engineers' store. In 0.5 mm increments, any hardware store.
Sets of bits in 1 mm increments might be found on a market stall. Bits at size increments of 1 mm are most commonly available, and lowest price. The price and availability of particular size bits does not change uniformly across the size range. This reflects both the smaller drilled hole diameter tolerance possible on smaller holes and the wishes of designers to have drill bit sizes available within at most 10% of an arbitrary hole size. In smaller sizes, bits are available in smaller diameter increments. For example, British Standard BS 328 defines 230 sizes from 0.2 mm to 25.0 mm.įrom 0.2 through 0.98 mm, sizes are defined as follows, where N is an integer from 2 through 9:įrom 1.0 through 2.95 mm, sizes are defined as follows, where N is an integer from 10 through 29:įrom 3.0 through 13.9 mm, sizes are defined as follows, where N is an integer from 30 through 139:įrom 14.0 through 25.0 mm, sizes are defined as follows, where M is an integer from 14 through 25: Standards organizations define sets of sizes that are conventionally manufactured and stocked. Metric drill bit sizes define the diameter of the bit in terms of standard metric lengths. The case that holds them in an indexed order (by size), via a graduated series of holes, is called a drill index. Metric drill set, 1.0–6.0 mm by 0.1 mm, jobber length.
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