![]() 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 bit sizes Metric drill set, 1.0–6.0 mm by 0.1 mm, jobber length. Ī comprehensive table for metric, fractional wire and tapping sizes can be found at the drill and tap size chart. The British Standards on replacing gauge size drill bits with metric sizes in the UK was first published in 1959. In nearly all other countries, metric drill bit sizes are most common, and all others are anachronisms or are reserved for dealing with designs from the US. ![]() In the U.S., fractional inch and gauge drill bit sizes are in common use. They can be made in any size to order, but standards organizations have defined sets of sizes that are produced routinely by drill bit manufacturers and stocked by distributors. Drill bits are the cutting tools of drilling machines. ![]()
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