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{{Short description|Standard cutting tool sizes}}
[[Drill bit]]s are the cutting tools of [[drill|drilling machines]]. They can be made in any size to order, but [[standards organization]]s have defined sets of sizes that are produced routinely by drill bit manufacturers and stocked by distributors.
 
In the U.S., fractional inch and gauge '''drill bit sizes''' are in common use. 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. The [[British Standards]] on replacing gauge size drill bits with metric sizes in the UK was first published in 1959. {{Citation needed|date=March 2022|reason=No British Standard or ISO code supplied.}}
 
A comprehensive table for metric, fractional wire and tapping sizes can be found at the '''[[drill and tap size chart]]'''.
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[[File:drill set 1.jpg|thumb|150px|Metric drill set, 1.0–6.0 mm by 0.1 mm, jobber length. 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 define the diameter of the bit in terms of standard [[Metric system|metric]] lengths. Standards organizations define sets of sizes that are conventionally manufactured and stocked. For example, British Standard BS 328 defines 230 sizes from 0.2 mm to 25.0 mm.
 
From 0.2 through 0.98 mm, sizes are defined as follows, where ''N'' is an integer from 2 through 9:
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The price and availability of particular size bits does not change uniformly across the size range. Bits at size increments of 1 mm are most commonly available, and lowest price. Sets of bits in 1 mm increments might be found on a market stall. In 0.5 mm increments, any hardware store. In 0.1 mm increments, any engineers' store. Sets are not commonly available in smaller size increments, except for drill bits below 1 mm diameter. Drill bits of the less routinely used sizes, such as 2.55 mm, would have to be ordered from a specialist drill bit supplier. 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.
 
There are also [[Renard series]] sequences of preferred metric drill bits:<ref>{{cite web|last1=Dasarathi-Cadem|title=Preferred sizes in engineering|url=http://cadem.com/cncetc/cnc-preferred-sizes/|website=CNC etc|accessdatedate=2 February 2017|access-date=6 September 2017}}</ref>
* R5 (factor 1.58) : M2.5, M4, M6, M10, M16, M24
* R10 (factor 1.26): M3, M5, M8, M12, M20, M30
 
Metric dimensioning is routinely used for drill bits of all types, although the details of BS 328 apply only to twist drill bits. For example, a set of [[drill bit#Forstner bitsbit|Forstner bits]] may contain 10, 15, 20, 25 and 30&nbsp;mm diameter cutters.
 
==Fractional-inch drill bit sizes==
[[File:2010-01-21 Craftsman Professional cobalt drill bit set.jpg|thumb|right|150px|Fractional drill bit set by [[Craftsman (tools)|Craftsman]]]]
 
Fractional -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. market.
 
ANSI B94.11M-1979 sets size standards for jobber-length straight-shank twist drill bits from 1/64&nbsp;inch through 1&nbsp;inch in 1/64&nbsp;inch increments. For [[Machine taper#Morse|Morse taper]]-shank drill bits, the standard continues in 1/64&nbsp;inch increments up to 1¾&nbsp;inch, then 1/32&nbsp;inch increments up to 2¼&nbsp;inch, 1/16&nbsp;inch increments up to 3&nbsp;inches, 1/8&nbsp;inch increments up to 3¼&nbsp;inches, and a single 1/4&nbsp;inch increment to 3½&nbsp;inches. 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 1/64th64 to 1/32nd32, but a much smaller percentage between 1 &nbsp;47/64th64 and 1 &nbsp;3/4.
 
Drill bit sizes are written as [[irreducible fraction]]s. So, instead of 78/64&nbsp;inch, or 1 &nbsp;14/64&nbsp;inch, the size is noted as 1 &nbsp;7/32&nbsp;inch.
 
Below 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). (Decimal places for .25, .5, and .75 are shown to [[thousandth of an inch|thousandths]] [.250, .500, .750], which is how machinists usually think about them ["two-fifty", "five hundred", "seven-fifty"]. 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".)
 
Charts like this, printed at [[poster]] size, are widely posted on machine shop walls for handy reference by machinists as they work. For some tasks it is faster and less annoying to glance down a column of numbers on a poster than it is to punch calculator buttons or do penciled arithmetic. It is also true that with long experience, talented machinists have much of the chart memorized anyway (with decimals to truncated thou). But the poster is there for reference and quick sanity check as needed. Tool distributors often give away such charts as [[promotional merchandise]].
 
For anyone looking to create such a poster or to tailor the design and colors to their own preferences, it is useful to realize that [[spreadsheet]] software makes this fast and easy to do. The tools that the software provides—formulas, cell formatting with rounding, autofill, and sorting by numerical order—obviate any manual calculation and data entry. For example, one can create the inch nominal rows for the chart—with columns for inch fraction, inch decimal, and corresponding millimeter decimal conversion—using formulas, cell formatting with rounding, and autofill; and one can then create the metric nominal rows below that, with the inch decimal conversions being provided by formula; and then one can sort the whole sheet by size order, automatically interposing the inch nominal and metric nominal rows by size order. Font and color can be chosen to make a poster-size chart that is easy to read from 1 or 2 meters away. Printing can be done on a desktop printer, with the letter-size sheets then being stapled together to make up the larger poster.
=== Decimal-fraction equivalents ===
[[File:Decimal-fraction equivalents--v0006.svg|frameless|rightcenter|800px|Decimal-fraction equivalents]]
 
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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). 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 and letter sizes are commonly used for [[drill bit#Twist drill bitsbit|twist drill bits]] rather than other drill forms, as the range encompassencompasses the sizes for which twist drill bits are most often used.
 
The gauge-to-diameter ratio is not defined by a formula,; butit is instead based on, buton—but is not identical to, theto—the [[Stubs Steel Wire Gauge]], which originated in Britain during the 19th century.<ref>{{Cite journal|last=Pöll|first=J. S.|date=June 1999|title=The story of the gauge|journal=Anaesthesia|volume=54|issue=6|pages=575–581|issn=0003-2409|pmid=10403873|doi=10.1046/j.1365-2044.1999.00895.x|s2cid=41519791|doi-access=}}</ref> The accompanying graph, with each step along the horizontal axis being one gauge size, illustrateillustrates the change in diameter with change in gauge, as well as the reduction in step size as the gauge size decreases. Each step along the horizontal axis is one gauge size.
 
Number and letter gauge drill bits are still in common use in the U.S. and to a lesser extent the UK, where they have largely been supersededsupplanted by metric sizes. Other countries that formerly used the number series have for the most part also abandoned these in favour of metric sizes.{{CN|date=October 2020}}
 
[[File:us drill sizes 2.png]]
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==Jobber-length drill==
Jobber-length drills are the most commonly found type of drill. The length of the flutes is between 9 and 14 times the diameter of the drill, depending on the drill size. So a {{convert|1//2|in|mm|1|abbr=on}} diameter drill will be able to drill a hole {{convert|4+1//2|in|mm|1|abbr=on}} deep, since it is 9 times the diameter in length. A {{convert|1//8|in|mm|3|abbr=on}} diameter drill can drill a hole {{convert|1+5//8|in|mm|3|abbr=on}} deep, since it is 13 times the diameter in flute length.<ref>{{Cite web|url=http://www.amesweb.info/Tooling/StraightShankJobbersTwistDrills.aspx|title = Jobber Length Twist Drill Sizes}}</ref>
 
The term ''jobber'' refers to a [[wholesaling|wholesale distributor]]—a person or company that buys from manufacturers and sells to retailers.{{CN|date=October (It was especially widely used in the 1850-1950 period.)2020}} Manufacturers producing drill bits "for the trade" (as opposed to for specialized machining applications with particular length and design requirements) made ones of medium length suitable for a wide variety of jobs, because that was the type most desirable for general distribution. Thus, at the time that the name of jobber-length drill bits became common, it reflected the same concept that names like ''general-purpose'' and ''multipurpose'' reflect.{{CN|date=October 2020}}
 
==Aircraft-length drill==
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==Center drill bit sizes==
[[File:CenterDrills123456.jpg|right|thumb|200px|Center drills, numbers 1 (bottom) through to 6 (top)]]
[[Drill bit#Center and spotting drill bitsbit|Center drills]] are available with two different included angles; 60 degrees is the standard for drilling centre holes (for example for subsequent centre support in the [[lathe]]), but 90 degrees is also common and used when locating holes prior to drilling with twist drills. Center drills are made specifically for drilling lathe centers, but are often used as spotting drills because of their radial stiffness.
 
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Spotting drills are available in a relatively small range of sizes, both metric and imperial, as their purpose is to provide a precise spot for guiding a standard twist drill. Commonly available sizes are 1/8", 1/4", 3/8", 1/2", 5/8", 3/4", 4&nbsp;mm, 6&nbsp;mm, 8&nbsp;mm, 10&nbsp;mm, 12&nbsp;mm, 14&nbsp;mm, 16&nbsp;mm and 18&nbsp;mm. The drills are most ordinarily available with either 90° or 120° included angle points.
 
==See alsoReferences==
{{Reflist}}▼
* [[Drill and tap size chart]]
 
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{{Woodworking}}
 
==References==
▲{{Reflist}}
 
{{DEFAULTSORT:Drill Bit Sizes}}
[[Category:Woodworking]]
[[Category:Machining]]
[[Category:Hole making]]
[[Category:Mechanical standards]]
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