Drill bit sizes
From Wikipedia, the free encyclopedia
Drill bits are the cutting tools of drilling machines. They can be made in any size to order, but standards organisations 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 Standard on replacing gauge size drill bits with metric sizes in the UK was first published in 1959.
A comprehensive table for metric, fractional wire and tapping sizes can be found at the drill and tap size chart.
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[edit] Metric drill bit sizes
Metric drill bit sizes define the diameter of the bit in terms of standard metric lengths. Standards organisations define sets of sizes that are conventionally manufactured and stocked. For example, British Standard BS 328 defines sizes from 0.2 mm to 25.0 mm.
From 0.2 through 0.98 mm, sizes are defined as
- N · 0.1 mm,
- N · 0.1 + 0.02 mm,
- N · 0.1 + 0.05 mm, and
- N · 0.1 + 0.08 mm, with N an integer from 2 through 9.
From 1.0 through 2.95 mm, sizes are defined as
- N · 0.1 mm and
- N · 0.1 + 0.05 mm, with N an integer from 10 through 29.
From 3.0 through 13.9 mm, sizes are defined as
- N · 0.1 mm, with N an integer from 30 through 139.
From 14.0 through 25.0 mm, sizes are defined as
- M · 1 mm,
- M · 1 + 0.25 mm,
- M · 1 + 0.5 mm, and
- M · 1 + 0.75 mm, with M an integer from 14 through 25.
In smaller sizes, bits are available in smaller diameter increments. This reflects both the smaller drilled hole diameter tolerance possible on smaller holes, and also the wishes of designers to have drill bit sizes available within at most 10% of an arbitrary size hole.
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.
Metric dimensioning is routinely used for drill bits of all types, although the details of BS328 apply only to twist drill bits. For example, a set of forstner bits may contain 10, 15, 20, 25 and 30 mm diameter cutters.
[edit] Fractional-inch drill bit sizes
Fractional inch drill bit sizes are still in common use in the US. In the past, they were popular elsewhere, but now have been largely discarded in favour of metric sizes.
ANSI 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 morse taper shank drill bits, the standard continues in 1/64 inch increments up to 1 3/4 inch, then 1/32 inch increments up to 2 1/4 inch, 1/16 inch increments up to 3 inches, 1/8 inch increments up to 3 1/4 inches, and a single 1/4 inch increment to 3 1/2 inches.
One disadvantage of this scheme of sizing is that the size increment between drill bits is very large for the smaller sizes, 100% for the first step. The implication is that number gauge drill bits have to be used to bridge the gaps.
Another disadvantage is the convention in labelling the bits. Rather than an integral number of 64ths of an inch, drill bit sizes are written down as irreducible fractions. So, instead of 78/64 inch, or 1 14/64 inch, the size is always written as 1 7/32 inch. This can lead to confusion and mistakes unless great care is taken.
[edit] US number and letter gauge drill bit 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 favour of metric sizes. A complication of using a gauge rather than a standard measurement of length is that the user always has to refer to a table of gauge sizes. For example, to drill a hole which is 10 thou (thousandth of an inch = .001) bigger than a 1/8 inch bolt, the user must first work out that 1/8 inch is 125 thou, so the hole size needed is 135 thou, and then must look up in a reference book which gauge drill bit has this diameter - a number 29.
Number drill bit gauge sizes are analogous to, but different from, american wire gauge. Here's a conversion table.
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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. 
[edit] Jobber length drill
Jobber-length drills are most common type of drill. The length of the flutes is ten times the diameter of the drill, that is they will drill a hole that is 10D deep.
[edit] Long series drill bits
The image shows a long series drill compared to its diametric equivalents, all are 11/32 inch in diameter. The equivalent morse taper drill shown in the middle is of the usual length for a taper shank drill. The lower drill bit is the jobber or parallel shank equivalent.
[edit] Centre drill bit sizes
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.
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[edit] See also
 Metalworking:
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Die head | Drill | Drill bit | Drill bit shank | Drill bit sizes | Drill and tap size chart | Drilling | Jig borer | Pin chuck | Screw thread | Taps and dies | Tap wrench |
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