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]

Drill bit sizes are written as irreducible fractions. So, instead of 78/64 inch, or 1 14/64 inch, the size is noted as 1 7/32 inch.

Such precipitation of carbides is the main reason for the HVOF-sprayed coatings embrittlement and failure during high temperature service.

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 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".)

Carbide CoatingNear me

Some of the systems are at work in Universities and National Labs, but most of them are used in production thermal spray shops.

The shortest standard-length drills (that is, lowest length-to-diameter ratio) are screw-machine-length drills (sometimes abbreviated S/M). They are named for their use in screw machines. Their shorter flute length and shorter overall length compared to a standard jobber bit results in a more rigid drill bit, reducing deflection and breakage. They are rarely available in retail hardware stores or home centers.

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 1/2 in (12.7 mm) diameter drill will be able to drill a hole 4+1/2 in (114.3 mm) deep, since it is 9 times the diameter in length. A 1/8 in (3.175 mm) diameter drill can drill a hole 1+5/8 in (41.275 mm) deep, since it is 13 times the diameter in flute length.[3]

The image shows a long-series drill compared to its diametric equivalents, all are 11⁄32 inch (8.7313 mm) 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.

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 mm, 6 mm, 8 mm, 10 mm, 12 mm, 14 mm, 16 mm and 18 mm. The drills are most ordinarily available with either 90° or 120° included angle points.

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 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. 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.

Tungstencarbide coatingprocess

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.

Extended-reach or long-series drills are commonly called aircraft-length from their original use in manufacturing riveted aluminum aircraft. For bits thicker than a minimum size such as 1⁄8-inch (3.175 mm), they are available in fixed lengths such as 6, 8, 12 or 18 inches (152, 203, 305 or 457 mm) rather than the progressive lengths of jobber drills.

Carbide coatingtypes

Metric drill bit sizes define the diameter of the bit in terms of standard 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.

Chromiumcarbide coating

The combustion temperature in the Kermetico HVAF process is well below the melting point of Cr3C2, which eliminates the possibility of the carbides dissolution into the nickel-chromium matrix during coating application.

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.[2] 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. Each step along the horizontal axis is one gauge size.

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.

The term jobber refers to a wholesale distributor—a person or company that buys from manufacturers and sells to retailers.[citation needed] 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.[citation needed]

The corrosion resistance is provided by the NiCr matrix while the wear resistance is provided mainly by the carbide ceramic phase.

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.

Tungstencarbide coatingthickness

HVOF tungstencarbide coating

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 Forstner bits may contain 10, 15, 20, 25 and 30 mm diameter cutters.

As a result, there is no precipitation of carbides within the nickel-chromium matrix during the service of the HVAF coating and the metal matrix remains ductile.

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.

Tungstencarbide coatingtemperature limit

We proudly design and produce our coatings and our HVAF thermal spray equipment in California and install it all over the world.

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.

Kermetico HVAF applied chrome carbide coatings have excellent resistance to elevated temperature erosion and sliding wear.

Usually, this coating is used at elevated temperatures where tungsten carbide coatings do not work or when faster and a less costly finishing is necessary.

While tungsten carbide coatings provide better wear resistance when temperatures are under 510°C (950°F), high velocity sprayed chrome carbide coatings mitigate abrasive and erosive wear at temperatures up to 750°C (1380°F).

Thermal sprayed chromium carbide coatings combat high-temperature wear due to the high wear resistance imparted by the hard carbide particles and the high temperature oxidation resistant nature of the Cr2O3 oxide formed over both phases of CR3C2-NiCr.While tungsten carbide coatings provide better wear resistance when temperatures are under 510°C (950°F), high velocity sprayed chrome carbide coatings mitigate abrasive and erosive wear at temperatures up to 750°C (1380°F).The corrosion resistance is provided by the NiCr matrix while the wear resistance is provided mainly by the carbide ceramic phase.

TungstenCarbide coatingspecification

Thermal sprayed chromium carbide coatings combat high-temperature wear due to the high wear resistance imparted by the hard carbide particles and the high temperature oxidation resistant nature of the Cr2O3 oxide formed over both phases of CR3C2-NiCr.

Drill bits are the cutting tools of drilling machines. 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.

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 supplanted by metric sizes. Other countries that formerly used the number series have for the most part also abandoned these in favour of metric sizes.[citation needed]

In smaller sizes, bits are available in smaller diameter increments. 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.

Uniform high hardness and ductility, low stress and porosity lead to a substantial increase of service life of parts protected with Kermetico HVAF Cr3C2-NiCr coatings in comparison with HVOF. Our partner in India deposits chrome carbide coatings as well.

A specific characteristic of a Kermetico HVAF chromium carbide coating is that it remains ductile during the long-term service at high temperatures, in particular, above 600oC (1,100°F) .