Available in any size, metric drill bits are the most commonly found.  In the United States, gage and fractional are used as standard drill bit sizes.  Drill bits are the cutting element of drilling machines.   They can be custom made to any size, however, the standards organizations have created set sizes which are now industry standards and sold in hardware stores and made by drill manufacturers such as Irwin, Makita and Hilti.

Metric sizes measure the diameter of the drill bit using standard metric lengths.  Standards organizations such as the British Standards Organization, or BSO, define the sizes and it is those sets of sizes that we commonly see in retail outlets today.  In 1959 the BSO published the new metric drill bit sizes when the industry moved from gage measurements to metric.

The smaller sized drill bits are made in smaller increments of diameter.  The availability and price of drill bits does not move in uniform with the range of sizes.  A set of drill bits at size increments of 1 mm are most commonly found in shops or DIY stalls and they are usually the most cost effective.  Builders merchants usually stock 0.1 mm increments.  Hardware shops usually stock 0.5 mm increments.

Other than bits under 1 mm diameter, it is unusual to find drill bit sets in sizes smaller than this.  Unusual sizes like 2.50 mm for example are required for specialized tasks and need to be specially ordered through a supplier.  With number gage drill bits, it is common to easily access every size range on the market.

In the USA, fractional inch drill bits are commonly used just as they were in other parts of the world.  But metric dimensioning is increasingly favored.  There are however some disadvantages to the fractional inch incremental measurements such as, for the smaller drill bit sizes, the increment between the measurements is quite large at 100%.  This means that number gage bits are needed to compensate for the gap.

Regarding letter gage and US number sizes of drill bit, both of which are commonly used in the US today, the operator is required to used a chart of drill bit sizes to work out the size of bit needed for the task in hand.  With metric, this is not necessary.

Number gage is similar but different from the American wire gage system used for the diameters of nonferrous, round and solid wire that conducts electricity.  Number gage is used from size 1, the largest measurement, to size 80, the smallest.

Letter and number gage drill bits are normally always twist drill bits.  The gage can be used to measure other types of bit however, the twist drill bit is commonly used in a size range covered buy  the gage.  There is no actual set formula that is used by the gage to diameter size conversion, however it is seen as a useful measure.

So, there are many different ways to measure drill bits, but the metric system is widespread.  Companies that supply metric drill bits and drill accessories include Hilti, Makita and Irwin.   There are many types on the market including tile drill bits and brad point bits, long and short bits, glass and metal, ceramic, flexible, step drill bits.

Cobalt drill bits can be used on extremely hard, high tensile strength materials like hardened stainless steel (precipitation-hardened), Inconel alloys, cast iron, and titanium but they are also used on softer materials like plastic. They are often used in the aviation industry.

There are two grades of cobalt bit. M35 has 5% cobalt added to raise resistance to heat and M42 has 8% of cobalt added to utilize its wear resistance properties as it is less prone to breaking and chipping compared to other bits. The most popular types of cobalt drill bit are jobber length, numbered measured, fractional measured, 6 inch and 12 inch extended length.

Bosch cobalt drill bits include an alloy that enables bits to heat to 1,100 degrees Fahrenheit and drill for an extended length of time at these higher temperatures. These temperatures that would normally destroy other bits made from less heat resistant material. They last longer in highly abrasive materials. The overall bit hardness can be increased to 68 HRC which is ideal for drilling in abrasive or tough materials. A thick web design increases the rigidity and prevents the drill bit from snapping when drilling in very hard and abrasive materials. The higher red hardness of cobalt allows speeds of up to 30% higher than other drills.

Twist drill bits are made from a range of materials including cobalt, carbon, black oxide and titanium. The functionality of the drill dictates which of these materials is included – each is designed for a particular purpose. Some materials are hard and require a drill that is resistant to wear and tear and high temperatures. Some materials are softer, so softer metals can be used in making the bit. Each bit has its specific purpose. Bits with black oxide in their formula are for general purpose use for heavy duty drilling in carbon, plastic, soft cast iron and aluminum.

Cobalt alloys hold their hardness at extremely high temperatures, more so than standard bits but their main disadvantage is that they are more brittle and can therefore snap more easily than regular drill bits. Cobalt bits are not coated, they are made solely of cobalt. When manufacture is almost complete, the bit is baked at high temperatures in a special industrial oven which changes the surface color to a subdued golden color. This is mainly done to enable color identification from other bits. Even if the gold color wears away through use or sharpening, it is still solid cobalt steel through and through.

Irwin is a popular drill manufacturer who use M42 grade (contains 8% cobalt) in their bits. A correctly sharpened a cobalt bit is still as good as a brand new bit – its performance is not affected. However, sharpening other coated or tipped drill bits such as titanium for example, the performance is decreased in effectiveness as the coating is worn away.

Companies that supply drill bits and accessories include Makita, Hilti and Irwin as mentioned. Along with cobalt drill bits, there are many other types on the market including tile drill bits and brad point bits, long and short bits, glass and metal, ceramic, flexible, step drill bits.