A cobalt drill bit is a drill bit made for special drilling applications that is composed of steel with a high cobalt content compared to that of other steels, of usually between 5% and 8%. While this makes the drill bit more brittle than traditional steel drill bits, it also makes it much more durable as well as giving it an abrasive quality that helps it to cut through hard materials repeatedly. Ranking cobalt drill bit hardness in comparison to other types of specialized steels puts it above that of high-strength steel (HSS), but below that of titanium steel. Common applications for using a cobalt drill bit include factory settings where a fixed piece of stainless steel or iron needs to be cut repeatedly. The bits work best for linear, controlled cutting of hard materials, and is not ideal for softer materials like wood or plastic because drilling fragments tend to clog up the drilling hole and put angular stress on the bit.
One of the chief advantages of cobalt drill bits over many other types of high-strength drill bits is that they do not obtain their hardness through just a surface coating of hardened alloys. Since a cobalt drill bit's hardness is uniform throughout the entire material, it can be repeatedly sharpened as needed. This is in particular contrast to titanium drill bits, which are better at cutting hard materials than a cobalt drill bit, but the titanium surface of such drill bits cannot be sharpened without destroying the cutting edge. In comparison to HSS drill bits, a cobalt bit can cut on average about 2,200 holes for every 2,000 holes that a HSS drill bit can cut before needing to be sharpened.
Industry names for types of drill bits with cobalt content are M40CO, M-35, or M-42. The M-35 drill bit contains 5% cobalt and the M-42 bit contains 8% cobalt. The higher the cobalt content, the harder and more brittle it becomes. Cobalt bits also are able to withstand higher temperatures without deforming than typical HSS drill bits can. While ordinary iron on the MOHS hardness scale is rated at an equivalent level of 4 and stainless steel at a level 5, cobalt steel is at the higher end of all steels that can have a hardness rating up to 6.3. Among the main industries where using a cobalt drill bit is common is in the aviation manufacturing industry due to the fact that titanium and strengthened steel components are often a part of aircraft construction. They are also used to manufacture rifle parts where they can bore holes to precise levels.
When a metal drill bit containing cobalt is manufactured, it often contains other elements like carbon and titanium for hard surface drilling and black oxide for softer drilling in aluminum and rigid plastics. The surface of the bit is also heat-treated in the manufacturing process, which gives the metal a brownish-gold tint that makes it easy to distinguish from other types of drill bits. Even though cobalt bits are superior in many ways to other types of drill bits, it is still recommended where possible to cool the drilling location with oil while cutting to enhance the life of the drill bit. Since such drill bits are designed to handle high temperatures during the cutting of tough materials, they are rated to withstand prolonged heat of up to 1,100° Fahrenheit (593° Celsius). The ability to withstand such high temperatures without breaking down makes the cobalt drill bit about 30% faster in drilling applications than other types of high strength bits.