Boring? Well if we are going to talk about the twist drill what better way to name the article?
The twist drill, so called because the piece of bar that was originally used in its production had two grooves cut into opposite sides and was then twisted to produce the flutes and was the invention of an American gentleman, Steven Morse in 1861. Nowadays the flutes are more likely to be cut into the bar by passing the rotating steel bar past a grinding wheel.
The final application will determine what material is used to make the drill bit and whether or not it will also be coated. Coating the drill bits adds to their lifespan, however they cannot be realistically re-sharpened as the new edges will not have the benefit of the coating that was applied when it was first produced. This factor isn’t really going to be a consideration for the householder or jobbing builder who is more prone to buying a new set of drill bits once they’ve gone past their usefulness, however for major industrial applications, re-sharpening is probably a more practical consideration. The coatings used range from a relatively cheap black oxide to a reassuring expensive diamond or zirconium nitride. Perhaps the most popular coating is titanium nitride which is the coating of choice for High speed steel (HSS) bits.
The majority of the ones in domestic use will tend to be made from high speed steel which is a form of tool steel. The grade of tool steel chosen is normally from the M series and could be M2, M35 or M42. These are alloys with varying levels of carbon, chromium, tungsten, molybdenum, vanadium and in the case of M35 and M42, cobalt. The higher cobalt containing grade is M42 with about 8% Co and a designation of HSS-Co.
In an effort to find a solution that is workable in most situations the point angle of the mass produced drill bits is normally set at about 118 degrees. Obviously this one stop solution isn’t as good as having the correct angle set for each material and when greater degrees of accuracy are required for more delicate or demanding applications drills will be purpose made and set accordingly.
A derivative of the twist drill bit is the lip and spur drill bit. In order to prevent the twist drill bit from wandering it is usual to drill a small pilot hole first. The lip and spur drill bit has been designed for use on wood and instead of the chisel type nose typically found on the twist drill bit, it had a spike and four “shoulders” that help keep the drill bit central as it works its way through the uneven grain structures and wood fibres. They are also better at drilling through soft plastics and given their unique design don’t require a point angle.
The other main derivative is the masonry drill bit. These are, more often than not, made from mild steel and are tipped usually with tungsten carbine which is brazed onto the shank of the drill bit. Given that these drill bits are used to push their way into concrete and brick often with the assistance of the drills “hammer action” special care if given to the design of the shanks so that they fit securely into the drill’s chuck.
Just another little spotlight on the many alloys and metals we find in everyday situations.
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