Twist Drill Bits: Flutes, Point and Coating

A twist drill is named after how it was first made: two grooves were cut in opposite sides of a round bar and the bar was twisted to make helical flutes. Today the flutes are usually ground: the bar turns while it moves past a grinding wheel, the way helical gears are cut. Drills have also been forged with their flutes, from blanks heated by induction. The common hardware-store drill has a 118° point, which works in wood, metal and plastic. A black oxide finish, made in a hot bath of sodium hydroxide with nitrates at about 141 °C that turns the surface to magnetite, adds heat and rust resistance; a titanium nitride coat, a few micrometres thick, can make a high-speed steel bit last three or more times longer.