Taps for tapping screw-holes in watchwork should be made of good steel wire. First soften it by heating to a red, and allow to cool. Then file to a slow taper and thread it cautiously, using plenty of oil. When a full thread has been cut. file it triangular, and smooth the flats with a pivot tile. Harden it by heating to a bright red and plunging in oil or water. Brighten the flats with a smooth emery stick, and lay the tap on a brass plate held over a lamp flame until the brightened flats show a pale straw colour.