Fig. 190 shows three twist drills and a countersink which are much used with drilling machines. Drill C is made of a twisted bar of highspeed steel. Drills A and C have taper shanks. They are placed in a taper socket and the socket is placed in the end of the drill spindle. Drill B and the countersink have straight shanks. They are held in a small chuck shown in Fig. 191. Small drills have straight shanks. The countersink is used for reaming the end of a hole to the shape of a lathe center.
Fig. 191. - Drill Chuck.
A vise, of which Fig. 193 is one type, is necessary for holding small work accurately on the drill table.
When the drilling of holes in duplicate pieces of work is repeated many times, a jig is made with the holes drilled through it in correct position and this is used as a guide for drilling by placing it over the work and thereby avoiding the necessity of marking the holes on each piece. Work of this kind is quickest done on a multiple spindle drill. Jigs in simplest form are made from plates of cast iron or rolled steel. Many forms of jigs, however, are very elaborate, consisting of cast-iron boxes with guide holes drilled through the sides at the desired angles and protected by bushings. A piece of work to be drilled is placed in one of these box jigs and secured in a particular position to insure drilling similar holes alike in every piece.