Twist Drills. The operation of milling a twist drill is shown in Fig. 16. The drill is held in a collet, or chuck, and, if very long, is allowed to pass through the spindle of the spiral head. The cutter is brought directly over the centre of the drill, and the table is set at the angle of spiral.
The depth of groove in a twist drill diminishes as it approaches the shank, in order to obtain increased strength at the place where the drill generally breaks. The variation in depth is conditional, depending mainly on the strength it is desirable to obtain, or the usage the drill is subject to. To secure this variation in the depth of the groove, the spiral head spindle is elevated slightly, depending on the length of the flute and diameter of the drill.
The outer end of the drill is supported by the centre rest, and when quite small, should be pressed down firmly, until the cutter has passed over the end.
The elevating screw of this rest is hollow, and contains a small centre piece with a V groove cut therein to aid in holding the work central. This piece may be made in other shapes to adapt it to special work.
Another, and very important operation on the twist drill, is that of "backing off" the rear of the lip, so as to give it the necessary clearance, to prevent excessive friction during drilling. In the illustration, Fig. 17, the saddle is turned about one-half degree as for cutting a right-hand spiral, but as the angle depends on several conditions, it will be necessary to determine what the effect will be under different circumstances. A slight study of the figure will be sufficient for this, by assuming the effect of different angles, mills and the pitches of spirals. The object of placing the saddle at an angle is to cause the mill E to cut into the lip at c', and have it just touch the surface at e'. The line r being parallel with the face of the mill, the angular deviation of the saddle is shown at a, in comparison with the side of the drill.
From a little consideration it will be seen that while the drill has a positive traversing and rotative movement, the edge of the mill at e' must always touch the lip at a given distance from the front edge; this being the vanishing point, if such we may call it. The other surface forming the real diameter of the drill is beyond reach of the cutter, and is so left to guide and steady it while in use. The point e, shown in the enlarged section, shows where the cutter commences, and its increase until it reaches a maximum depth
