in turn drives a pinion geared to another rack in the sliding bed of the attachment, thus giving the necessary longitudinal movement. In the cut the former is shown in position on the face of the worm wheel.
The attachment is sometimes driven automatically by means of a round belt leading from a small jack-shaft to a three-step cone pulley fastened on the end of the worm shaft. The pulley is clutched to the worm so that either hand or automatic feed may be used by the simple movement of a lever. Illustrations of the use of this attachment are to be found in Chapter IX.
Scales and Verniers for Milling Machines. Scales and verniers are useful on such work as boring jigs, fixtures, or wherever extreme accuracy is required and it is necessary to make fine adjustments of the table. The scales are graduated to 40ths of an inch, and the verniers read to thousandths of an inch. A machine with all of the table adjustments fitted with scales and verniers is shown in Fig. 45.
Spring Chucks. Fig. 46 shows an unassembled spring chuck. This chuck is convenient for holding wire, small rods, straight shank drills, mills, etc. The collet holder is of steel, ground to fit the standard taper hole of the machine spindle, and has a hole its entire length. The front end is fitted to receive a spring collet, which is held in place by a cap nut that forces it against the taper seat and closes the chuck centrally. A nut is provided for withdrawing the collet holder from the spindle.
In addition to the attachments already mentioned in this chapter, there are many minor fixtures frequently used in milling operations. These are spoken of in connection with general notes on milling in Chapter VII.
Fig. 46
