The object of having three pair of gauge tongues is to enable the obtaining of three degrees of fit; thus with a piece placed between d k the hole may be bored to fit the piece easily, with it placed between e l the fit may be made barely movable, while with it placed between f m the fit may be too tight to be a movable one save by pressure or driving, each degree of fit being adjusted by means of the screws e f g.

The tool is fed by moving spindle o by hand, the screw p being adjusted so that its end abuts against stop q, when the hole is bored to the requisite depth; r is simply a guide for the piece s, which being attached to o, prevents it from rotating.

In watch manufactories special chucks and appliances are necessary to meet their particular requirements. There is found to exist, for example, in different rods of wire of the same nominal diameter, a slight variation in the actual diameter, and it is obvious that with the smaller diameters of wire the split chucks will pass farther within the mouth h of a, [Fig. 651], because the splits of the chucks will close to a greater extent, and the cones on the chucks therefore become reduced in diameter.

If then it be required to turn a number of pieces of work to an exact end measurement, or a number of flanges or wheels to equal thicknesses, without adjusting the depth of cut for each it becomes necessary to insure that the successive pieces of work shall enter the chucks to an equal distance, notwithstanding any slight variation in the work diameter at the place or part where it is gripped by the chuck.

To accomplish this end what is termed a sliding-spindle head is employed. In this the outer spindle has the end motion necessary to open and close the chuck, the chuck having no end motion.

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Fig. 666.

The construction of this sliding-spindle head is shown in [Fig. 666], in which a wire chuck is shown in position in the spindles; l is the live spindle passing through parallel bearings, so that it may have end motion when the nut m is operated. The inner spindle n to which the chucks are screwed is prevented from having end motion by means of the collar p and nut q at the rear bearing. When nut m is rotated and n is held stationary by means of the pulley p, l slides endways, and the chuck opens or closes according to the direction in which the nut moves the spindle l.