Fig. 525.
The means for giving motion to the feed screw and for enabling a quick change from the coarse roughing feed to a finer finishing feed to the cutting tool without requiring to change the gears or alter their positions, is shown in [Fig. 525]. a and b are two separate pinions bored a working fit to the end of the driving shaft s, but pierced in the bore with a recess and having four notches or featherways h. The end of the driving shaft s is pierced or bored to receive the handled pin i, and contains four slots to receive the four feathers j which are fast in i. In the position shown in the figure these feathers engage with neither a nor b, hence the driving shaft would remain motionless, but it is obvious that if pin i be pushed in the feathers would engage b and therefore drive it; or if i were pulled outwards the feathers would engage a and drive it, because a and b are separate pinions with a space or annular recess between them sufficient in dimensions to receive the feathers. The difference in the rate of feed is obviously obtained through the difference in diameters of the pair of wheels a, c and the pair d, b, the lathe giving to the lead screw the slowest motion and, therefore, the finest feed.
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Fig. 526.
The means for throwing the carriage in and out of feed gear with the feed screw and of providing a hand feed for operating the tool in corners or for quickly traversing the carriage, is shown in [Fig. 526], in which s represents the feed screw and b a bracket or casting bolted to the carriage and carrying the hand wheel and feed mechanism shown in the general cut figure.
b provides a slide way denoted by the dotted lines at b, for the two halves n and n′ of the feed nut. It also carries a pivot pin shown at p in the front elevation, which screws into b as denoted by p′ in the end view; upon this pivot operates the piece d, having the handle d. In d are two cam grooves a a; two pins n, which are fast in the two half-nuts n n′, pass through slots c c in b, and into the cam grooves a a respectively.