Fig. 502.
The construction of the bearings which carry the lead screw in the S. W. Putnam’s improved lathe is shown in [Fig. 502], in which a represents the bearing box for the headstock end of the lathe, having the foot a′ as a base to bolt it to the lathe shears. l represents the lead screw, having on one side of a the collar l′ and on the other the nut and washer n and n′. The seat for the change wheel that operates the lead screw is at l′′, the stop pin l fitting into a recess in the change wheel so as to form a driving pin to the lead screw. The washer n′ is provided with a feather fitting into a recess into l so that it shall rotate with l and shall prevent the nut n from loosening back as it would be otherwise apt to do. End motion to l is therefore prevented by the radial faces of l′ and n′.
At the other end of the lathe there are no collars on the lead screw, hence when it expands or contracts, which it will do throughout its whole length under variations of atmospheric temperature, it is free to pass through the bearing and will not be deflected, bent, or under any tension, as would be the case if there were collars at the ends of both bearings. The amount of this variation under given temperatures depends upon the difference in the coefficients of expansion for the metal of which the lead screw and the lathe shears are composed, the shears being of cast iron while lead screws are sometimes of wrought iron and sometimes of steel.
The bearings at both ends are split, with soft wood placed in the split and a screw to close the split and adjust the bearing bore to fit the journal, in the manner already described with reference to other parts of this lathe.
The construction of the swing frame for carrying the change wheels that go between the driving stud v, [Fig. 494], and that on the seat l′′, [Fig. 502], are as follows:—
Fig. 503.
[Fig. 503] represents the change wheel swing frame, an edge view of which is partly shown at w in [Fig. 494]. s is a slot narrower at a than at b. Into this slot fit the studs for carrying the change wheels.
By enabling a feed traverse in either direction the lathe carriage may be traversed back (for screw-cutting operations) without the aid of an extra overhead pulley to reverse the direction of rotation of the lathe, but in long screws it is an advantage to have such extra overhead pulley and to so proportion it as to make the lathe rotate quicker backwards than forward, so as to save time in running the carriage back.