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Fig. 504.

The mechanical devices for transmitting motion from the lead screw to the carriage are shown in [Fig. 504], representing a view from the end and one from the back of the lathe. b is a frame or casting bolted by the bolt b to the carriage a of the lathe. c is a disk having a handle c′ and having rotary motion from its centre. Instead of being pivoted at its centre, however, it is guided in its rotary motion by fitting at d d into a cylindrical recess provided in b to receive it. c contains two slots d and d′ running entirely through it. These slots are not concentric but eccentric to the centre of motion of c. Through these slots there pass two stud bolts e and e′ shown by dotted lines in [Fig. 504], and these bolts perform two services: first by reason of the nuts f and f′ they hold c to its place in b, and next they screw into and operate the two halves g and g′ of a nut.

Suppose, now, that the handle c′ be operated or moved towards arrow e, then the dot at f being the centre of its motion and the slots d and d′ gradually receding from f as their ends g are approached they will cause e to move vertically upward and e′ to move vertically downward, a slot in b (which slot is denoted by the dotted lines h) guiding them and permitting this vertical movement.

Since e and e′ carry the two halves of the nut which envelops the lead screw l it is obvious that operating c′ will either close or release the half nuts from l according to which direction it (c′) is moved in.

The screws h and h′ screw tightly into b, and the radial faces of their heads are made to have a fair and full bearing against the underside of the shears, so that they serve as back gibs to hold the carriage to the shears and may be operated to adjust the fit or to lock the carriage to the bed if occasion may require. This lathe is made with a simple tool rest as shown in the engravings or with a compound slide rest. In some sizes the rest is held to the carriage by a weight upon a principle to be hereafter described. The bed is made (as is usual) of any length to suit the purposes for which the lathe is to be used.

The next addition to the lathe as it appears in the United States is that of a compound slide rest.