1038

THEORY AND PRACTICE OF ENGINEERING.

Book IT.

Fig. 1591.

SCREW-CUTTING MACHINE.

To cut a screw with a double thread, one is first cut, the driver is then applied to a holein an iron chuck, directly opposite its first position, by which a semi-rotation is given tothe new screw, without producing a motion in the cutting frame, the cutter being appliedto a point midway between two of the first formed threads ; a third thread, or any number,

8CREW-CCTTINO.

may be cut in the same manner by this machine, by commencing at a third or two-thirdsor any other distance from the original position. The cutter is secured in its place byplates fastened by nuts and screws.

Another process is generally adopted for large screws, which consists of moving the slide-rest that contains the cutters, in the same way as performed by the lathe; the tool holder

m p

SCREW-CUTTING MACHINE.