The more recent development of the thread-cutting mechanism of engine lathes aims to arrange the change-gears so that any desired thread may be cut without removing or replacing any of the gears. To accomplish this, all the necessary change-gears are permanently located on the lathe, and any one of them may be brought into use as required, by shifting one or more levers or equivalent devices.

One of the most prominent of these devices is shown in Fig. 99, which illustrates at the left, an end and, at the right, a front elevation of the device applied to an engine lathe. Motion is communicated from the work spindle by means of the gear A on the head shaft, and through the gears B and C, to the supplemental shaft H, upon which is fitted a forked sliding arm G. This sliding arm G carries a pinion D splined to the shaft H, and also a connecting pinion E journaled in it and capable of engaging either one of the sets of change-gears F, which are fixed upon the lead-screw J, by sliding the lever to the right or left, raising it until the gears engage properly, and permitting the pin on the thumb-lever K to enter one of the series of holes shown in the gear casing and thus secure the lever G and connecting pinion E in proper position to transmit motion for the supplementary shaft H to the lead-screw J.

An index on the outside of the gear casing gives the necessary information as to the position of the lever G for any desired thread. No calculations are necessary. Size of Lathe. In this country, the size of a lathe is designated by the greatest diameter it will swing over the guides, and by the length of the bed. The one illustrated is known as a 14-inch by 8-foot lathe. In England, the distance from the guides to the center is the unit of size, and, in a few cases, the greatest distance between centers is considered to be the length of the lathe. Thus a 15-inch lathe in England would be a 30-inch lathe in the United States.