This section is from the book "Lathe Design, Construction And Operation, With Practical Examples Of The Lathe Work", by Oscar E. Perrigo. Also available from Amazon: Lathe Design: Construction And Operation.
By the term "roughing lathe" we understand that the design is heavy and massive with a very powerful driving mechanism, lateral and cross feeds and a very rigid tool holding device. Such a lathe is seen in Fig. 21. While it is somewhat analogous to the forge lathe it is usually understood to be of much less capacity. And while the forge lathe, being for handling forgings almost exclusively, holds the work on centers, the roughing lathe should be made with a large hole in the spindle so that work may be "roughed out" from the bar as well as when held on centers, or with one end in a chuck and the other on a center. And here it encroaches upon what may be considered the field of the so-called "rapid reduction lathe"; with this difference, however, that in the former lathe the work is simply roughed out, while in the latter it is supposed to be not only roughed out or rapidly reduced to near finished sizes, but in many cases entirely finished, or finished to dimensions suitable for being finished by grinding.
Fig. 21. - A Roughing Lathe.
In the third class we commence with the complete engine lathe, with thread-cutting mechanism, back geared or triple geared, with a compound rest which in the larger sizes is capable of power feed at all angles. Such a lathe should also be supplied, especially in the larger sizes, with a tool-rest to attach to the front wing of the carriage on the left-hand side for turning the full swing of the lathe. The larger lathes, particularly those that are triple geared, should have a tail-stock arranged with two sets of holding-down bolts, by means of which one set may be loosened and the tail-stock set over for turning tapers without removing the work from the lathe, as the other set of bolts still holds the tail-stock to its place on the bed. There should also be a tail-stock moving device consisting of a rack attached to the bed, with which is engaged a pinion fixed to a shaft journaled in a bracket attached to the tail-stock base. By means of a crank on this shaft the tail-stock can be easily moved to any desired point upon the bed.
In lathes of 42-inch swing and larger, this arrangement should be back-geared by the introduction of a second shaft, the gears being in ratio of 2 to 1. In lathes of 60-inch swing and larger this ratio should be 3 to 1. The tail-spindle in the smaller lathes has the usual screw and hand wheel for moving it back and forth. In large lathes this is inconvenient and laborious. The hand wheel should be placed in front of the tail-stock and near the center, being mounted upon a short shaft at right angles to the spindle and journaled in a bracket fixed to the tail-stock. Upon this short shaft is also a miter gear engaging with another fixed to a shaft parallel to the spindle and extending to the rear end of the tail-stock where it passes through another bracket and has fixed upon it a spur pinion which engages a spur gear fixed to the tail-spindle screw, and by which mechanism it is operated. The ratio of this spur gear and pinion is usually 2 to 1 on lathes of 42-inch swing, and proportionately more on larger lathes. By the use of this mechanism the operator may stand opposite the tail center in adjusting his work and easily reach the hand wheel controlling the movement of the spindle, which would otherwise require an assistant to operate.
In the triple-geared head-stocks of this class of lathes it is customary to attach the face-plate to the main spindle by a force-fit and key instead of making it readily removable by a coarse thread, for the reason that it is to be driven by means of a very large internal gear bolted to its rear side and engaged by a pinion fixed to a shaft driven by the cone through a suitable system of triple back gearing. In this case the cone is not placed upon the main spindle, but upon a separate shaft placed sometimes in front and sometimes in the rear of it. The front position is the more convenient for the operator in making the necessary changes of speed.
It is upon this class of lathes that many improvements have been made in the last few years in the thread-cutting devices, the original idea having been to avoid removing and replacing "change-gears" when threads of different pitches were required to be cut. The first attempt in this line, so far as the patenting of a device shows, was made by Edward Bancroft and William Sellers in 1854, and taken up by various inventors with more or less success but never brought prominently into the market until the patent was granted to Wendel P. Norton in 1892, when somewhat later on the mechanism was adopted by the Hendey Machine Company, since which time it has been manufactured with much success. In the meantime many other devices for the same purpose have been devised and built, so that now every tool room and nearly every machine shop making any pretense to modern equipment possesses lathes having some one of these "rapid change gear attachments" included in their design or arranged to be attached when desired by the customer.
In the development of the engine lathe proper, much attention has been paid to the supports for the bed, and instead of the former pattern of light and, later on, heavy legs, substantial cabinets of liberal dimensions and weight, have been designed and are now used upon nearly all such lathes, the only exceptions seeming to be upon those where the selling price renders economy in the use of cast iron essential; upon lathes too small and light to justify their use; and upon lathes built by the more conservative manufacturers who have not yet come to consider this class of improvements as necessary to the efficiency of their machines.
A precision lathe is designed to be a lathe in which fineness and exactness in all its parts is the prime consideration rather than a great range of work or capacity, or from which a large output may be realized. It is therefore not necessary that it should be very heavy or massive except in so far as its weight may render it capable of greater precision. While the entire design and construction of the lathe is as exact as possible, the effort is also made to provide against all conditions and causes that shall be detrimental to its one object, that of turning out its work in as perfect a manner as possible.