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.
The form and proportions of the lathe bed having been duly considered, its different component parts illustrated and described, and these detail matters criticised and commented upon, the next part of the lathe to be dealt with would naturally seem to be the legs, cabinets, or like supports upon which the bed is to rest.
The usual height of the centers of a lathe from the floor is about 43 inches, and in designing lathes this height is maintained without regard to the capacity or swing of the lathe until its swing becomes so large that with the bed resting on a properly built foundation on a level with the floor, it becomes necessary to raise this height sufficiently to obtain a bed of proper depth and a head-stock of sufficient swing to meet the requirements.
Fig. 46. - Early Form of Cast Iron Legs with Braces.
Therefore the smaller the capacity of the lathe the higher will be the legs or other supports under the bed.
In the early style of wooden beds, these supports were simply legs of square timber bolted to the bed and either vertical or spread out at the floor, according to the notion of the builder. When cast iron beds came to be used the legs were also of cast iron and of rather frail design. Later, when the necessity for more rigidity was found desirable, not only the beds but their supporting legs were made heavier.
In a shop near Boston was found a lathe provided with an example of the earlier form of cast iron legs strengthened by cast iron braces as shown at A, A, A, A, Fig. 46. The lathe was 12-inch swing and of the hand lathe pattern, with a wooden cone pulley on the spindle, probably built about 1840. The legs were quite light, the different members being about 3/4 inch thick and 2 inches wide. The braces were of the same dimensions and secured at the ends by 1/2-inch "tap bolts" of the old square-head style, the ends of the braces being thickened somewhat to accommodate them.
How this lathe happened to endure the wear and tear of shop use for so many years without the legs being broken is a mystery. Their frail and slender appearance beside the modern deep bed, supported by heavy cabinet legs, is an object lesson in the practical evolution of the American lathe.
With the continually increasing weight and rigidity of the lathe beds to meet the hard service of modern shop methods and highspeed steels, first represented by the Mushet tool steel, it became necessary to furnish much better supports for the lathe beds, and the fact was apparent that these supports must extend for a greater distance along the length of the bed than the older form of legs ever had. At this time there were several of the different machine tools supported on a "cupboard base," or a base of rectangular form* having a door giving access to its interior for the purpose of stowing away tools, change-gears, wrenches, and like articles. This form of base was prominently used in the Universal Milling Machine. Whether the "cabinets" for supporting a lathe bed were suggested by this use of them or not does not appear, although it seems probable. We know that wooden cupboards had been used under lathes, being fastened to the legs and used for the same purposes as the cabinets or cupboards formed in the bases or, as sometimes called, the "standards" or columns of the later machines.
At the present time a number of lathe builders still use the old-style legs, made heavier and with the material better distributed for strength, and, as a rule, the top portion of the leg extending farther along on the under side of the bed for the purpose of giving better support.
It is also the case that the cabinet form of bed supports is used more upon expensive lathes, such, for instance, as those designed more particularly for tool room and precision work. For turret lathes and screw machines they are also much used, and are often cast as an integral portion of the bed itself instead of being made as a separate piece and bolted on.
Cabinet supports for lathe beds are made in various forms by the several builders, some of which will be illustrated in this chapter. These will be such as have some general features common to nearly-all of them, and in addition a few of the forms having special features.
The correct principle governing the dimensions of cabinet supports should be properly understood. Obviously, the reasons for substituting cabinets for the earlier form of legs was to obtain a better support. It was certainly possible to so design the leg as to amply support the weight of the lathe and all that could be put upon it by way of work to be done by it. The disadvantage was that a leg placed at each end of the bed and extending only a short distance along under it left a long stretch of bed with no support at all. This necessitated "center legs" and, in a long lathe, two or three of them. Under these conditions it was a difficult matter to so set up a lathe that these center legs should all sit level and support the bed in a correct, level, straight line.
Fig. 47. - Lathe Bed Supported by Old Style Legs.
These difficulties are in a great measure avoided in the lathes provided with cabinet supports. In Fig. 47 the effect of the old-style legs is seen. Attention is called to the fact that the head-stock is only supported by the leg at the outer end, while the point at the front journal where the heaviest weight comes has no support whatever from the leg. The same may be said in a lesser degree of the rear end, where the tail-stock has only partial support in a similar manner. And when the tail-stock is moved out of its extreme rear position the case is much worse and identical with that of the head-stock. This condition will, of course, necessitate the use of a center leg, which if not supported upon the floor or foundation in a perfectly correct position will do as much harm as good. If it is too low it will be of no benefit since the center of the bed may sink under the weight, and strain of the work upon the carriage. If it is too high the lathe will be thrown out of line.