It will have been noticed in the engravings of the cross sections or beds thus far given, that the “side plates” or outer walls have been uniform on the two sides and across the ends. Also, that the bed is very much strengthened by the track or flat upper member. To obtain a casting of nearly uniform shrinkage throughout, and to diminish as much as may be the unequal strains, as well as to add to the strength and stiffness of the bed, the lower edge has been re-enforced by an additional thickness for a short distance from the lower edge. This has been made of different forms by different designers, but is substantially as shown in these engravings.
In Fig. Ideal Form of Bed to Resist Torsional Strains is shown an ideal form of bed combining great strength and stiffness with a minimum amount of material when its rigidity is considered. Much is said in machine tool design of the “box form,” and while in some instances its merits may have been overrated it certainly is a form possessing most excellent qualities of strength, stiffness, and power to withstand torsional strains as well as to rigidly support heavy loads. It is for these reasons that this bed is designed as it is, and for these reasons it seems fair to call it an ideal form. The entire length of the sides or “side plates,” are double, or of the “box form” tied together at frequent intervals so that the outer and inner wall properly support each other. To “balance the casting,” there is not only an additional thickness of metal at the bottom of the outer wall, on the outside, but an inwardly projecting flange along the inside of the inside wall at the bottom. As far as possible the casting and all its component parts are of as nearly as may be the same thickness, so as to reduce to a minimum the internal strains of the casting as it cools after being “poured.”
A further reference to the form and disposition of the cross-braces or cross-ties is made a little further on in describing these members of the casting.
Thus far the cross section of the bed, and its component parts of the side plates, the track or top portion and the V’s, have been
shown, in addition to the front elevations and the various forms of beds for supporting the weight of the head-stock, the tail-stock, and the carriage. The next feature to be considered will be the “cross-bars,” or “cross-ties” as they are sometimes called.
The cross sections of these various forms are shown in Fig. Forms of Cross-Ties or Braces at A, B, C, D, and E, which give the principal forms in common use. At A is the simple form or single bar, set on edge and used in the earlier forms of cast iron lathe beds for many years. The desire to get some form more rigid laterally led to the addition of a horizontal rib, first on the top edge only and then on the bottom also, making the I-beam section shown at B. This was for many years considered quite sufficient for the purpose until the desire for more strength and stiffness led to the adoption of the “box form” shown at C. Later on this form was still further strengthened by the addition of outwardly projecting ribs or flanges at the bottom edges forming the section that is shown at D. To this form has since been added the top ribs as shown at E, and the question has, for the time at least, been solved, of making as strong and rigid a cross-bar as is possible.
It will be noticed that wherever these forms are with double walls the internal space is closed at the top. This occurs, first, as the bed is cast bottom side up, and it is more convenient to pour the molten iron into this form and have a solid casting; it gives a better appearance to the top of the cross-bar in the finished lathe; and a cross-bar open at the top would furnish a receptacle for dirt, chips, and small articles that would occasionally drop into it.
These cross-bars were located at right angles to the length of the bed as shown in plan in Fig. The Usual Manner of Placing the Cross-Ties, their distances apart in
the earlier forms of beds being two or three times the width of the center of the bed. This distance was gradually reduced as the beds were made heavier and stronger, until ten or fifteen years ago it was frequently the case that the cross-braces were spaced considerably less distance apart than the width of the bed, particularly in the wider beds used for heavy lathes, say from 36-inch swing and larger. This method of locating them prevailed in the use of the forms shown in cross section at A, B, and C, Fig. Forms of Cross-Ties or Braces.
As still stronger and more rigid beds were called for, the braces were placed at an angle, generally crossing each other, and of the form and proportion shown in plan in Fig. Angular Bracing with the Cross-Ties. In this case it was usual to use the forms shown in cross section at B, C and D, Fig. Forms of Cross-Ties or Braces. The angle at which these were set was varied by different builders, that here shown being 45 degrees, and the most usual angle used.
In Fig. Ideal Manner of Arranging Angular Bracing with Cross-Ties is shown a plan of the ideal bed, a cross section of which is shown in Fig. Ideal Form of Bed to Resist Torsional Strains. These cross-braces are made of the sectional form shown in Fig. Forms of Cross-Ties or Braces, at E, and are placed at an angle of 30 degrees with the side of the bed, and in the illustration the spaces between the walls of the braces as well as the bed are shown, and also the proper spacing from the head end of the bed. It will be readily seen that such a form of casting insures great stiffness and rigidity and guarantees the casting against torsional
strains, as well as against unequal strains as the casting is cooling. As a matter of design in providing a rigid bed this form seems to realize all the desirable qualities that leave nothing more to be desired. Yet it is possible that in the continual development of the lathe, better methods and stronger beds will be brought out, for what we consider to be of ample strength to-day may be relegated to the scrap-heap a dozen years from now.
The form of the “track” or upper portion of the lathe bed has much to do with the form and strength of the carriage which it supports. In the early form of wooden beds, with two V’s formed from wrought iron bars set upon edge and chipped and filed to the inverted V form, with the head-stock, tail-stock, and carriage all resting upon them, the carriage had, of necessity, to be made with scant bearing on the V’s, that is, very narrow, measured along the length of the bed, as it could not pass the head-stock and the tail-stock as the “wings” of the carriage do in the later forms of bed with four V’s, or their equivalent. Consequently, the head center of the lathe had considerably more “overhang” than it has at present, in order to permit the tool to be worked up near the lathe center; and the same was true of working up closely to the tail-stock center.
With the advent of cast iron beds four V’s were usually provided for. Whether the idea of four V’s came in with the cast iron bed is not certain, as it is entirely possible that some ingenious machinist fitted the wrought iron strips, not only to the inside but to the outside of the two wooden beams composing the bed, and so
accomplished the same results as to providing a “wing carriage,” capable of passing the head-stock and the tail-stock as we have them to-day.
The lathe bed with four V’s and the carriage suitable for it is shown in Fig. A Carriage on a Bed with Inside V’s, by which it will be seen that the portion of the carriage coming over the inside V’s at A must be cut away so as to clear them entirely, as the carriage must rest wholly upon the outer V’s. The necessity for this cutting away to clear the inside V’s is a source of weakness to the carriage, and the only way to compensate for it is to make this part of the carriage broader, which does not add much to its strength, or to make it deeper, which lessens the capacity of the lathe by decreasing its possible “swing over the carriage.”
In Fig. A Carriage on a Bed when Inside V’s are Omitted is shown the effect when the inside V’s are omitted, and the carriage at A may be made of much greater strength without raising its top line so as to decrease the swing over the carriage. It is clear that so far as the convenience of design and the strength of the carriage is concerned this form of bed is preferable to the one having four Vs. There is one disadvantage, however, which occurs in fitting the head-stock and the tail-stock to this vertical inner surface of the “track” at B, B. The head-stock, being fixed to the bed, may be tightly fitted and remain so, but the tail-stock,
from its being a movable part and frequently run back and forth, will in time wear sufficiently to throw its center out of line with the center of the head spindle. This disadvantage may be obviated by making these vertical surfaces B, B, slightly inclined.
This inclination to the inner surfaces of the track of the bed is shown in Fig. Form of Carriage for Ideal Form of Bed, which gives the form of a carriage when designed to fit the ideal form of bed shown in Figs. Ideal Form of Bed to Resist Torsional Strains and Ideal Manner of Arranging Angular Bracing with Cross-Ties. In this case
the full strength of the carriage is maintained and a second support is furnished it inside of the outer V at the front and back by the contact of flat, horizontal surfaces in the place where the inside V would be in the form of bed having four V’s. This construction shortens very much the “span” of the carriage between supports and consequently renders it much more stiff and rigid, adapting it to much more severe strains in heavy work than either style of carriage preceding it. In fact it is the strongest carriage now known, in proportion to its weight.
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.
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. Early Form of Cast Iron Legs with Braces. 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 ¾ inch thick and 2 inches wide. The braces were of the same dimensions and secured at the ends by ½-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.
These difficulties are in a great measure avoided in the lathes provided with cabinet supports. In Fig. Lathe Bed Supported by Old Style Legs the effect of the old-style legs is seen. Attention is called to the fact that the headstock 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.
In sharp contrast to these conditions is the bed shown in Fig. Form and Proportions of Cabinet Supports. In this case the front cabinet is of a length on the bed equal to the length of the head-stock, hence the front bearing of the head spindle has a support of solid iron down to the foundation, or floor upon which the cabinet supports rest. The tail-stock is similarly supported by a cabinet occupying the distance equal to its length upon the bed. An argument in favor of this method of supporting the bed is not necessary as the conditions are self-evident.
But there is still another reason why the cabinet support is the more rigid, and that is the fact that with the long distance on the bed to which the cabinet is firmly and solidly bolted comes additional stiffness and rigidity, not only in a vertical direction for sustaining weights, but also to withstand the torsional strains to which every lathe bed is subjected, and which are multiplied rapidly as we load the lathe with heavier work, take heavier cuts, and use high-speed tool steel, by which much greater speed may be used.
The next matter to be considered is the form of the cabinet, although this is a secondary consideration, the first being that we have the cabinet and that it reaches out under the bed to the practical length as shown in Fig. Form and Proportions of Cabinet Supports.
For small lathes, say from 12 to 20-inch swing, the cabinet is frequently made nearly square While this is wrong in theory, as has just been explained, it is an improvement upon the old-style leg. The form shown in Fig. Form and Proportions of Cabinet Supports is substantially that used by Lodge & Shipley in their smaller lathe. Its peculiar feature is the strength, vertical end walls, without projections at the base, while the regular projection is made in the front and the rear. This form is less expensive in its pattern work and somewhat easier to mold, but its appearance is not as good as the one shown in Fig. Ideal Form of Small Lathe Cabinet
which has equal projections on all four sides and at the top and bottom, thus giving it a more symmetrical appearance. It may have only three sides enclosed, the side walls turning the corner for only an inch or so, and this side be placed underneath the lathe bed, as is now done by some of the builders. But as this cut-away portion would come directly under that point of the head-stock where the most support is needed, it is of doubtful utility to cut it away, or to reduce the support of solid iron at this point.
Lathes for light work, of 12 to 18-inch swing, may be supported by square cabinets, but if for heavy duty and continuous hard work the cabinets should be considerably longer than they are wide and support the bed as shown in Fig. Lathe Bed Supported by Old Style Legs.
In Fig. Cabinet and Cupboard is shown a form for head and tail cabinets, or ” Cabinet and Cupboard/’ for medium-sized lathes, say from 20 to 28-inch swing. These answer the conditions as represented in Fig. Form and Proportions of Cabinet Supports, and are not excessively expensive. They also furnish one closed cabinet and an open cupboard, both of which are available for storing tools, gears, and similar articles. The arched opening at A affords a
convenient space for introducing a lever or bar for the purpose of moving the lathe. This arch should be placed in the cabinets of all but the smallest ones, and even in them a small arch suitable for the use of a crowbar will be found convenient.
In either of the styles of cabinets shown the shelves may be cast in, but the usual method is to cast strips upon which the ends of wooden shelves may rest, thus making not only the pattern work but the foundry work more simple and economical.
In Fig. The Lodge & Shipley Form of Cabinet for Large Lathes is shown the form of cabinet used by Lodge & Shipley for large lathes and which gives an excellent support to the bed and its superstructure.
In Fig. The Hendey-Norton Form of Cabinet for Large Lathes is shown a similar cabinet used by the Hendey-Norton Company, differing from the last one in having the inner end cut away. This cabinet does not, of course, admit of the introduction of shelves. In the larger lathe, say from 30 to 40-inch swing, inclusive, doors are not usually provided, as the height does not admit of it. Above 40-inch swing the bed usually rests directly upon the foundation.
The cabinets here shown are given simply as examples, but they give a good idea of the forms used by most of the modern lathe builders at the present time, and the reasons for their continued and enlarged use. It is altogether probable that the future will witness an increase rather than a decrease in the use of the cabinet for supporting machines of all kinds where it is possible to introduce them, on account of their great rigidity in proportion to the weight of cast iron used, as well as the fact that they furnish a safe and convenient receptacle for tools.