Lathe bed design

LATHE BED DESIGN

BY JOSEPH G. HORNER

CONTENTS
-    The Sections of Lathe Beds
-    The Longitudinal Forms of Lathe Beds

Machinery's reference series
1913, The Industrial Press, New York
    

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CHAPTER I

THE SECTIONS OF LATHE BEDS

All the early lathe beds were made of wood. Engravings showing some of these wooden beds may be seen in old works on turning. They are to be found now only in some of the lathes used by wood-turners, and in some pattern-shops, although in the latter case, lathes with iron beds are now almost exclusively used. Fig. 1 shows a wooden lathe bed or stand. Different methods were used for attaching the bearers or shears to the uprights. At a very early date the wear of the top surfaces of the wooden bearers was prevented by screwing thin flat iron plates onto them. Strips of wrought iron were also fitted, having curved edges chipped and filed to shape, as shown in Fig. 2. There was not a great deal of durability in these shears, but the chief objection to this construction was that when the timber warped, as it was bound to do in the course of time, it pulled the iron strips with it, and threw the headstock, tailstock, and rest out of alignment.

The first all-iron beds were of triangular section, the form probably originating with Henry Maudslay. The bed was built of two bars of triangular section, secured in brackets bolted onto the legs. There was a very good reason for the adoption of this form of bed in preference to any other. There were no planing machines at that period in the latter part of the eighteenth century so that it was an important consideration to be able to reduce the chipping and filing to a minimum on a single bar. Besides, if the two upper faces were true, it made no difference whether the bottom one was true or not, because there was clearance between it and the tailstock and rest.

Lathe beds with a single shear of triangular section have often been built, although they are seldom seen now, except in the lathes used by watch- and clock-makers. These beds are sufficiently rigid for light duty, and chips do not lodge on them. The triangular-section lathe bed also possesses the virtue of insuring self-alignment of the tailstock and rest, which bear on the upper edges only. The essentials of the triangular bar section have been revived and perpetuated in the Pittler bed referred to later but in a modified and stronger, stiffer, and steadier form. The Pittler bed consists of a bar of trapezoidal section. The bar is hollow, and the lead-screw, passing through the hollow section, is thus protected. In some watchmakers' lathes, the essential features of the triangular bed are retained, but the lower side is of convex form. Some lathe beds are of cylindrical cross-section, either solid or hollow. All these types are simply variations of the single bar type, and are illustrated later in this treatise. Mention may also be made of square and rectangular beds, the latter being employed in a few of the peculiar French lathes used for screw threading.


Early Development of Lathe Beds

Since a single triangular bed was not stiff enough to resist the torsional stresses of heavy cuts, which produce vibration and cause the bed to spring, an early development was that of using two deep parallel bars or beam sections, cast separately and bolted together. In the next stage the two bars were cast in one piece with connecting ribs. It was still, however, necessary to reduce the labor of chipping and filing to the least amount consistent with the practical requirements of the time; hence the form shown in Fig. 6, in which the top vees of the triangular bars were still retained, represented standard practice, with or without the internal stiffening ribs which were cast to increase the rigidity in the lateral direction. Then modifications of the design in Pig. 6 were introduced as shown in Pig. 7, where one vee is dispensed with, but the other retained for guidance. In Fig. 8, the width of the flat bearing surface is increased. This type of bed also made it easier than with two vees to fit the parts to a nicety. This construction is, for the same reason, employed instead of two vees in many lathes and grinding machines to-day. At last both bearing faces were made flat as shown in Fig. 9, and the longitudinal means for guidance offered by the vees was, therefore, abandoned. The lateral play was then prevented by making tenons on the heads fit between the edges of projecting internal ribs, as shown in Fig. 9. All finished surfaces were still kept narrow, however, until, after the invention of the planer, they developed into the present forms.

As the slide-rest developed, the battle of the vees and flats became intensified. The older upstanding vees are still retained with modifications as the only guiding elements in standard American practice. At a comparatively recent date slight modifications have been made in some forms, in which a flat is combined with the vees; but the principal difference which exists even now is that of using either two or four distinct vee-ways. In the latter design, Fig. 3, the two inner vees guide the sliding tailstock, and the two outer ones, the carriage or saddle of the slide-rest. The inner vees are frequently sunk below the level of the outer ones to increase the swing of the lathe, and to enable a greater thickness of metal to be put into the carriage. Both vees are truncated or flattened more or less on the top. The type having two vees only, serving both for the carriage and the tailstock, is now chiefly used for the smaller classes of lathes. In England, the vee-beds have been long since abandoned, except in a very few cases where an Anglo-American design is aimed at.

Some of the early lathes with vees anticipated the modern forms of front-slide lathes. Figs. 4 and 10 illustrate beds of this type, as constructed by Richard Roberts, of Manchester, from about 1817 to 1820. They were probably the first of that type, and they do not differ essentially from modern lathes of a similar kind. In these illustrations two variations are shown. In one, dependence is placed on the guidance of one vee only, and the lower edge of the front slide bears against a plain face. In the other, a bottom vee is included, with a setting-up strip. Note in Fig. 4 that the centers of the heads are brought forward in front of the bed center. The remainder of the design is in harmony with the practice of that period.


THE LONGITUDINAL FORMS OF LATHE BEDS

The remarks made in the previous chapter relative to the flexure and torsion of lathe beds need not he repeated here, but we shall consider, with the aid of the representative illustrations, how flexure is best resisted, and how the longitudinal shapes are modified to serve different functions. The principal differences which are made in the forms of lathe beds are those arising from variations in dimensions, while subsidiary differences are produced by special designs of lathes, or additional functions, or by the particular class of work which is done in the lathe. Thus, the shapes of beds of similar dimensions for ordinary screw-cutting or engine lathes and those for turret lathes are very often radically different. In the one case provision has to be included for the screw-cutting and feeding devices, for the carriage motions, and for the tailstock, while in many turret lathes these features are absent, and the bed is plainer, with provision only for clamping the turret base and the cross-slide. On the other hand, the arrangements for lubricating the cutting tools and work often intro- duce complications into the design of turret lathe beds, and the casting is of a more elaborate character below the bed proper around the top of the legs or standards.

The length of a bed has an important influence upon its construction and the number of supporting points, and if a gap is included, this also modifies the form to a considerable degree. The number of sup- ports ranges from the single cabinet standard in some small lathes, and the two standards or legs in those of ordinary dimensions, to the three or more supports in longer beds. A continuous bed of full depth for the whole length is employed in lathes for heavy work and large swing, and is supported solidly on concrete foundations. The truth and rigidity of a lathe bed depends to a certain extent upon how it is fastened down. Ordinarily, beds are bolted rigidly to their foundation, which may be a wooden floor or a stone or concrete base.

Many years ago Prof. Sweet suggested the adoption of a tripod support for lathe beds, and this suggestion has been acted upon in practice. One end of the lathe is bolted down by the usual means, and the other is pivoted on a pin which passes through lugs in the bed and in the leg. The only support at that end is the pin on which the leg is free to adjust itself. Many firms also adopt the three-point support principle without any pivoting device: sometimes there are three points of contact with the foundation, and sometimes the bed is united to the legs at three points. The effect of an untrue foundation is thereby neutralized. Another method of affording good support is that of casting the bed with, or bolting it to, a single column of box form, which makes the lathe self-contained, and obviates any risk of distortion or winding. This construction is employed both for small ordinary lathes, and for turret lathes up to fairly large dimensions.


Legs or Supports for Lathe Beds

When legs are used to support the bed, it is the custom of some makers to spread the legs under the head to a greater extent than those under the right-hand end, to resist the vibration, which is more pronounced at the headstock end. Other firms do not put ordinary ribbed legs at all under the headstock, but prefer a boxed cabinet support, even when there are legs at the other end. The principle of this seems faulty, since, if it is considered necessary to put a box support under the headstock end, the use of a flimsy support at the other end of a heavy bed appears unreasonable. Many makers view the matter in this light, and place the bed on equally solid and substantial supports at both ends; sometimes the supports are of identical pattern, but frequently they are a little larger at the headstock end, in order to afford more cupboard room for tools and appliances.

The practice of placing the supports a certain distance inward from the ends, mentioned in the previous chapter, is followed in many instances, and a further development of this principle is found in the case of some lathes, particularly those with gaps, where the metal of the boxed bed is carried down to a considerable depth under the head- stock, gradually tapering off towards the ends. A great many turret lathes have their supports placed some distance inward from the ends of the bed, and the underside of the latter is often tapered or curved upward from the outside of the legs to the ends of the bed.


Gap Lathes

The question of forming a gap in a lathe bed has long been the subject of controversy. A gap lathe bed is practically as common in England as a straight bed. Theoretical considerations have been urged against it, chiefly on the ground that the bed is weakened, because its continuity is broken; but an English lathe maker would argue that the metal which is removed can be more than compensated for by extra metal placed underneath and beyond the gap, and in the heavier lathes by metal brought down to the ground in the form of a broad foot. The real objection to a gap is its unalterable dimensions it is wider than is required for some jobs, and not wide enough for others. The fitting of the bridge-piece is also liable to become slightly inaccurate when a lathe has done much service, but this can be rectified. Thirty or forty years ago such lathes predominated over all others, but gradually, with the growth in specialization, they were displaced, to some extent, by straight-bed lathes on the one hand, and by regular facing lathes, and vertical turning and boring mills, on the other.

The movable gap is used to a moderate extent, in medium and large sizes of lathes, and would be adopted more extensively but for the fact of the ever-growing specialization. The breadth of gap is adjustable within a wide range, or it may be closed up entirely, the object being, of course, to support the carriage as close as practicable to the cutting point of the tool under all conditions. The most serious defect in gap lathes, perhaps, is the fact that the lead-screw has to be kept low down to be out of the way. In the movable-gap lathes an- other difficulty arises in the driving of the lead-screw, which has to be done from gears at the right-hand end of the bed.


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