Fig. 954.
It may also be observed that if the lathe bed be worn it will usually be most worn at the live centre end, where it is most used, and a tool set above the centre will gradually fall as the cut proceeds towards the live centre, entering the work farther, and therefore reducing its diameter. This can be offset by setting the tailstock over, but in this case the wear of the work centres is increased, and the work will be more liable to gradually run out of true, as explained with reference to turning taper work. Sir Joseph Whitworth recommends that the tool edge be placed at the “centre” of the work, while at the same time on a line with the middle of the body of the steel. To accomplish this result it is necessary that the form of the tool be such as shown in [Fig. 953], in which w represents a piece of work, r the slide rest, a the fulcrum of the tool support, the dotted line the centre of the work, and the arrow the direction in which the tool point would move from its deflection or spring. Now take the conditions shown in [Fig. 954], and it will be perceived at once that the least tool deflection will have an appreciable effect in causing the tool point to advance into the work in the direction denoted by the arrow. This would impair the cylindrical truth of the work, because metals are not homogeneous but contain in forged metals seams and harder and softer places, and in cast metals different degrees of density, that part laying at the bottom of the mould being densest (and therefore hardest) by reason of having supported the weight of the metal above it when cooling in the mould.
This brings us to another consideration, inasmuch as supposing the tool edge to be set level with the work centre (as in [Figs. 951] and [953]), the arc of deflection of the tool point will vary in its direction with relation to the work according to the vertical distance of the top of the tool rest (r in [Figs. 953] and [954]) from the horizontal centre of the work.
Thus the vertical distance between the point a in [Fig. 953] and the work centre is less than that between a and the horizontal work centre in [Fig. 954], as may be measured by prolonging the dotted lines in both figures until they pass over a, and then measuring the respective vertical distances between a and those dotted lines. It is to be noted that this distance is governed by the vertical distance of the top of the tool rest r from the work centre, but where this distance is required or desired to be reduced a strip of metal may be placed beneath the tool and between it and the slide rest.
It will now be obvious that to produce work as nearly cylindrical as possible, the tool edge should stand as near to the slide rest as the circumstances will permit, which will hold the tool more firmly and prevent, as far as possible, its deflection or spring from the cut pressure. Both in roughing out and in finishing, this is of great importance, influencing in many cases the depth of cut the tool will carry as well as the cylindrical truth of the work.
We may now present some others of the ordinary forms of tools used in the slide rests on external or outside work, bearing in mind, however, that these are merely the principal forms, and that the conditions of practice require frequent changes in their forms, to suit the conditions of access to the work, &c.
Fig. 955.