Fig. 621.
In [Fig. 621], for example, is a section of a lathe shear, with a slide rest shown in place, and it will be observed that the wear of the V upon the lathe bed, and of the V-groove in the slide rest, will cause the rest to fall in the direction of arrow a, and that a given amount of motion in that direction will have less effect in altering the diameter than it would in any other direction. This is shown on the right hand of the figure as follows: Suppose the cutting point of the tool is at a, and the work will be of the diameter shown by the full circle in the figure. If we suppose the tool point to drop down to f, the work would be turned to the diameter denoted by dotted arc g, while if the tool were moved outwards from a to c the work would be turned to the diameter e. Now since f and c are equidistant from the point a, therefore the difference in the diameters of e and g represents the difference of effect between the wear letting the rest merely fall, or moving it outwards, and it follows that, as already stated, the diameter of the work is less affected by a given amount of wear, when this wear is in the direction of a, than when it is in the direction of b. When the carriage is held down by a weight as is shown in [Figs. 577] and [578], there is therefore no lost motion or play in the carriage, which therefore moves steadily upon the shears, unless the pressure of the cut is sufficient in amount, and also in a direction to lift the carriage (as it is in the case of boring with boring tools); but to enable the carriage to remain firm upon the shears under all conditions, it is necessary to provide means to hold it down upon the Vs, which is done by means of gibs g, g, which are secured to the carriage, and fit against the bottom of the bed flange as shown.
Now since lathes are generally used much more frequently on short than on long work, therefore the carriage traverses one part of the shears more than another, and the Vs wear more at the part most traversed, and it follows that if gibs g are set to slide properly at some parts they will not be properly set at another or other parts of the length of the shears; hence the carriage will in some parts have liberty to move from the bed, there being nothing but the weight of the carriage, &c., to hold it down to the Vs. Now, the wear in the direction of a acts directly to cause this inequality of gib fit, whereas that in the direction of b does so to a less extent, as will appear hereafter.
Meantime it may be noted that when the carriage is held down by a suspended weight the shears cannot be provided with cross girts, and are therefore less rigid and more subject to torsion under the strain of the cut; furthermore the amount of the weight must be sufficient to hold the carriage down under the maximum of cut, and this weight acts continuously to wear the Vs, whether the carriage is under cutting duty or not, but the advantage of keeping the carriage firmly down upon the Vs is sufficiently great to cause many to prefer the weighted carriage for light work driven between the lathe centres.
Fig. 622.
[Fig. 622] represents the flat shear, the edges being at an angle and the fit of the carriage to the shears being adjusted by the gibs at a a, which are set up by bolts c c and d d. In this case there is a large amount of wearing surface at b b, to prevent the fall of the carriage c, but the amount of end motion (in the direction of b, [Fig. 621]), permitted to the carriage by reason of the wear of the gibs and shear edges, is greater than the amount of the wear because of the edges being at an angle. It is true that the amount of fall of the carriage on the raised V is also (on account of the angle of the V) greater than the actual amount of the wear, but the effect upon the work diameter is in this case much greater, as will be readily understood from what has already been said. The wearing surface of the raised V may obviously be increased by providing broader Vs, or two Vs instead of having four. This would tend to keep the lathe in line, because the wear due to moving the tailblock would act upon those parts of the shear length that are less acted upon by the carriage, and since the front journal and bearing of the live spindle wear the most, the alignment of the lathe centres would be more nearly preserved.
