A compromise between the flat and the raised V-shear is shown in [Fig. 628], there being a V-guide on one side only, as at j. When the carriage is moved by mechanism on the front side of the lathe, and close to the V, this plan may be used, but if the feed screw or other mechanism for traversing the carriage is within the two shears, the carriage should be guided at each end, or if the operating mechanism is at the back of the lathe, the carriage should be guided at the back end, if not at both ends.
In flat shear lathes the tailstock is fitted between the inside edges of the two shears, and the alignment of the tailstock depends upon maintaining a proper fit notwithstanding the wear that will naturally take place in time. The inside edges of the shears are sometimes tapered; this taper makes it much easier to obtain a correct fit of the tailstock to the shears, but at the same time more hard to move the tailstock along the bed. To remedy this difficulty, rollers are sometimes mounted upon eccentrics having journal bearing in the tailstock, so that by operating these eccentrics one half a turn, the rollers will be brought down upon the upper face of the shears, lifting the tailstock and enabling it to be easily moved along the bed to its required position.
Fig. 629.
In many of the watchmakers’ lathes the outer edges are beveled off as in [Fig. 629], the bearing surfaces being on the faces b as well as on the edges a. As a result, edges a are relieved of weight, and therefore to some extent of wear also, and whatever wear faces b have helps the fit at a a.
Fig. 630.
In the Barnes lathe, as in several other forms in which the lathe is made (as, for example, in screw-making lathes) the form of bed in [Fig. 630] is employed. The tailblock may rest on the surfaces a, a′, b, c, d, and e, or as in the Barnes lathe the tailstock may fit to angles a b, but not to e d, while the carriage fits to b e, and c d, but not to a, the intention being to equalize the wear as much as possible.