The guide for supporting the work when the tool is employed in the slide rest, is distinguished as the back stay; it is required for the support of rods and long screws, and has to travel along the work at the same rate as the tool. The pedestal therefore can no longer be fixed upon the bearers as in the previous examples, but is attached to the saddle plate carrying the slide rest; and the rod or screw to be turned is also always supported by the popit head. The guide portion of the back stay for metal turning, is very generally made after one of the forms indicated by figs. 133, 134 or 137. Fig. 142 being another and stronger arrangement.
The pedestal of the back stay for the lathe fig. 114, is continued below in the form of a bridge, which avoids any interference with the traverse of the lower slide of the slide rest. It is bolted on either side to the saddle plate, the bolts passing through elongated holes, fig. 143, giving it a small power of transverse adjustment. The backstay itself, is formed of two pieces or jaws of metal, jointed together at one end, kept apart by a steel spring, and compressed upon each other by a screw in a bridle attached to the lower jaw. It is attached to the upright, by a bolt, nut and spring washer. The bolt is less in diameter than the width of the opening between the upper and under jaws, through which it passes, and the washer, which is wide, concave and rather thin, to act as a spring under the nut, bears by its ends upon both jaws. The upper jaw carries a piece of hardened steel filed with a notch, forming a triangular opening with the lower; the two jaws closing upon and touching the work at three points. The size of the opening is contracted as the work progresses, by the pressure of the bridle screw, and varied to receive work of different diameters, by exchanging the steel piece in the upper jaw for others, fig. 144, with larger openings or notches.
The arched support is first fixed to the saddle of the slide rest, the jaws of the back stay are then opened and placed upon the work and the bolt passed between them and through the upright. The upper jaw then rests upon the work and upon the fixing bolt, which latter is at first screwed up by the fingers only. The jaws having been previously oiled, are then adjusted to fit or rub upon the work by the bridle screw, after which the attaching bolt is further screwed up. The backstay can be placed on either side of the upright, as may be more convenient for the work; but relatively to the cut, it is always placed so that its support falls upon a point, slightly to the right of that which receives the thrust of the tool. It therefore always bears upon a surface, just previously completed by the tool, and meets with no impediment to its traverse.
The backstay shown by fig. 140, is sometimes used with the slide rest for ornamental turning, for the support of long and delicate pieces during their ornamentation with the drilling instrument or revolving cutting frames. A slip of hardwood, shown in section fig. 141, with a longitudinal groove or a series of notches, cut to roughly fit the form of the work, and altered from time to time as that progresses, is carried in a rectangular trough about three inches long. The wood is held in its place by two thumb screws from above, bearing upon its side, and is pushed forward from behind, by two similar screws equidistant from the center of the length of the trough. The latter, is mounted on a cylindrical stem, capable of vertical adjustment and provided with a fixing screw. The socket in which the stem works, being attached to and removable from the rest bottom, by screwing into a short series of holes tapped in its upper surface.
The guide is first adjusted vertically to the height of center, and then, while the binding screw in the socket is still sufficiently slack, the slip of wood is advanced into contact by its pushing screws; it is then secured by the two thumb screws above, after which the stem is fixed by the binding screw in the socket. The circular or swing motion of the stem within its socket, together with the independent action of the two pushing screws, allows the guide to be very gently and exactly adjusted for the support of the most delicate or fragile ornamental work.