The slide rest previously described, although applicable to the purposes of ornamental turning, has one disadvantage. It is necessary that the various pieces of apparatus to be used with it should have a foundation plate with chamfered edges to fit accurately between the guide bars. This is often inconvenient, and adds to the difficulty of making, and consequently to the cost of such pieces. In addition to this drawback, it may happen that one of these fittings by being more frequently used becomes more worn than another, so that the guide bars require constant re-adjustment, and their accuracy and parallelism become impaired. To obviate these and similar inconveniences the slide rest is now commonly made like [Fig. 277], and a tool receptacle, [Fig. 278], is fitted to slide between M, M, and is so arranged as to hold securely the universal cutters and other apparatus required for ornamentation or for plain turning. These are all made with a rectangular bar fitting the longitudinal channel in the middle of the receptacle, and are secured by the following simple contrivance. It will be seen by the drawing that the central channel is widened at A, A, and that a groove or saw-cut B runs along the inside from end to end. This groove is continued in a similar manner on the side next to the reader. [Fig. 280] represents an ordinary tool holder, with a rectangular shank A, and clamping screw B, by which the tool c is secured. The part A is laid in the central channel, and a small piece of metal shaped like [Fig. 279] is inserted in one of the open spaces, A, A of the receptacle and slid along with its lower flange in the saw-cut until clear of the enlarged part of the channel. It is thus retained, and the clamping screw which passes through its centre is brought to bear upon the piece to be fixed, which is thereby securely held in its required position. Two of these holdfasts are generally used at the same time. If the main bar of the tool holder is not quite thick enough to be clamped, then it is only necessary to lay a small plate below it. By the above simple means, the necessity for fitting each individual piece of apparatus to work upon the chamfered guides is done away. In order to ensure the position of the sole of the rest at right angles to the lathe bed a kind of saddle, A, [Fig. 281], is used. This is of cast iron or brass, accurately planed on the upper surface, and has a projection fitting between the bearers of the lathe. The usual holding down-bolt passes through the hole in the centre, securing the saddle and the rest at the same time. The usual arrangement of a kind of double socket, the inner one rising at pleasure by being tapped into the outer, has already been described, and serves for accurate adjustment of the height of the rest. It is convenient, in addition, to have a stop or set screw under the bed of the rest, and a similar one on the top of the socket, so placed that when the frame is swung round it shall stop precisely at right angles to its former position. Thus, if the tool is first required to be used upon the side, and then upon the face of the object to be turned, these two positions are obtained at once, and can, if necessary, be alternated without any re-adjustment of the moving parts of the rest by the aid of the set square. The receptacle-holder is generally advanced by the hand lever, [Fig. 279], one pin of which fits into the hole in the guide-bar as seen in the drawing, while the other falls into a short slot e, made in the upper surface of the receptacle, or of the piece of apparatus to be used in it. Of course, this arrangement may be reversed, one or both pins being fixed to the rest and its receptacle slide, and the holes made in the lever. Sometimes, however, a slower and more regular movement is required than it is possible to give in this way, and the lever is replaced by the leading screw C, D, [Fig. 278], the head of which is removable, and can be replaced by a small winch handle. This screw is tapped into the lug cast upon the receptacle, and its point is of the form shown. The latter fits into a hole in the pillar A, [278], and is retained by a pin, which falls into the groove, D, [Fig. 278], and prevents the screw from advancing or receding without carrying the sliding plate with it. The pin being removed, the screw will no longer act in this way, and the slide may be moved by the lever instead. The other screw, E, F, of fine pitch, serves to regulate the advance of the receptacle, and consequently the depth of cut of the tool—a round head with divisions on its edge is attached to one end, which abuts against the pillar B, [Fig. 277], which latter has a mark on its top to act as an index. Thus the advance of the tool can be regulated to a great nicety, and successive predetermined and different depths may be reached and repeated at pleasure, as is sometimes necessary. C, C, [Fig. 277], is one of a pair of stops which can be fixed by their screws at any two points of the bed of the slide rest. These serve to regulate the distance which the top slide and tool holder are intended to traverse, as in drilling a number of flutes of equal length, and many similar works. They are usually made of gun-metal, the screws of iron or steel, or of a metal called homogeneous, which may be described as between the two, and, being pleasant to work, is worthy of notice. It is absolutely necessary that the slide rest for ornamentation should be made with the greatest nicety. The slides must work equally smoothly from end to end of their traverse. The pitch of the screws must be not only fine, but even and regular, and the screw itself of precisely the same diameter from end to end, else it will work loosely through its nut in one place, and jamb in another. It is extremely pleasant to feel the exquisite smoothness and oiliness, for no other word will express it, of the movements of sliding parts in the workmanship of Munro or Holtzapffel, especially if compared with inferior work. Good amateur's work indeed is often far superior to that which is sometimes advertised, and perhaps a few hints may not be out of place here, relative to the construction of this necessary addition to the lathe.
Figs. 277, 279, 280.
Fig. 278.
Fig. 281.