Lapidary apparatus for amateurs is in some cases made precisely similar to that described in the preceding section, but more generally the laps are driven by a foot wheel and treadle; the general arrangement of the apparatus then closely resembles the horizontal grinding machine shown in fig. 1039, page 1157, in which the upper surface of the lap is not obstructed by a vertical spindle, as in the lapidary's bench, fig. 1141, and the same spindle is employed for all the different grinding and polishing wheels.
The less difficult process of lapidary work, such as producing a flat or a rounded surface on a pebble, may be executed with facility on the horizontal grinding machine, which is frequently converted into a simple lapidary apparatus by the addition of a slicer and trough to catch the water thrown off the lap. But for the more elaborate processes of lapidary work, such as cutting a stone into thin slices and facetting gems, other additions have been made, in which the principal difficulties of manipulation are removed by the introduction of guides for slitting and facetting, and the apparatus then assumes the more complete forms shown in figs. 1190 and 1192.
Fig. 1190 represents the amateur lapidary apparatus fitted with the crane, or swinging arm, for presenting the stones to the slicer. The foundation of the apparatus exactly resembles that described in page 1157, except that the rectangular trough, indicated by the dotted lines in fig. 1190, occupies the place of the upper platform in fig. 1139, and the lower platform has an enlargement at its right-hand corner for the support of a strong cylindrical pillar that carries the socket a; this socket slides vertically upon the pillar, and admits of being fixed at any height by means of the binding screws b b. The swinging arm, c, is supported on the socket by two center screws, one of which is seen at d; these center screws allow of the horizontal motion of the arm, which has near the front a rectangular opening, in which is fitted a ball and socket-joint; and a binding screw at the extremity of the arm serves to retain the ball at any desired angle.
The ball is perforated with a square hole, into which is fitted the square stem of a metal cup, for the reception of the stone to be sliced. The cup and ball are shown separately in fig. 1191; and, as there seen, the stem of the cup passes through the ball, and is fixed by the milled nut beneath. The arm is drawn towards the slicer by the weight e, a line from which passes over the pulley f, fixed on the end of the frame, and is attached to a small stud screwed into the under side of the arm.
In slitting a stone with this apparatus, it is first cemented in a cup of appropriate size, the stone and cup being both heated nearly to the fusing point of the cement. The stone is placed with the intended line of division as nearly horizontal as convenient, and when the cement is set, the cup is inserted in the ball and fixed by the milled nut. The position of the ball is then adjusted until the line of division is exactly horizontal, and the precise height is regulated by sliding the socket upon the vertical pillar. When the adjustments have all been satisfactorily made, the binding screws are tightened, and the weight is attached to the line; this completes the preparations so far as the adjustment of the apparatus is concerned; and when one cut has been made, if the stone is required to be cut into parallel slices, it is only necessary between every cut to shift the socket a, upwards, a distance equal to the thickness of the intended slice.
Of course the slicer requires to be charged with diamond powder, and lubricated with the oil of brick, as explained at page 1310; and the weight should also be proportioned to the size of the stone. To allow of the ready adjustment of the weight, it is made in detached disks, with central holes that fit upon a cylindrical rod, to which the line is attached. The rod is flattened near its upper end, and every disk has a radial mortise extending from the central hole to the edge. In applying the weight, the mortise is passed over the flattened part of the rod, and the central hole is slipped down the cylindrical portion of the rod, which is too large to pass through the mortise.
In fig. 1192 the apparatus for cutting facets is represented, the upright pillar at the back is here employed to support the platform g, and upon this is mounted the gim peg h, which may be employed for cutting facets in the same manner as that used by practical lapidaries; but the instrument for cutting facets shown at l, is a more exact contrivance, and far better adapted for the purposes of the amateur, as the facets may be cut to any required angle with great exactness, and the operator has only to determine the size of the facets by suspending the process when each facet is sufficiently developed.
The basis of the instrument is exactly the same as those for setting straight and angular turning tools, shown in fig. 1042, p. 1159, and fig. 1047, p. 1165; indeed the same instrument may be made to serve all three purposes when fitted with suitable beds or sockets for the reception of the different objects to be ground. In the instrument for grinding facets the bed g, shown in fig. 1042, is replaced by a frame, which is mounted in the same manner, and is therefore capable of being set to any angle, either vertical or horizontal.
The frame, shown separately on a larger scale in fig. 1194, carries a steel spindle, which is capable of revolving within bearings, and may be fixed at any position by the binding screw, n. A hole bored up the center of the spindle extends nearly through its length, and into this is fitted the cylindrical stem of the cement cup, p, intended for the reception of the stone. The cement cup is retained in the spindle by the small binding screw, q. For determining the position of the spindle, a small pulley, r, is fixed on its front end; the edge of the pulley is provided with two circles of graduations, the one containing 96 divisions, the other 60; and to enable the divisions to be read off with accuracy, an index is fixed on the top of the frame.