For circles of small diameter, the cutters are made as hollow cylinders of sheet iron of various diameters, and each attached by screws to a circular disk of cast iron, as shown in section in fig. 1096. The cutter is screwed on the lower end of the spindle, just the same as a chuck on a lathe mandrel, except that the spindle is placed vertical instead of horizontal. To ensure free access for the sand and water beneath the cutter, one or two notches, about three-quarters of an inch wide, are generally made in the lower edge.
For large circles, the apparatus is made stronger than that shown in fig. 1094, and the vertical spindle is fitted at its lower extremity with a circular plate, to which is bolted a wooden cross, shown in plan in fig. 1097, and in elevation in fig. 1098, the cross has radial grooves about 18 inches long near the outer extremities of the four arms. The cutters consist of detached plates of iron from 6 to 18 inches long, of various widths, according to the thickness of the work. The cutters are curved as segments of a cylinder, of the particular diameter they are required to cut, and are each rivetted to a clamp that passes through the radial groove, and is retained by a wedge. The number and length of the cutters is solely a matter of convenience, as a single cutter, when put in rotation, would make a circular groove, and several cutters are only employed in order to expedite the process. But every different diameter requires a different curve in the cutters, and which must all be placed at exactly the proper distance from the center of rotation.
The horizontal bench upon which the marble is laid, is generally a temporary structure, adjusted to suit the thickness of the object to be sawn. Works of large diameter are seldom more than one or two inches thick, but those of small diameter are frequently much thicker, and sometimes three or four thin pieces are cemented upon each other, and cut at one operation. Short pillars are sometimes sawn out of an irregular block in a similar manner, instead of being chipped and turned. And it has been proposed that long cylinders, and tubes of stone, should be cut with cylinders of sheet iron of corresponding length, put in rotation, and supplied with sand and water.
Marble works of small and medium size, are ground flat upon horizontal revolving laps, after the same general method as that pursued by the lapidary, but with a proportionate increase of size in the lap, which is supplied as usual with sand and water. The laps for marble works are made as circular plates of cast iron, from 6 to 14 feet diameter, and about 3 inches thick when new; they are mounted in various ways upon vertical spindles, so that their upper sides or faces may be about 2 feet 6 inches above the ground. Across the face of the lap, or as it is called the sanding plate, one or two strong square bars of wood, faced with iron, are fixed so that their lower sides may just avoid touching the face of the lap, and their edges present perpendicular faces, from 5 to 6 inches high, at right angles to the face of the lap. The wooden bars serve as stops to prevent the work from being carried round by the lap, and also as guides to ensure the work being ground square.
The piece of marble is laid flat upon the lap, with the face to be ground downwards, and the side of the work in contact with the guide bar. Water is allowed to drip upon the plate from a cistern fixed above, and small quantities of sand are thrown on as required. During the progress of the work the workman leans upon the marble, the position of which is shifted occasionally to expose both the work and the lap to an equal amount of wear, and prevent the formation of ridges, but which is less likely to occur with iron laps used for grinding large surfaces of marble, than when small objects are applied upon lead laps, as by the lapidary and mechanician.
The one side of the marble having been reduced to a flat surface, the work is turned over to grind the adjoining face, and the first face is held in contact with the perpendicular side of the guide bar, in order to present the second face of the work to the lap exactly at right angles to the first. When two pieces of similar size are to be ground each on the one face and two edges, as for the upright sides of a chimney-piece, the two pieces of marble are cemented together back to back with plaster of Paris, (a process that is called lining), and the pair are ground as one piece on all four faces; in this case the flat sides are first ground parallel to each other, or of equal thickness on the two edges, and the latter are then ground square by placing the sides in contact with the guide bar.
When the lap is of moderate size, one guide bar only is employed, and it is fixed across the diameter of the plate, which then allows of two workmen being employed on the opposite sides; but large grinding plates sometimes have two or three bars placed at equal distances across the face, and four or six workmen may then be employed at the same time upon separate pieces of marble.
The sand and water are continually thrown from the lap by the centrifugal force, and the large sizes of the works sometimes applied, prevents the use of a rim standing up above the level of the lap to catch the wet, as used by lapidaries. Every workman, therefore, stands within a kind of trough like a box, about three feet high, without a top or back; the troughs serve as a protection to the workmen, who would otherwise be exposed to a continued shower of sand and water.
The surfaces of large slabs are in some cases ground upon revolving plates; in this case the axis is placed entirely beneath the surface of the plate, somewhat as in fig. 1039, and the slab is traversed by two men over the face of the plate to grind it equally, but the machine next described is better adapted for largo slabs of marble requiring tolerable accuracy.