37. - Metal Wheels or Laps, made of nearly every metal and alloy in common use, have been more or less employed in the mechanical arts, as vehicles for the application of several of the polishing powders, but of all laps, notwithstanding their variety, those of lead slightly alloyed, and supplied with powdered emery, render the most conspicuous service. Generally the plane or flat surface of the lap is employed, at other times the cylindrical edge, as by cutlers, but the portion actually used is in either case called the face of the lap.

38. - Lapidaries, Marble Workers, sometimes also mechanicians and others, place the spindle vertically, so that the lap revolves in a horizontal plane, and in which case the lower end of the spindle is supported in a center fixed to the cross rail of the wood frame or bench, the upper in a bracket or overhanging arm extending from the platform, and beneath the latter is placed the pulley by which the spindle is driven. In some cases the upper center is dispensed with, and the spindle works in a metal collar just beneath the lap - after the manner of a lathe, if we conceive the mandrel to be placed perpendicularly.

The lap in all these cases revolves within a shallow trough, extended two to six inches above the lap, in order to catch the emery and water that are thrown off. The emery is usually applied dry, the lap having been previously moistened with a small brush dipped in water or with a mop made by twisting a wire around a few rags, the wire serving also as the handle, the dry emery powder then readily adheres to the lap, and less water is required than if the emery and water were previously mixed. In some cases the lap is screwed upon the mandrel of an ordinary turning lathe like a chuck, but which is hazardous, lest the emery should find its way to the collar of the lathe mandrel.

39. - Cutlers' Laps are fixed on spindles placed horizontally, in fact in the same form that serves for their grindstone and other apparatus. Cutler's laps measure from about i to 20 inches in diameter, the best razors being smoothed on laps of 4 to 6 inches diameter, and commoner razors on those from 10 to 12 inches, which act the more expeditiously but leave a thicker edge.

40. - Differences of Construction in Laps. - The lap is in some cases a thin disk of metal fixed by means of a screwed nut against a shoulder on the spindle, but it is better with lead laps to employ an iron plate cast full of holes to support the softer metal. The casting mould may in this case be either an iron disk with a central screw to fix the iron center plate at the time of pouring, or the mould may be made of sand and in halves after the usual manner of the foundry. In either case the iron plate should be made as hot as the fluid metal, which by entering the holes becomes firmly united to the iron especially if the holes are largest on the reverse side or that away from the lead.

41. - Cutlers' narrow Cylindrical Laps are sometimes similarly cast upon the edges of cast-iron wheels or disks, but it is far more usual to make a wooden center on account of its lightness. In order that the wood center may not contract nor lose its circular form, it is made in four quarters or of more pieces, with the grain pointing to the center; the pieces are united by two circular disks of wood or metal, nailed to the sides, after this the edge is turned to the required width and cylindrical, with a groove in the center and a chamfer on each edge, to retain the lead.

A better construction is followed by Mr. Lund, he makes his wheels of common Honduras mahogany, or rather a species of cedar, in about thirty-two sections, and arranged in two layers or disks, sixteen in each, so as to break joint. No nails are used, and although the parts are only united by glue they are found to endure the transitions from heat to damp to which they are often exposed. In uniting them they are glued joint by joint, and quickly arranged together upon a flat board, and when nearly a half circle is combined, a few nails are driven round the margin to allow the last wedge or sector to be driven in tight; when two such sectors are dry the last wedge is fitted into the space between them with the trying plane, and driven in tight to make out the last joint, the parts being restrained from slipping away by the use of a few nails as before, lastly the two circles when flattened are glued together and compressed by several hand screws.

The mould for casting laps, is in general an old grindstone in the center of which is placed the wooden disk, and around the latter is built up at the distance of 1 or 2 inches a border of soft clay. The metal, usually 1 part tin and 4 or 5 parts lead, (the lining of tea chests being preferred, on account of the tin with which it is alloyed and soldered,) is then melted and poured in, but the heat should be barely such as to scorch white paper. The lap when cold is fixed on the spindle, and its edge is turned true, the horse being used as the support for the turning tool.

42. - The Cylindrical Edge of the Lap, and which alone the cutler employs, is called the face, and the dressing or coating of emery, which is never used by cutlers with water, is called the head, terms applied in common to his other wheels. In order to make the smooth metal retain the fine emery, it is scored or scratched with a pointed knife, by which two series of slight oblique furrows are scored in the face of the lap, to produce a faintly but coarsely checkered surface.

43. - In Lapping Razors and Large Articles, fine emery and oil are mixed up in a cup, a small quantity is spread on with the thumb whilst the lap is nearly at rest, the emery is then pressed in the lap with a spoiled razor blade, or a short bar of razor steel, (that from which the blades are forged,) whilst the lap is in motion, and when the lap is charged the work is drawn steadily across from end to end and entirely off the lap, to reduce it to an uniform surface. After having preparatively lapped about one dozen of razor blades on both sides, which is called the first course, the process is repeated with finer emery, or else "to fine the lap," the head is rubbed off with a piece of felt, or with thick woollen cloth, and the surface of the metal is rendered as fine as possible, with a smooth piece of flint, or with a steel blade; and the lapping is completed in the last course on the nearly naked lap, a stick of charcoal being commonly used still more to deaden the emery before the flint is applied, and the charcoal moreover gives a black polish that could not otherwise be left from the lap.