STONE-CUTTING AND DRESSING MACHINES. Stone is a substance which in none of its varities is easily operated on by machinery, owing chiefly to its brittleness, its unequal hardness, and the natural cracks which so frequeinly impair its solidity. Accordingly, though many ingenious machines have been invented for working stone. it is as yet only in some of the plainer kinds of work that they can be said to have entirely superseded hand operations.
Some stones and slates are soft enough to be cut with ordinary toothed saws much in the same way as wood is cut. More generally, however, the sand-saw is employed, which we shall presently describe in noticing marble-cutting. For the cutting of com mon kinds of stone, which are not to receive a fine polish, a machine, which promises to be. very efficient, has been recently patented by Mr. George Hunter of Maentwrog, Caer narvon, and is now in operation at various large quarrieS, both of stone and slate. The cutting portion consists of a circular disk, round the circumference of which a number of pointed steel tools are fixed into sockets. thus giving it the appearance of a largo toothed saw. This machine will cut sandstone at the rate of 5 to 6 in., slate at 3 in., and soft limestone at 3 in. per minute, supposing these to be in blocks each 2 ft. thick.
So far as the sawing or slicing of stones is concerned, the tendency of late years is to rely on the use of the diamond—the dull black variety which is of no use as a gem. Some American stone-cutting machines have saws with teeth set with these diamonds, and are said to cut ordinary sandstone at the rate of 75 sq. ft. per second for each saw. Machines for dressing the face of stones by means of a series of chisels in imitation of the handwork of the mason have recently been tried and have given fair results.
It is considerably more than a century since machinery for sawing and polishing marble was first established at Ashford, near Bakewell, in Derbyshire, that county being still the seat of the principal marble manufacture of England. Marble is cut into slabs by means of a series of thin plates of soft iron used like saws, but having no teeth. The saw-blades are fixed into a rectangular frame, to which a reciprocating horizontal motion is given. The block of marble to be cut rests on a carriage below the frame, and a small rill of mixed sand and water is constantly falling into the saw-cuts.
After the marble has been sawn into slabs it is cut up into narrow pieces, when so required, by means of small circular saws with smooth edges, sand and water being employed as above.
The sawn slabs are next submitted to the grinding process. This, for pieces of moderate size, is usually done upon a large circular cast-iron plate, called a or mounted upon an upright spindle, and supplied with sand and water. The workman places the piece of marble with its face downward upon the grinding-bed, and exerts the proper amount of pressure. The marble is held in its place by means of
guide-rods stretched across the plate. Slabs too large to be manipulated la this way are ground with plates of iron operating upon their surface.
The marble, when properly ground, is polfshed on a polishing bed or table, with an arrangement for securely fixing it while the rubbing is being proceeded with. The polishing rubbers are sometimes blocks of wood faced with felt, and sometimes bunches of hemp compressed between two side-plates. They are attached to a swing frame with a pendulum-like motion, which draws them backward and forward over the surface of the marble. Flour emery is used to charge the rubbers in the first instance, and putty-powder (oxide of tin) for the finishing polish. Instead of emery sometimes the tine-grained stone known as water of Ayr stone is used to prepare the marble for the putty-powder.
Cylindrical objects, such as columns or vases, are first formed roughly into shape with a hammer and chisel, and then turned, with a pointed steel tool, upon a lathe, to which a slow motion is given. When thus brought to an accurate form, a rapid motion is given to the and the tool-marks ground away by the use of coarse, and then fine, and still finer sandstones—the polishing being completed with emery and putty-powder while the object is stIll upon the lathe.
Machinery is also applied to the production of flat objects with curved and molded outlines. The machine for this purpose operates by the use of a rotatory cutter, which is gulled in its action by a template formed accurately to the intended shape of the article.
The cutter is of steel or stone, and is attached to the lower end of a spindle driven by beveled wheels. There is a flange which allows the cutter to penetrate the marble till it reaches the template and no further. In the process of cutting the marble is con stantly drawn up against the cutting-tool by two weights, the one pulling the table in one direction, ihe other the carriage on which the table rests, in ,a direction at right angles to the former, thus compelling the cutter to follow- the outline of the template. The shape of the cutting-tool is, of course, exactly the reverse of the molding to be formed.
In the cutting and polishing of granite, the machinery and processes are so nearly the same as those employed for marble, that it is unnecessary to describe them sepa rately. Suffice it to say, that all objects to which the sawing apparatus cannot be ap plied, require to be worked to shape with great care by means of steel chisel§ and iron mallets, which small portions at a time. Owing to the great hardness of the material, any defect in the chiseling greatly increases the labor of polishing. So slow, indeed, are the operations with granite, that a saw-blade will not cut through an inch in depth during a whole day, and a good-sized sawn slab will take a week to polish.