Saws of six to ten inches, are sometimes used in machines such as that shown on page 756, for very small pieces of ivory veneer and for slicing up wooden mosaic works, but it is more usual to employ larger saws for miniature leaves, say those from fifteen to twenty inches diameter, and consequently larger machines are also required, which are driven either by a hand fly-wheel or other motive power. The principal variations between veneer saw-benches, and those for ordinary and thicker works, is in the parallel guide, which, for veneers, is made fully as high as the width of the block to be sawn, by screwing a parallel piece of wood or metal against the vertical face of the parallel rule, and cutting it off in a circular arc, exactly to agree with the curvature of the saw, and without extending at all behind it. In many cases the parallel guide is constructed with a set-screw, that it may be adjusted for distance very minutely, after which it is fixed as usual. When, therefore, the block of ivory or wood is placed against the parallel rule, and pressed towards the saw by hand, the thin leaf bends away as cut from the block, or yields sufficiently to pass behind the saw without impediment.
In bevilled or veneer-saws for ivory, the teeth should be finer, and the rate of motion slower than for wood, say, three-fourths the speed, as when a considerable velocity is used the saw becomes heated, and this, from the gelatinous nature of the material, causes the sawdust to adhere to it; the heat also tends to split the thin leaves of ivory. These sources of mischief are avoided by giving to the saw-blade a subdued rate of motion, and keeping it moderately anointed with tallow or lard.
Some idea of the delicacy of veneer-saws for ivory, will be given by the inspection of the annexed scale, which shows the average numbers of veneers or leaves cut from each solid inch of ivory: -
When the width of the ivory is
Each inch of ivory is cut into
The leaves from 1 to 2 inches wide and 2 to 8 inches long, are used for memorandum-books, the larger sizes for miniature leaves, the lengths of which are about one-third more than their widths. When scraped and prepared ready for the artist, the 30, 27, or 16 leaves, respectively measure about half an inch in total thickness, showing the waste in sawing and scraping to be equal to about one-half the original material. The leaves might be cut still thinner, but this would be objectionable as regards their intended purposes.
The bevilled or veneer-saws, when used for wood, have greater diameter, coarser teeth, are used without grease, and at a higher velocity than for ivory; but the single-plate veneer-saws are not frequently made of the full-size named in the table, nor are they used for wood exceeding about six inches wide, or that has not been previously squared into small pieces.*
In the larger applications of the veneer-saw, it is built up of segments or separate plates of steel, screwed to the edge of a metal disk or chuck. Some few of the smallest segment-saws are even less than two feet diameter, and those not exceeding about four feet diameter are generally used in the ordinary saw-benches, with fixed horizontal platforms, the work being then fed by hand as usual.
But when the segment veneer-saw exceeds about four feet diameter, the horizontal platform or table is rejected, and the guidance of the wood is entirely effected by machinery, called the drag; the arrangement of this construction, which is known both as the veneer-mill and the segment-saw, is shown in the perspective figure 804, page 812. The veneer almost always proceeds from the edge of the saw, through a curvilinear trough parallel with the back of the saw; but in the figure the veneer is represented as if bent almost at right angles, so as to quit the saw in front; this construction is far less common, but was selected for the present illustration, as it affords a more conspicuous view of the entire process.
* A manufacturer, experienced for thirty year* in cutting miniature leaves, generally employs single-plate saws from sixteen to twenty inches diameter. He also uses a segment-saw, measuring the larger diameter, when new, and composed of six segments, attached to a gun-metal chuck, the edge of which is very thin, and the center enlarged into a boss cut with a hollow screw, for its attachment to the saw-spindle, which runs in a collar and center, exactly after the manner of a lathe-maudrel. He prefers about eight to ten points per inch, and an average velocity of about 000 to 700 revolutions per minute; in topping the teeth, he uses a steel turning-tool, and sets the teeth before sharpening them.
He adds, that when the blocks of ivory are out into lengths, prior to being sawn into veneers, loss occurs, because the central and wider leaves require to be longer than those from the same block, which are exterior and narrow. Sometimes the entire tooth, or a large portion of it, is cut into veneers with the large segment-saws, having the drag (to be described); this is better as regards the cutting of the leaves into squares; but the apparent economy is again lost, as these saws being intended for wood, have coarser teeth, and will not leave such smooth surfaces as the saws exclusively used for ivory, neither will they produce more than about fourteen or fifteen veneers from each inch of ivory.
In the veneer-saws furnished with the drag, the axes run in massive brass bearings, which are fixed on brick or stone piers; the edges of the larger saws dip below the ground into a pit lined with brickwork or masonry.
The axis of the saw is connected or disconnected with the steam-engine at pleasure, by means of a fast and loose pulley; and in bringing the saw to a state of rest, the brake-wheel at the end of the axis is strongly grasped by a friction-hoop, as in some cranes. Between the driving pulleys and the cone for the saw is placed a bevelled pulley, for a catgut band or rope that is used in feeding the cut, as will be hereafter explained. The saw, which is the all-important part of the machine, is made of great strength, and consists of three parts, shown in the section of the edge, fig. 803, of which the shaded part c to c is of cast-iron, the white part s to s of soft steel, and the black h to h of hardened steel.