It may be considered that in the last section, the remarks on the structure and use of the circular saw-bench, were concluded, so far as concerns its ordinary application to the conversion of timber into scantling, or squared pieces of various sizes. But it still remains to notice, in continuation, some of the miscellaneous and large applications of circular saws, which so far as admissible, will be introduced in the order formerly adopted, as the subdivisions 7, 8, 9, and 10, will be repeated, to which will be added the sawing of curvilinear works, and some other less classifiable matters.
Part of the contrivances for these works, are merely additions to the ordinary saw-bench, others are machines expressly constructed for their respective purposes; but to save unnecessary subdivision, they will be collectively and briefly noticed; as the principles rather than the mechanical details will be advanced, together with references to such published descriptions of them as have come under the author's notice. Two contrivances for obtaining an accurate base to work from, in pieces not originally straight, will be first referred to.
The late Mr. Smart, in obtaining the first true side in irregular pieces three or four feet long, intended for the staves of casks, attached the pieces to an external fence or guide. The wood was grasped by its extremities, somewhat as between the centers of a lathe, in a kind of trough made of two boards united at right angles; one end of the trough had a solid block of wood, that could be fixed at variable distances; the other end had an iron bar, roughened at its extremity, and brought up by a rack and pinion, so as to stick into the ends of the wood, the grasp being secured by a ratchet.
The trough was considerably longer than the length of the wood to be sawn, and two studs projected from its extremities beyond the side of the work. These projections were made to rub against the face of the parallel rule, and avoiding the saw, to direct the cut exactly in a right line, and produce, on the irregular wood, one flat surface that might serve as the base for the subsequent operations.* The same end is differently obtained, and on larger pieces of timber, in the following method.
* See Trans. Soc. of Arts, Vol. 47, plate 8.
In the Ravensbourne wood-cutting mills at Deptford, battens 10 or 12 feet long, and intended to be sawn and planed fur flooring-boards, are grasped by their upper and lower edges, and without strain, by screw-teeth or dogs built out from a carriage which runs in V bearings; the slide is carried by a self-acting rack and pinion movement, past a circular saw revolving in a vertical plane, which skims the side of the batten, and leaves it as straight as the V slide itself. The traversing carriage or drag of this machine, is closely analogous to that of the veneer saw to be hereafter noticed.
7. Sawing grooves, rebates, and tenons. - These works may be accomplished in the large way, in the modes already described on page 761. The flooring boards of the warehouses in the St. Katherine's Docks, London, were grooved on each edge upon an ordinary saw-bench, for the reception of strips of hoop-iron used as tongues to prevent dust falling through the joints; and the frames for doors are occasionally grooved for the panels in the same manner, but with thick saws. The still wider rectangular grooves in the blocks for wood pavement, are cut out with two ordinary saws on the same spindle, having two or more intermediate chisels, to cut the bulk of the removed wood into chips.
The mortises in the shells of ships' blocks, for the reception of the sheaves, are cut by small double circular saws; a hole is first bored through the shell at each end of the mortise, and the saws are then made to penetrate from each side, and nearly complete the mortise, in a less expensive manner than with the mortising engines in Portsmouth Dockyard.
The squares or tenons of the steel pins for harps, by which the strings are tuned are also cut by means of two thick saws, separated to the extent of the side of the square; the pin is presented twice to the saws, the second position being at right angles to the first. The equality in size of the squares is also ensured by this method, so that they all fit the same tuning key.
Rebates may of course be cut upon the ordinary saw bench at two processes, as before explained, but they are also made by two saws mounted on separate spindles, and placed in the exact directions of the two cuts required; one saw spindle is a little before the other, to avoid the contact of the teeth. The angular grooves or rebates in the blocks for wood pavement, are thus made at one operation, in a machine with two saws at right angles to each other.
The combination of two saw spindles was first employed by the late Mr. Smart, in cutting the tenons for the construction of his patent hollow mast. The small pieces of wood were first squared on all sides to the proper measures, each small block was then rebated, first on the one angle, it was then turned over, and the formation of the second rebate completed the tenon. Another part of the same machine carried a mandrel and center bit, so that by the aid of a guide, the holes in the tenons could be also made exactly true and alike.*
Two saws mounted on the same spindle are used in cutting the teeth of combs, which may be considered a species of grooving process. One saw is in this case larger in diameter than the other, and cuts one tooth to its full depth, whilst the smaller saw, separated by a washer as thick as the required teeth, cuts the succeeding tooth part way down, on the same principle as in the studda, and rack saws, figs. 703 to 706, page 723.
A few years back, Messrs. Pow and Lyne invented an ingenious machine for sawing box wood and ivory combs. The plate of ivory or box wood, is fixed in a clamp suspended on two pivots parallel with the saw spindle, which has only one saw. By the revolution of the handle, a cam first depresses the ivory on the revolving saw, cuts one notch, and quickly raises it again; the handle incompleting its circuit, shifts the slide that carries the suspended clamp to the right, by means of a screw and ratchet movement. The teeth are cut with great exactness, and as quickly as the handle can be turned; they vary from about 30 to 80 teeth in the inch, and such is the delicacy of some of the saws, that even 100 teeth may be cut in one inch of ivory; the saw runs through a cleft in a small piece of ivory, fixed vertically and radially to the saw, to act as the ordinary stops, and prevent its flexure or displacement sideways. Two combs are usually laid one over the other and cut at once; occasionally the machine has two saws, and cuts four combs at once.