A circular saw is nothing more than a circular plate of steel, having teeth upon its circumference, and made to revolve upon an axis with great rapidity, by means of bands or straps. The saw itself may move either on a horizontal, a vertical, or an inclined plane; and as the timber may be laid upon a plane inclined in any given direction, it may be sawn in lines, mak ing any angle whatever, or at any given distance from one another. When the saw is fixed at a certain angle and at a given distance from the edge of the frame, all the pieces of wood may be cut exactly of the same size, by pressing them against the edge as the saw is cutting them. The following is a description of the circular saws at Rothiemurchus in Inverness-shire.
There are two kinds of saws made use of in the Rothiemurchus saw-mill, circular saws and upright ones. A circular saw is a thin round plate of steel, toothed on the circumference fixed on a revolving axle. An upright saw is the common saw made use of by sawyers, fixed in a frame moving vertically. In both cases, the log to be sawn is fixed to a frame, which is moved against the saw. Each of these con structions has peculiar advantages. The upright saw, it is evident, cuts only in descending; there are also two points in every stroke at which it is stationary, the one when it is at its height, the other when at its lowest. A large proportion of the time of each stroke is thus consumed without effect. A circular saw cuts during the whole of its revolution; and it is found that a much greater velocity can he given to a circular motion, which is equal and constant, than to an up right one, which is necessarily unequal. A circular saw is thus much more expeditious than an upright one. It is, however, much more limited in its appli cation, as it can only work in wood of less depth than the radius of the saw. The size of wood is further controlled by the thick plates of metal which are made use of to fix the saw on its axle. These jiatinges, as they are called, it is found by experience, require to be about one-third of the diameter of the saw. Be sides this, the size of the saw itself is controlled by the thickness required to give the plate sufficient stiff ness. A circular saw, too, one-eighth of an inch thick, would occasion much loss in saw draft. In this mill no saws have ever been used above three feet diameter.
The application of upright saws is controlled only by the length of stroke given to the saw frame and its size; the stiffness of the saws is given by stretching them tightly between the upper and lower ends of the frame, and is therefore in a great degree independent of the actual size of the saw.
The intention in this mill is to saw by circular saws, where they would not occasion too great a loss insaw draft; and in this case only, to saw by upright ones.
The circular saws run about 1000 turns per minute, and will cut 10 inches deep on 3-16ths of an inch of saw draft. The upright saws make about 120 strokes per minute, and will run on rather less saw draft.
The circular saws will cut a 12 foot log in less than a minute; the upright saws in equally fine work, will not cut above 15 inches in the same time.
In the process of making deals with circular saws, the first operation is called slabbing; it is to reduce the tapering round log into a plank as thick as the deals are intended. to be broad. This is done by run ning two saw drafts parallel to each other through the log, which produces a flat-sided round-edged piece of timber of equal thickness, but tapering in breadth. The slabbed log is then cut into deals of the required thickness, by running the saw through it as often as is necessary at right angles to the former saw drafts.
In this mill the slabbing is performed by two saws fixed on the same axle, S, S, Figs. 2. and 3. the dis tance between which regulates the thickness of the slabbed log. A plank or batten B, so narrow as to pass freely between the saws when at the least requir ed distance from one another, is laid parallel to the face of the saws in the centre between them. A broader plank D, E, is laid parallel to this batten on the outward side of each saw, as far apart as to pass the saws freely when at the greatest required distance from one another. These three planks are connected together by cross pieces A, C, at the ends, and be ing laid on rollers R parallel to the axle of the saw, form a table, moveable backwards and forwards, limited in either range by the connecting cross-pieces coming against the saws. This travelling table is moved by a pinion working in a rack, which is insert ed on the under side of one of the broad planks. The power is raised as necessary, by a smaller pinion working on the larger, and put in motion by the hand of the workman by means of a winch W. On the farther end of the batten is fixed a head-stock H, a few inches high; and a grapple GJ, turning on joints at J, and having a cross-head at G, into which three teeth are inserted, G, G, G. Fig. 4 shows this grapple on a larger scale.