Convenient as the circular saw is when fitted as an adjunct to the lathe, its use is confined to pieces which are rectilineal. Curved lines cannot be cut by its means, and as it must frequently happen that portions of the proposed work are composed of arcs of various dimensions, it becomes necessary to provide the means of cutting them out. We may remark here, that although the circular saw, and that of which we are about to speak may be fitted to mount on the ordinary lathe bed; it is better for many reasons to have a separate stand, made like that of the lathe, but smaller, and fitted with crank, treadle, and flywheel, to serve for the various purposes of sawing, grinding, or polishing; the latter operations especially soiling and tending to damage the lathe. The above description of the methods of mounting circular saws will answer for a separate stand, as will the following details of saws for curvilinear work. In respect of the latter we have to provide for the perpendicular motion of the blade, which is necessarily thin and narrow, and also for stretching the blade so as effectually to prevent it from bending or buckling—guides are not required in general, as the work is moved about by hand in all directions according to the intricacies of pattern to be traced. For plain circular pieces, however, a very simple expedient is sometimes used, which will be described in its proper place, when treating of Bergeron's scie mecanique. The guides for parallel motion are various, and a selection may be made from the following [Fig. 191]. No. 1 is the arrangement used by Professor Willis, and detailed in Holtzapffel's work. A, A are wooden springs, one above, the other below the platform B. C is a guide pulley, D an eccentric. The catgut band which gives motion to the saw may be passed round this, or affixed to a metal ring as in the eccentric of a steam engine—or may be attached to a ring slipped over the pin of a crank disc, as shown at E. This pin being adjustable, permits the traverse of the saw to be regulated, which gives it perhaps an advantage over the first method. In the above the motion of the saw is not truly in a right line, but the deviation in so short a traverse is unimportant. The reader may, perhaps, imagine wooden springs a somewhat primitive expedient, but this is by no means the case, as they will retain their elasticity longer than metal ones when they are subjected to the rapid vibration which they are called upon to undergo. No. 2 is the parallel guide, used by Mr. Lund, and also described by Holtzapffel; the metal springs, however, shown by the latter being here replaced by india-rubber, which is now formed into springs of various sizes and powers suitable for our present and many similar purposes. A, A, and D, here form guide pulleys, the saw, E, being suspended from the first by the two catgut bands, on the ends of which are the india-rubber springs, F, F. The lower end of the saw is attached to another catgut band which passes over the pulley, D, and thence to the eccentric or crank disc as before. The platform is at B, B. [Number 3] is an arrangement similar to the beam of Newcomen's engines. The arc at the end of the oscillating rod, and from the furthest point of which the saw is suspended, forms the guide for parallelism. Underneath the platform the pulley and eccentric may be used as before, and the saw is raised by the spring attached to the arc as shown. It will be evident on inspection that this arrangement is similar in principal to the last, as the arc forms part of a large wheel of which the centre is the point of oscillation. Watt's parallel motion, represented in the next diagram, is also suitable, the saw being attached to the centre of the short link—the springs being so contrived as to act upon the ends of the longer bars. With regard to the means of producing the necessary rapidity of movement, the above-described eccentric or crank disc can hardly be surpassed. In the saw patented lately by Mr. Cunningham, the disc is attached to the mandrel like a chuck, and the crank pin is connected to the oscillating rods that carry the saw by an intervening rod or link.
Fig. 191, no. 1 and 2.
Fig. 191, no. 3 and 4.
Fig. 192.
The whole is represented in [Fig. 192], which is copied from the inventor's circular. There is a satisfactory parallel motion, and an india-rubber ball with a small tube attached is pressed at every stroke to blow away the sawdust. The whole plan and details are as good probably as can be devised, and as an addition to the lathe this saw is invaluable. Another form of mechanical saw to work with the foot, but without any flywheel, is figured by Bergeron, and is thence copied into Holtzapffel's book, and would therefore have been omitted here were it not that the price of Holtzapffel's work places it beyond the reach of many whom it is specially qualified to instruct; and that the former is in French and has not been translated. Therefore, as the arrangement of saw is exceedingly good, the writer has determined to introduce it here. Its construction is simple enough to be within the reach of any amateur in carpentry, and the only metal work required consists of a few iron rods screwed at the ends, such as the village blacksmith can readily supply. The saws are precisely those sold as turn or web-saw blades. It must be understood that the use of this tool is not the same as that to which fret or buhl saws are applied, but merely the cutting of boards in strips or curved pieces, such as the felloes of small wheels, circular plates to be finished in the lathe, as bread platters, or such other curvilinear works as the chair or pattern maker is accustomed to cut out with the several sizes of frame saws.
A, B, [Fig. 193], is a stout bench with cross bar, C, underneath, cut away to allow of the movement of the treadle and its rod. On the top of the bench is a pillar, D, to support the spring bow E, by which after depression the saw is raised to its original position. F, F, and G, G, are guide rods (not continuous). The lower ones are fixed to the cross bar, c, and under side of the bench. The upper form the sides of a rectangular frame, H, H, of which the top and bottom bars of wood are dovetailed at the back to slide up and down the chamfered bar behind them, K. The frame thus allows of being raised or lowered, not only to suit work of various thicknesses, but also to act as a stop to prevent the saw from lifting the work as it ascends. The lower bar is extended on one side as in the figure, and is divided into inches, and on this graduated part is a slide with a point below, which can be fixed by a screw. This is, as the drawing plainly shows, intended for the guidance of the wood in cutting circular pieces. The saw is similar to the ordinary framed saw used by chair makers, but has two blades, and one central stretcher. The saw for curved work is narrow, that for straight cutting is broader. Near the latter a parallel guide is fixed, as described when treating of circular saws. This simple contrivance, although planned so many years ago, is of great value, and deserves to be far more generally known. To the joiner and cabinet-maker it would form a most useful addition to the usual tools of the workshop.