Strained Saws.—There are more varieties of strained saws than of unstrained, the former class being divided into (i) those kept in tension by a spring and (2) those strained in a gate or sash, and hence having a positive pull which is practically unvarying each way.
String-strained Saws, which are of small size, are generally used for cutting curves in thin stuff Of this type are the " scroll-," "jig-," or " fret-" saws, which may have either (1) a plain wooden lever, (2) leaves of steel fastened at one end, (3) a regular leaf-spring held in the middle, or (4) fiat spiral springs whose axes bear levers having fastened to them the levers attached to the saw cross-head. In some power-driven machines, where the blade is held at both ends, the strain is greatly variable during the stroke, as the tension is put upon the blade partly by the amount of pull upon it and partly by a spring, which may be a simple lath or a more complicated leaf, attached at that end which would be called the tip of a pull-cutting saw. In either case the work to be done is the same: keeping the blade as nearly as possible at the same tension during the entire cut ting-stroke and preventing it from buckling and fouling during its return. Some jig-machines have the table and bed separate from the guides and from the tension device, while in others the cross-head is driven by a pit man from a shaft below. For cutting bevelled lines the table needs to be pivoted and should tip in a plane revolving about an axis that will make it form an angle with the side of the saw; so that, as the operator stands facing the saw-teeth, the table tips either to the right or to the left. Both ends of the blade may be much more effectually held in a "frame" or "sash." In this case the tension, usually given by taper-keys or wedges, is applied permanently to the blade of the saw, any variation being that caused by the buckling of the eross-members of the frame.
The is practically the same as an ordinary singlc-blade sash saw, but the gate is much wider in proportion to the length of the saw and stroke, taking in from 64 to 84 inches swing for a thickness of 6 inches of material sawed and having a stroke of from 4 to inches. Such a machine (pi. I I, fig. 2) has the gate driven from both sides (or ends) by pitmans from au overhead shaft.
Gam; lumber-sawing it is important to save both material and labor. Circular saws, on account of the width of the kcrfs they cut, waste too much material, and as they cannot be made with thin blades, and as not every one can run a band-saw, the gang sash, by reason of the extremely thin saws it can use, offers the best means not only for saving material (in which respect it is equal, if not superior, to the band saw), but also for saving time and labor by making several cuts at once, in which latter respect it has the advantage of the band. But the ordinary gang sash is a reciprocating machine which if not properly balanced will shake itself to pieces and endanger its surroundings. Increase of weight, required for heavy sawing, only makes the matter worse; for, if unbal anced, the heavier it is made the weaker it is. In one type of compen sating gang the sash (fig. 3) and blades are counterbalanced by a weight attached to the opposite side of the crank; the weight of the two pitmans driving the counterpoise is equal to that of the pitman driving the sash, and the counter-weight exactly balances the sash. As one pound goes up another goes down with the same velocity, and the mill runs smoothly.
The machine is self-contained, and it is claimed that if set upon beams of a mill it will run quietly at three hundred revolutions per minute. One disadvantage of the gang sash is that when sawing cants which are slabbed only top and bottom there is no opportunity for selecting from the face of each cut the best clear stock, the sawyer being likely to let the best of the log go to form part of each board, the grade of which may be lowered by knots and shakes, whereas were they sawed off separately with a circular or band-saw he could pick " the fat and the lean." some purposes a jig-, scroll-, or fret-saw is better than a band, as it will make both inside and outside cuts; furthermore, the band-saw table, tilting but one way, allows draught upon one side only, while the jig gives draught all around. As a thinner blade can be used on a jig- than on a band-saw, the former will do curved work more smooth ly and accurately, and less time is required to start and stop a jig- than a band-saw. To secure greater rigidity in machines having the table sepa rate from the upper part, the guys, tightened by turnbuckles, are ex tended from the lowest possible point of the vertical post to points upon the ceiling. In jigs it is desirable to have at all points of the stroke an equal strain upon the blade, and to be able to vary the tension. In some machines the deflecting strain is produced by two steel springs (each made of a series of graduated lengths), to one end of each of which are attached straps extending to a segmental pulley. As these straps are wrapped around this pulley, the leverage is lessened as the strain upon the springs is increased, thus compensating for the distance the saw travels (pi. TT, fig. 4). The deflecting springs are attached in a sliding cross-head adjusted to suit the thickness to be sawed. In jigs, if the blade is held in a clamp, it will be unnecessary to punch or drill holes in the blade, and this is also an advantage when using broken blades (fig. 5). In one make the upper end of the blade is held in a clip attached by a link to the outer end of a lever with its other end keyed upon a shaft bearing a strong flat spiral spring; this single lever lies in the plane of the saw and about at right angles to its length. Where there is a spring-head it is desirable to have it slide on the standard, so that the same strap-length may be used for blades of different lengths. The hold-down and the slides should be adjustable independently of each other for thickness of stuff and for length of blade. To keep the teeth cool and the working-line visible, every jig should have a blower provided with a rubber tube, to conduct the air to the saw-kerf, while some of the blast may be directed against the lower guides, to keep them cool and free from sawdust. As on inside work it is necessary to stop and start frequently, some machines have instead of fast and loose pulleys a friction-brake, which will save about half the time on such inside work, while in others the machine can be stopped when run ning at full speed, and the blade can be quickly removed and replaced. An tmstrained or " mulay " jig does not hold the blade at the top, but simply guides it. Kept close to the work is an adjustable guide, which holds the sides and back of the blade between steel plates adjustable to the thickness of the different blades used. There being so little recipro cating weight, such machines are adapted for use on the upper floors of buildings.