In sawing up pieces of plank-wood, the broad surfaces left by the pit-saw will in general be found sufficiently accurate for their guidance in that plane, so that the edges alone then require examination, and one of these is sometimes corrected with a jack-plane, for greater exactness.
When the saw has been put in rapid revolution, and so that the teeth near the operator descend, the work is laid flat on the platform and against the parallel guide, and is then gradually advanced towards the saw. If the work be thrust forward too quickly, the saw may be altogether stopped from the excessive work thrown upon it, and if it be not advanced at an uniform rate, the markings left by the saw will present corresponding irregularities.
In dividing a piece of wood that is long' compared with its width, it occasionally springs open as a fork when sawn, so that the outside or guiding edge of the work, from having been originally straight becomes a little concave. This is sometimes allowed for by making the face of the parallel guide to consist of two straight lines, a little distant one from the other, instead of one continuous line, by fixing a thin plate to the principal piece by countersunk screws. The set-off in the guide usually occurs a little behind the cutting edge, and allows the work to escape the saw, so as not to be scored by the ascending teeth at the back part of the plate, and which are otherwise apt to catch up the work, if small, and throw the pieces in the face of the operator.
It usually happens that many similar pieces are cut in immediate succession; in such cases, the succeeding piece is frequently made to push forward that which is nearly sawn through, by which mode the risk of hurting the fingers with the saw is avoided; otherwise the piece is thrust towards the conclusion with a stick of wood, having a rectangular notch at the end.
The jointed platforms are very convenient, as they can be turned up to shoot off any accumulation of work or sawdust, and also for the removal of any little pieces of wood, which may occasionally become wedged in the cleft beside the saw.
7. Sawing grooves, rebates, and tenons. - When the platform of a circular saw machine does not admit of any change of elevation, as in that shown on page 765 and many others, the quantity the saw projects through the table can only be varied by selecting saws of different diameters, or by placing supplementary beds of different thicknesses upon the platform; the latter method generally interferes with the action of the parallel rule. But in the machine, fig. 736, constructed in iron, the hinged platform may be adjusted by the regulating screw in front, so that the projection of the saw through the table may, if required, barely exceed the thickness of the wood to be operated upon, or the saw may be only allowed to cut to a limited depth, and to form a groove either in the side or edge of the work.
By making two incisions on the contiguous faces of the wood, the solid angle may be removed, as in the formation of a rebate, fig. 737, the same cuts again repeated would form the tenon, fig. 728; but this process requires that the end of the wood should have been previously cross-cut exactly square, in the mode explained in the following subdivision of this chapter.
8. Sawing or cross-cutting the ends of pieces, either square or bevilled; or those in which the angular variations are in the horizontal plane. The most general guide for cutting the ends of work either square or oblique, is shown in fig. 736, and also in plan in figs. 740, 741, and 742 ; it is applicable to every angle. An undercut groove is made in the platform parallel with the saw, for the reception of a slide that carries a semicircular protraetor, which latter is graduated, and may be fixed at any angle by the thumb-screw passing through its semicircular mortise into tin-slide beneath. The slide has sometimes V grooves made in its two sides, and the platform is then in two parts with bevilled edges, corresponding with the V grooves. The work to be sawn is held by the fingers in contact with the straight fence of the guide, and the two thus grasped are slid together past the saw.
The guide for angles is represented in fig. 740, in the position for cutting rectangular pieces from the end of a long bar, and the edge p p of the parallel guide, then serves as a stop for the width of the blocks thus removed. By the similar employment of an oblique position, such as that shown in fig. 741; rhomboidal pieces of any angle and magnitude, may be as readily produced.
"When the pieces are not cut from the end of a long rod, but are small, and only require to be reduced to any exact size, it is more convenient, to affix the stop for width upon the fence or the semicircular protractor, as in fig. 741, and in this manner small pieces can be easily sawn into regular or irregular polygons of any particular angles and numbers of sides.
In cutting mitres, as for picture-frames, the once piece would be cut by placing the semicircular fence in the position, fig. 741, but for the other piece of the mitre, it is necessary to place the semicircle as in fig. 742, so that the guide may precede the work that is to be sawn; consequently, unless the slide will admit of being withdrawn from the groove, and replaced the other end foremost, there should be two holes for the thumb-screw, and two indexes for the graduations.
Although the oblique fence may be placed at the smallest angle, and even parallel with the saw, yet when the pieces are required to be thin and acute, it is more generally convenient to prepare with the apparatus, fig. 740, a wooden guide of the particular angle, and of the form shown in fig. 739; p, being the parallel rule; g, the guide or bevilled block, and w, the work. A separate wooden block is necessarily required for every angle, and the parallel guide is still available in determining the general width or thickness of the works.