HOW A SAW CUTS
An Explanation of the Construction and Operation of Hand Saws. Why cannot a board be cut by a blade with a knife edge when such a blade is used in the same manner as a saw?
Of course we understand that an instrument of this kind will sever exceedingly thin stock. But when it comes to a board of ordinary thickness, the knife edge will merely score to a certain depth. This depth depends upon the length of bevel. The thicker portion of the beveled edge, coming in contact with the sides of the scoring, prevents the blade from entering further. The blade merely glides back and forth. If additional pressure is applied in an effort to force the cutting edge further into the work, the friction becomes so great that the blade will be jammed or wedged in the board. The reason for this is, that there is no displacement of wood to allow the cutting edge to reach more of the wood continuously, or to allow the back of the blade to slide without binding.
Saw Action Similar to that of Chisel. The nearest approach to the cutting action of a saw is the action of a chisel. However, instead of cutting out small pieces of wood like sawdust, the chisel separates and removes a long shaving, by what may be appropriately termed "paring.'' or ''slitting." Its comparatively broad sharp edge separates the fibres of wood lengthwise with the grain and does not cut on the sides. The wedge-shaped end merely raises the shaving or chip and the chisel, pushed ahead, tears the fibre at the sides. The angle at which the chisel is held, the amount of bevel on the cutting edge, the pressure exerted, and the hardness of the stock are the factors which govern the thickness of the shaving.
The actions of both the knife and the chisel are employed in the cutting done by a cross-cut saw. A cross-cut saw moves successive pieces of material, not long shavings but small particles called sawdust, by scoring, cutting, and tearing.
The similarity comes to the rip saw in that its teeth are practically a series of small chisels.
The hand saw for cross grain cutting possesses practically V-shaped teeth. The teeth are set or bent over slightly to clear the body of the blade in the kerf. Although the back of each tooth is beveled as is the front, it is the outside edge of the front of the point that does the cutting.
How a Hand Saw for Crosscutting Cuts. Take a cross-cutting hand saw, properly set and sharpened, each tooth of uniform size, shape, set, and bevel. Make with this a light short cut across a smooth piece of lumber. One can see that the extreme points on both sides of the cutting width of the saw first made parallel scorings the width of the set. These scorings are similar to the line cutting of a knife across the face of the wood, thus starting the cut. Then as pressure is applied, the teeth enter deeper and deeper, gradually bringing into action the cutting edge on the outside front of the points. The forward motion of the blade causes the points and cutting edges to strike the fibre at a right angle to its length, severing it from the main body of wood on each side of the blade, and paring the ridge of wood between scorings. A continuation of the thrust pressure carries the teeth in farther until the full bite is taken. With the points scoring continuously on each stroke and the outside edge of the tooth cutting, the beveled front edge of each tooth performs its duty, chisel-like, of crumbling up and dislodging the upper portion of wood left between the cutters. At each thrust of the saw the pieces of wood are carried out of the kerf in the throats or gullets between the teeth, until finally the board is completely divided.
How a Rip Saw Cuts. The ripping of lumber, that is cutting or splitting it lengthwise with the grain, requires different action on the part of the saw tooth from cross-cutting. Consequently, the rip tooth is of another shape. Although a cross-cutting saw can rip lumber more easily than a rip saw can cut across the grain, the former operation would be slow and arduous work. The form of tooth in a cross-cutting saw does not properly sever the fibres in ripping because the line of the cutting edge runs with the fibre instead of across it. This being the case, the saw cannot cut freely nor entirely clear itself in the kerf. The rip saw tooth has a straight front. Its cutting edge strikes at practically a right angle to the fibre of the wood, but severs it at only one place- the front of the tooth wedging out the piece of wood.
This may be more clearly understood by making a direct comparison with the cross-cutting tooth. As previously stated, this scores with the point and cuts with the knife edge on the outside front. The rip tooth, with its straight front and cutting edge on top, strokes down. The comparatively wide cutting edge, cutting across the long fibres of wood, enters deeper and deeper. The wedgelike body of the tooth presses against the partially severed piece of wood until, unable to stand the strain, the fibres on the sides and bottom of the small section tear apart.
Piece after piece, each successive tooth cutting its portion of half the width of the kerf, is thus separated from the main body of the board and carried out in the gullets of the teeth at each thrust of the saw. In this manner, the rip saw practically chisels out the kerf in small sections, leading to a complete division of the board.
On a cross-cutting saw the pointed teeth, set alternately to right and left, leave a shallow groove, which runs along the' cutting edge from butt to point. This groove is deep enough to allow a needle to slide in it. This is not true with the rip saw. It will be noticed, on glancing down the cutting edge of the rip saw, that the square topped teeth extend entirely across and beyond either side of the blade. The inside of the tooth on the night barely overlaps the inside of the tooth on the left.
Only Small Part of Saw Tooth Actually Cuts. It is a common supposition that the entire tooth of a saw cuts. As a matter of fact, however, the actual cutting is done, with the cross-cut saw, by the points, and front cutting edges which extend only to where the right and left teeth overlap; and, with the rip saw, by the chisel-like edge of the teeth.
Because they are set alternately night and left, each tooth individually severs only half the width of the kerf. This division of duty by the numerous teeth in the hand saw makes possible the performance of quicker work, involving less effort and driving power than if the full width of kerf were cut by each tooth.
So it will seem that, no matter what sort of a saw, the cutting edge of each tooth makes an incision across the long wood fibre. Then the base of the tooth plows out the small pieces thus separated. This is done continuously with each stroke of the saw. The saw enters farther with each thrust, the kerf becomes deeper, until a complete separation of the board is accomplished.
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