Grinding. A new chisel, or one that has become considerably dull, must be ground. With the handle of the chisel in the right hand, and the fingers of the left hand resting on the blade near its cutting edge, apply the chisel to the stone, Fig. 50, as shown by the dotted outline a, and then raise the right hand until the proper angle is reached, a position indicated by the full outline b. See that there is a good supply of water, and, as the grinding progresses, move the tool gradually from one side of the stone to the other.

Fig. 50

Assuming that the stone is in fairly good order, the tool should be applied relative to its motion, in the manner shown by a and b, Fig. 50, the motion being in the direction of the arrow d. If the stone is not round or does not run true, there is danger that the cutting edge may dig into it, to the injury of both stone and tool. Under such conditions, it will be best for the operator to move round to the other side, and hold the tool in the position indicated by c. The first position is preferable, chiefly because of two reasons: first, the tool may be held more steadily; and, secondly, there is less tendency toward the production of a "wire edge." As the extreme edge becomes thin by grinding, it springs slightly away from the stone, and allows the chisel at points still farther from the edge to become thin, thus resulting in an extremely delicate edge which must be removed before the tool can be made sharp. In the effort to remove this wire edge, it frequently breaks off farther back than is desired, and the process of whetting is prolonged. With the chisel held at c (instead of b, the proper position) the direction of the motion relative to the tool aggravates this tendency of the light edge to spring away from the stone.

The grinding process is complete when the ground surface reaches the cutting edge - a condition readily determined by holding the tool to the light. If it is still dull, there will be a bright line along the cutting edge. When this line has disappeared, the tool is as sharp as it can be made by grinding, which, if persisted in, will only result in a wire edge. The action of the grindstone, however, is too severe to produce a good cutting edge, and the chisel, after being ground, must be whetted (107-110).

48. To whet the chisel, apply it to the oilstone A, Fig. 51, in the position shown by the dotted outline b, and as it is moved back and forth along the length of the stone, as indicated by the arrows, gradually bring it to the position shown by b'. That is, the angle between it and the stone is to be increased until the cutting edge c comes in contact with the stone; this position can be recognized by the sensation imparted to the hand, and the behavior of the oil with which the stone is lubricated. At first thought, it may seem that the bevel ab, Fig. 52, which was produced by the grinding, should be maintained in whetting; but to do this would require so much time that one corresponding very nearly to ab, as cd, is taken.

Fig. 51

Fig. 52

Great care is necessary on the part of one unskilled to avoid giving the tool a rocking motion on the oilstone; if this is indulgedin, the edge will appear rounded, as shown by Fig. 53, and will be no sharper than if it had the form indicated by the dotted outline abc. When sufficiently whetted, the cutting edge, if held to the light, will show a dull, grayish hue. If a bright line appears along the edge, it is not yet sharp. The whetting turns a light wire edge over on the flat face, an exaggeration of which is shown by a, Fig. 54. This cannot always be seen, but may be detected by the finger; it is removed by a single stroke of the blade with the flat face on the stone, as shown by a', Fig. 51. It is necessary, however, that every precaution be taken to prevent the production of a bevel indicated by the dotted line c, Fig. 54, and opposite that already existing. To guard against this, the chisel should be applied to the stone in the manner illustrated by the outline a, Fig. 51 (111-115).

A tool must be whetted often enough to keep the edge in good condition; it is dull whenever it fails to cut well. When, by frequent whetting, the whetted surface becomes so broad as to require considerable time in the production of the edge, it should be reground, and the process just described repeated.

This method of sharpening the chisel will, in general, apply to the gouge, drawing-knife, and all similar tools.

Saws.

49. The efficiency of any saw is measured by the amount of force it absorbs in making a given cut or "kerf." For example, if one saw severs a 4" X 4" timber with half the force required by another, it is evident that the second saw is only one-half as efficient as the first. Almost every element that enters into saw construction has its effect on the efficiency of the tool. Chief among them is the thickness of the blade, which, of course, determines the width of the kerf; for a wide kerf will require the removal of more material than a narrow one, and the force absorbed in each case must bear some relation to the amount of material removed. In recognition of this fact, the people of some eastern countries use saws designed to cut when drawn towards the operator, a method of handling that allows great thinness of blade - too great to stand the thrust by which our saws are driven into the work. But the result is that the Chinese saw, for example, which is represented by Fig. 55, accomplishes its work with remarkable ease. The shape of such a saw, however, and the awkward manner of applying force to it, probably more than neutralize the advantage gained from its delicacy, although in the abstract, the thinner the blade the better the saw.

Fig. 53

Fig. 54

50. The form of our own saws is not the result of chance, but, on the contrary, has been developed after a careful study of the conditions under which they are required to work. Other things being equal, pushing a saw gives better results than pulling it. Under a thrusting force, it is found necessary to make the blade sufficiently thick and strong to resist bending tendencies, but with no surplus material to add unnecessary weight. In view of these facts the outline of the blade is tapered, as shown by Fig. 56. The blade is thicker also at the handle than at the point. To assist in giving it clearance in the kerf, it is tapered from the teeth to the back. This difference in thickness is accomplished in the process of manufacture,by grinding the rough blade after it has been hardened. Imperfections left by the hardening or the grinding process, may be detected in the finished saw by bending the blade, as shown by Fig. 57. If it is uniformly ground and hardened, the curve will be regular as shown; if it is thick in spots, or if it varies in hardness, the curve will be uneven, as indicated by the dotted line.

Fig. 55

Fig. 56

Fig. 57