Читать книгу The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking - Paul N. Hasluck - Страница 48

BEFORE describing edge tools and their uses, it will be well to discuss the action of such typical tools as the chisel and knife; though the latter is not generally found amongst woodworkers’ ordinary tools. It is desirable to examine the manner in which the power applied to a chisel is used, and for simplicity’s sake it is better to conceive the cut as being made upon some homogeneous material, such as lead. Taking first the ordinary method of paring, in which the face of the chisel is in contact with the mass of the material, Fig. 130 shows the force A that impels the chisel; B is the resistance of the material to the crushing action of the edge of the chisel; C is the reaction of the paring to the pressure exerted by the bevel of the chisel; and D is the reaction of the body of the material to the pressure exerted to the face of the chisel. To obtain a proper conception of the actual cutting edge, it must be borne in mind that the edge of even so delicately sharpened a blade as a razor presents a jagged, rough appearance when viewed through a microscope, and theoretical or mathematical conception of an edge is never attained in practice. It is the greater or less approximation to it—or, in other words, the sharpness or bluntness of the chisel—that determines the amount of the force B (Fig. 130). The passage of a thin parallel blade with a square edge through, say, a bar of soap is obtained entirely by the crushing and displacement of the material, and the thinner the blade the less the resistance, and therefore the smaller the force required. The edge of the chisel must be regarded as such a blade of exceeding thinness; but to allow the passage of the chisel blade, which has considerable thickness, the paring must be bent aside as it advances. In setting out the force B, the frictional resistance to the advance of the chisel may be included. In its resistance to being bent aside, the paring acts as a very short cantilever; so it is evident that the bending force which produces the reaction C increases rapidly as the paring becomes thicker.

Fig. 130.—Direct Paring with Chisel.