Читать книгу The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking - Paul N. Hasluck - Страница 134

Centre bits should be sharpened with a file and oilstone slip, the latter producing the finished edge. The nicker O (Fig. 398) must be sharpened from the inside only, and in such a manner that the leading part of the edge is highest, as indicated by S in Fig. 392. The router B (Fig. 398) should be sharpened mostly from the top side; the under side should not have too great angle, or it will cause the edge to cut too deeply into the wood, and thus take off thick shavings and produce roughness. The correct angle for the under side of the horizontal cutter or router is regulated for the most part by the length of the vertical cutter or nicker opposite to it. Its slant should be part of a spiral, which, when continued round the bit, would pass through the lower part (near the point) of the nicker, as shown by the dotted line in Fig. 395, and also in Fig. 396. Theoretically, this is quite correct, and under such conditions it would be quite impossible for a bit to overfeed itself. But besides the question of necessary clearance, there is a temptation to make the angle steeper, to help in drawing the bit forward. The length of the nicker and a little experiment will in each case soon determine the most desirable angle. The nicker, too, is occasionally topped off on the outside, as shown in Fig. 394. Faults of boring with a bit so formed are excessive strain on the operator, and a badly finished and more or less oval-shaped hole. The nicker should be, if not quite upright, very nearly so, right down to the point, as shown by the line K L in Fig. 398. This line is not quite parallel to the axis of the bit, being a little nearer to the axis at K than at L. Having sharpened the point of the. nicker from the inside with a file and oilstone slip, take away any feather-edge that may have burred over to the outside by rubbing on the oilstone as shown in Fig. 399, which illustrates the safest way of doing this. To keep the back of the bit to the proper height while it is being rubbed, place on the stone a piece of wood whose thickness will depend upon the size of the bit to be sharpened. The proper shape of the nicker, as seen from the side, is given by Fig. 392. In a new bit there is scarcely any curve, but just a straight slightly sloping edge; if it is correctly made and sharpened there should be a keen edge from S round the curve to about T (Fig. 392). The length of this edge should equal, or slightly exceed, the depth of the thickest shaving the router is capable of removing.

Fig. 399.—Sharpening Centre Bit Nicker.

Fig. 400.—Special Centre Bit.

Fig. 401.—Vertical Boring with Brace.

Fig. 402.—Horizontal Boring with Brace.