Читать книгу The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking - Paul N. Hasluck - Страница 133

It is intended to show here the chief faults commonly found in a new centre bit, and the best methods of rectifying them. Figs. 387 to 389 show a well-finished and properly made bit, in front elevation, side elevation, and plan respectively. The most common fault is shown at A (Fig. 390); the corner of the horizontal cutter or router operates out side the limits of the circle described by the vertical cutter or nicker. This produces a deep scoring round the inside of the hole, and generally gives the boring a ragged and unfinished appearance. It also puts unnecessary strain on the operator. The projection should be filed away until the outline of the lip corresponds with, and just clears, the inside of the hole, as shown in Fig. 389. The second fault to be noticed is that the outside of the nicker often is formed badly, having more clearance at the front side (R, Fig. 390) than at the back, this producing a continual wedging at the side of the bit. A correctly formed nicker should have, if anything, a little clearance at the back, as shown at M, Fig. 391, which is a section taken on the line I J (Fig. 392). Other minor faults are shown at Figs. 393 and 394. Often in finishing chisels, plane-irons, gouges, and other edged tools, the workman tops off the edge from the wrong side, possibly to prevent accident; the centre bit, too, suffers similarly. The edge of the horizontal cutter is made to slant upwards in the direction shown by C D (Fig. 393), instead of downwards, as at B (Fig. 395). This defect is remedied by using a fine file and an oilstone slip, placing the bit on the corner of the bench, as shown in Fig. 397. The slope at the bottom of the horizontal cutter E Fig. 396, is the most important influence affecting the cutting power of the bit. It may be regulated so that, no matter what pressure bears upon the brace, the rate of advance in boring is uniform. In fact, owing to the want of a little attention here, much labour is often lost, in vain efforts on the part of the workman to make the bit cut faster. On the other hand, the angle of slope may be made so steep that the horizontal cutter will outfeed itself, and endeavour to expel from the hole more material than has been separated from the main body of the wood by the vertical cutter. The hole is then ragged on the inside, and many of the fibres project from the sides, while in other places there are signs of fibres having been dragged and torn away. Frequently bits are not correct to the sizes stamped on them; often they are too large, for the reason already given. To remedy this, if the discrepancy is not more than 1/32 in. to 1/16 in., a little filing may be done at the side of the centre pin. If the bit is too large, the filing should be done on the side Q (Fig. 391); if too small, file on the side N. If too large by more than, say, 1/16 in., the bit should be softened by heat and the vertical cutter closed up a little, the bit afterwards being re-tempered.

Fig. 389.—Plan of Centre Bit in use.

Fig. 390.—Faulty Router of Centre Bit.

Fig. 391.—Section of Correctly Formed Centre Bit.

Fig. 392.—Centre Bit Nicker.

Fig. 393.—Faulty Centre Bit Router.

Fig. 394.—Badly Sharpened Nicker.

Fig. 395.—Centre Bit Cutters.

Fig. 396.—Action of Centre Bit in Good Condition.

Fig. 397.—Sharpening Centre Bit Router.

Fig. 398.—Centre Bit Cutters.