Читать книгу The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking - Paul N. Hasluck - Страница 132

Centre bits are the most commonly used bits, and the ordinary form with pin is shown by Fig. 385, another form with screw being shown by Fig. 386. Centre bits are useful for boring large holes, and are much superior to shell-type bits in the important point of boring exactly where the hole is required; hence in fitting locks small centre bits are useful for keyholes. Centre bits of the kind referred to are the most useful for holes up to about 1/2 in. The centre bit consists of a piece of steel so shaped as to fulfil these three requirements—a centre, a circle-cutter (the “nicker”), and a chisel (the “router”) to remove the core of the hole, and must be so formed as to act in the order named. In buying centre bits, bear in mind that the hole by no means agrees with the size of the bit; for instance, a 1/2-in. bit bores a hole at least 9/16 in. in diameter. The reason of this is that the pin is not quite in the centre, and so the circle cut is of larger diameter than the width of the tool. To gauge a centre bit, measure the distance between the nicker and centre of the pin, but not so as to get an oblique measurement. To be exact, bore a hole in a piece of waste wood similar to the kind being worked on, and measure the diameter of this. Large centre bits have two, sometimes even three, nickers on one side, and these bits are expensive. If it is required to cut out of thin wood circles of greater diameter than 3 in., they may be cut by a tool like the knife of a cutting gauge, fixed to an arm that rotates on a pivot fixed on a block of wood, acting like beam compasses.

Figs. 387 and 388.—Centre Bit.