This is an extract from the final copy of Issue 10. (Remember it’s free).
It might be assumed that every carpenter and cabinetmaker knows the right from the wrong ways of sharpening an auger bit; no doubt many do. But the auger bit is a little different from a good many other edge tools in that knowing the right way to sharpen really depends on knowing the idea on which the bit is constructed. The modern perfected bit represents a vast amount of experimenting with angles, cutting edges and screw threads which in the new bit are in exactly the right relation to each other. A little wrong use of a file easily destroys the correct planning of the manufacturer. For this reason, it may be profitable to many workers to summarise here some of the directions given from headquarters.
The first work of a bit is of course done by the screw on the tip. It is part of the business of this screw point to centre the hole, but even more is it required to lead the broader cutting edges – the lip and the spur -into the real work of boring. For ordinary uses, and for woods that are not excessively hard
or gummy, the worm (also known as the snail and lead screw) has a double thread, Fig. 1, of carefully calculated pitch. The so-called quick-boring bit, however, has a single-thread point,
Fig. 2, the thread having a steeper pitch than that of the double-thread point. Because of its more prominent and steeper single thread, the quick-boring point leads the bit effectively into the gummiest woods, or into the hardest, like lignum vitae. In softer woods this point insures very fast cutting. Bits of this type are therefore particularly well suited to an electrician’s work.
A third style, the square, or “diamond” point, Fig. 3, is sometimes provided for bits to be used on machines with forced feed.
The following directions for sharpening bits are gathered from the text of an essay on the subject just issued by the Russell Jennings Mfg. Co., Chester, Conn., maker of the well-known bits of that name, and which represents pioneer patents for the extension lip found in modern bits. Sharpening the point or worm is hardly advisable or even practicable except with the diamond point, though the skilful manipulator of a fine, three cornered file can do a good deal in the way of restoring a point that has got battered in a collision with a nail. But it is with the lips and spurs that most can be done in the way of sharpening, and it is important to sharpen these in a way that shall to the utmost maintain the efficiency of the brand-new bit.
The spurs of the bits should be sharpened with a flat, second-cut file used on the inside of the spur, never on the outside. The general shape of the spur should be maintained as in the new bit. In filing, it is not necessary to sharpen the back edge; simply thin the front edge until this edge is sharp, and file back far enough to keep the original shape. Filing only a small portion of the inside surface next to the dulled edge would leave a shoulder which would make the turning of the bit take up more force than would be required with proper sharpening and furthermore would reduce the cutting efficiency of the edge of the spur. For sharpening the lips, Fig. 4, the proper file is a half-round, second-cut. Use the flat side of this file on the side of the lip that is away from the screw point never on the side next to the point. The slope of the face of the lip that is next to the tip is essential to the proper action of the lip in diving into the wood and so must not be changed. In sharpening the edge of the lip, file away from the edge toward the shank of the bit. This leaves the edge clean and free from any feather edge.
For bits 5/8 inch and larger use a 6-inch file. Employ smaller files for bits of less than 5/8-inch diameter. A half- round file is necessary for the lip. The same file may be used for the spur provided care is taken not to let the edge of the file cut a furrow in the lip. Nothing but a really good file is of any use on a tempered bit.