Types of bits

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Bits are probably the most abused of all woodworking tools, for too often the wrong type is chosen, or the bit is incorrectly or over sharpened. The general run of bits are supplied with either square taper shanks (69:1) for use in the armbrace, or parallel round shanks (69:14) for hand-drills. A few types are also available with parallel shanks turned down to 1/4 in (6 mm) diameter (69:13) for use in the standard drill-gun, and with Morse taper shanks for lathe heads or specialized drilling-machines. In the main, parallel shank-bits are classed as machine-bits with a constant shank diameter of 1/2 in (12.5 mm), but the universally used Russell Jenning's auger- and dowel-bits are also supplied with 1/4 in (6 mm) and 3/8 in (9.5 mm) diameter shanks. Types of bit are as follows.

Jennings pattern auger-bit (69:1) The best smooth boring bit for general cabinetwork. Also available as the standard 5 in (127 mm) dowel-bit.

Solid centre auger-bit (69:2)

Extra strong general-purpose bit with greater chip clearance.

Snail Horn Countersink Bit

Single-spur solid centre-bits (69:3)

For deep boring, also difficult or stringy wood.

Scotch square-nose auger-bit (69:4) Designed for boring hard and rough timbers.

Solid nose auger-bit (69:5)

Unbreakable nose for tough, difficult drilling and boring at an angle.

Expansion-bit (69:6)

Primarily intended for softwoods, although it can be used in medium hardwoods provided care is taken and large-diameter holes are not attempted. Available in two sizes, each with an additional cutter (69:7) to give a range of diameters from1/2 in (12.5 mm) to I1/2 in (38 mm) and from 7/8 in (22 mm) to 3 in (76 mm).

For clean boring of short, small-diameter holes. Old-type centre-bit (69:9)

Always a good bit for shallow holes, and still used as a machine-bit for large diameters. The brad-point enables holes to be bored from either side of the wood, thus preventing splintering out on the underside.

Improved pattern centre-bit (69:10) A fast, clean-cutting bit for shallow holes.

Forstner bit (69:11)

Unequalled for boring any arc of a circle, and unaffected by knots or the run of the grain. As it is guided by its circular rim it gives a flat-bottomed hole, useful in some work.

Dowel-sharpener (69:12)

Useful for chamfering the ends of dowel-pegs for easy entry.

A revolutionary and inexpensive bit which relies on speed for its cutting ability, therefore its use is confined to hand drills and drilling machines.

Improved twist-drill (69:14) A wood-cutting version of the standard metal-boring twist-drill.

Snail-horn countersink (69:15)

Countersink for wood, with taper shank for hand-braces.

Rose-head countersink (69:16) General-purpose countersink for wood and non-ferrous metals, available with both taper and parallel shanks, and in 90° and 120° included angles.

Machine bit (69:17)

With brad-point in lieu of screw lead to the nose, as the latter will snatch at the wood if driven at speed, with risk of serious injury in hand-held boards. Brad-points are not self-driving and require feed pressure, therefore they are advisable for all high-speed drilling, unless the work can be very securely clamped down.

Twist-drills

The standard jobber's twist-drill primarily intended for metal boring is also used extensively by the woodworking trades for small-diameter holes, although it will never cut as cleanly as a correctly sharpened wood bit. Moreover, it has neither screw lead nor brad-point, and is always more difficult to position accurately. On the other hand it retains its direction, and is not deflected by knots or the run of the grain. The cheaper carbon steel varieties are good enough for wood provided they are not allowed to burn. Twist-drills are not suitable for use in the ratchet-brace, as they require a higher speed than it can achieve for clean cutting.

Record Ridgway Tools Limited, who take a justifiable pride in the excellence of their products, give the following instructions for the use and upkeep of wood-boring bits:

1 A good bit should draw itself easily into the wood, cutting a clean hole without undue pressure on the brace.

2 A bit should never be used to bore a hole at one operation deeper than the length of its twist or flute. Where deeper holes are required the bit should be repeatedly withdrawn and the chips cleared as they rise.

3 Generally speaking bits are sharpened more often than necessary, and the life of the bit considerably shortened by incorrect filing.

Sharpening bits

Basic rules as laid down by Ridgway are: (1) always sharpen from the inside or underneath, never outside or on top; (2) maintain original shape and angles; (3) use suitable smooth-file, never grind and always file lightly, removing as

70 Sharpening wood bits

little metal as possible and preserving the balance of the bit so that each side does the same amount of work. Figure 70 gives the procedure. In 70:1, A is the side wing, B the cutters, C the spurs and D the throat. The sequence of sharpening is as follows:

Sharpening the side wings (70:2). Rest the bit in the bench with the screw lead down. File with the file working through the throat of the bit. Never file the side wings on the outside or the clearance will be ruined (the appropriate file section is shown alongside each drawing).

Sharpening the spurs (70:3). Hold the bit nose uppermost with its twist firmly against the edge of the bench. File the inside of the spurs. Never file the outside as this reduces the clearance, causing binding and clogging of the bit.

Sharpening the cutters (70:4). Hold the bit as in (70:2) and file the cutting edges on the underside only, i.e. with the file working through the throat of the bit. The cutters must be at the same level so that they cut chips of equal thickness.

Another wood-boring tool which is always abused is the common bradawl. It should be carefully sharpened with a fine file and finished on the oilstone, and is used for boring fine holes for nails or the threads of screws but not the clearance hole for the actual shank of the screw which should be made with a shell-bit or twistdrill. The birdcage-pattern bradawl with square shank bores with a reamer-like action, and can be used on the thinnest wood without splitting. Reamers for use in the arm-brace can also be obtained, and are used for widening existing holes.

Plug-cutters

Cross-grain pellets or plugs for filling screw-holes can be cut with Stanley plug-cutters, available in three sizes for Nos. 6, 8 and 10 screws. These are virtually hollow drills cutting a circular path, of which the core or waste forms the required pellet. Larger sizes cutting standard 3/8 in (9.5 mm) and 1/2 in (12.5 mm)

71 Stanley plug-cutter diameter plugs can be obtained for use in the drill-press or drilling machine.

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Wood Working 101

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