The Technique of FURNITURE MAKING
Types of bits
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.
Jenning’s 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.
|
69 Wood-boring bits |
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 from 1/2 in (12.5 mm) to I1/2 in (38 mm) and from 7/8 in (22 mm) to 3 in (76 mm).
Shell-bit (69:8)
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.
Flat-bit (69:13)
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 selfdriving 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.
