The centre-bit is a useful tool, particularly for very thin stock. The spear-like point a (Fig. 481), acting as a centre, the point b cuts a deep ring, and the edge c, which is bent so as to form a flat chisel, scoops out the pieces of wood, and so a round and smooth hole is made. This bit does not cut very well with the grain. It can be sharpened with a small oil-stone. It is well to bore a trial hole with this bit in a piece of waste wood when exactness is required, because the spur is not exactly in the centre, so that the hole cut is a trifle wider than the diameter of the bit.
The expansion-bit has an adjustable contrivance that enables it to bore holes of various sizes, but such tools are hardly necessary for beginners, though very convenient and often used by carpenters.
The gimlet-bit is a common form, but is easily dulled and bent and is likely to split delicate work. The quill-bit is excellent, except for end grain. Shell-bit, gouge-bit, pod-bit, spoon-bit, duck's-bill-bit, etc., are names applied to simple tools good for boring small holes. They are easily sharpened with a stone, work quickly and leave a smooth hole, but do not cut so well in end grain. They are not as much in use as formerly, the twist-drill taking their place for many purposes.
Reamers, or tapering bits (half-round, square, octagonal, conical), are useful to enlarge holes and occasionally to make them conical. Reamers for metal are also useful.
For other forms of boring implements, see Awls and Twist-drill. See also Boring and Countersink.
Block-Plane.—See Plane.
Fig. 482.
Boards or Planks, Laying Exposed.—In laying boards or planks to be exposed to the weather, place them (unless they are from the middle of the tree) so as to have the outer side exposed—that is, the side farthest from the heart should be put outside or uppermost. If put the other way the action of the atmosphere, water, etc., will tend to separate and loosen the layers and fibres (Fig. 482).
Boring.—In boring with the bit-brace, after the bit has gone a short distance into the wood, stop and, keeping the brace in position, test carefully from in front and from one side to see whether the bit is at right angles to the surface. Repeat this test and alter the position of the brace as many times as may be necessary until you are sure that the bit is going through at the right angle. A common way to do this is to stand squarely in front of the work and judge by the eye whether the bit is at right angles with the work, and then to stand at either side at right angles to the first position and judge of the angle again. The direction of the bit can be tested more accurately by applying the square. Few people can bore accurately without some such test.