PLANE irons, chisels and gouges require occasional grinding and frequent sharpening. Grinding is done on a circular grindstone or emery wheel, with water as lubricant and cooling agent, to produce a uniform bevel of about 25 degrees. The stone or wheel should turn towards the edge of the tool, both to keep it flooded with water and to prevent the edge being burred.
Sharpening, that is, putting a fine cutting edge on the tool, consists in rubbing the tool on a fine-grained oilstone, or hone, at a rather larger angle, say 30 degrees. The stone should have no vein marks in it, but be of the same colour throughout. The best natural stones are Turkey and Arkansas, which give a very keen edge. Good composition stones also are much used.
The stone should be bought ready cased – sunk in a wooden block, with a cover to protect the projecting part. Wipe a stone clean before use, and lubricate it with olive, thin mineral, or paraffin oil, and replace the cover before putting it away.
The main difficulty that confronts the beginner when sharpening a plane iron or chisel is to make long strokes without altering the angle, so producing a rounded, instead of a flat, bevel. At first the strokes may be kept short, and the tool gradually advanced, so that a good proportion of the whole surface of the stone may do its fair share of the work. Using the central part only will soon wear a stone hollow.
The edge must be carefully watched and tested on a thin sliver of wood, such as a matchstick, till all parts of it cut cleanly. If what is called a wire edge – a tiny flap of metal – forms, the back of the tool should be pressed quite flat against the stone and rubbed a few times to remove this.
The final touch – often omitted by amateurs – is to strop the tool on a strap dressed with tallow and crocus powder till the edge is perfectly clean.
A chisel should be given a truly straight edge. The iron of a plane must be rounded off slightly towards the ends of the edge, in order that there may be no visible scoring of the wood.
For jack planes the rounding is con-siderably greater than for smootiiing and trying planes. It is effected by moving the iron laterally as well as to and fro, so that the corners are brought alternately near an edge of the stone with the rest of the blade overhanging. The pressure and grinding effect is then concentrated for a moment on a small part of the edge.
Gouges are of two kinds; firmer gouges, with the bevel on the outside; and paring gouges, which have the bevel on the inside. The first are sharpened like a chisel, except that the tool has to be rolled from side to side, to bring all parts of the edge into contact with the stone.
Paring gouges are sharpened with a special oilstone slip, which tapers from one edge to the other, and has rounded edges. The slip is held in the hand and the gouge rubbed up and down on the more suitable edge. The slip is also used for removing the wire edge from a firmer gouge.
Centre-bits need occasional sharpening to be able to do their work cleanly. The nicker, which severs the fibres round the circumference of the hole, must on no account be ground on its outside edge, which should be quite parallel to the centre line of the bit, or splay outwards a little. All filing and rubbing must be done from the inside edge. The tip not only tapers from the inside but is given a slight upward slant in the cutting direction, so that a fibre shall be severed gradually by a drawing cut.
The router is sharpened from the upper side only. If bevelled on its lower surface it would not be able to cut at all. A careful examination of a new centre-bit of good size will perhaps explain matters better than words can. The nicker should be sharpened only just sufficiently to produce a keen edge, as every sharpening shortens it a trifle; and when it is too short to cut ahead of the router the tool will become practically useless.