This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Chipping chisels for engineers seldom remain long in use, through the continual hammering and consequent vibration to which they are subjected for cutting metals, and because they are made of a granular tool steel which is too solid for chisels, and always breaks unless the cutting part of the chisel is too thick to possess good cutting properties. Every sort of steel which has been cast, but not afterwards made fibrous with hammering, should bo rejected, and pure iron bars, that were carbonized with charcoal without being afterwards cast, should be selected, the precise quality of anyone piece in all cases depending on the quality of the iron at the time of carbonization.
It is not possible for the tool maker to know how or of what materials his steel was made, but he is able to ascertain the quality of any piece by testing it, which should always be done previous to making a large number of one bar, or of one sort of steel. It is also necessary to test each bar, and sometimes both ends of one bar, because one end may be much harder than the other end, and the operator be deceived thereby.
The bar steel which is made for hand chisels is in the shape of four-sided bars, each having two flat sides and two curved convex ones; such a shape is produced with rolling, and is convenient for handling. A piece of such a bar, or a few inches at one end of it, is to be first tested by heating it to a bright red, and cooling it in clean cold water until the steel is quite cold; it is then filed with a saw file, or some other smooth file known to be hard, and if the steel cannot be cut, its hardening property is manifested. The next test consists in hardening it and allowing it to remain in the water till nearly cold, then taking it out and allowing the heat in the interior to expand the hard exterior; this will break it, if not fibrous enough to withstand the trial. A third test consists in making a grooving chisel of the steel, and hardening it ready for use. This is the proper test for all chisels, because it is easily and quickly performed; and it is advisable to make the cutting end rather thinner than for ordinary chipping, so that if it does not break nor bend while thin, it is reasonable to expect it would not break if thicker.
The forging of a chisel, whether a broad smoother or a narrow groover, consists in tapering one end, and next cutting off the cracked extremity which is produced whenever steel is forged thin and tapered. During the final reducing, the taper part is thinned with a flatter, and the flattening is continued till the end is below red heat. Hardening is next performed while the work is yet warm; this consists in gripping the chisel in tongs, and heating 5 or 6 in. of the steel to redness, then placing about 2 in. of the taper part slantways into water and moving it quickly to and fro till cold; it is then taken out and tempered, which is effected with the heat in the thick portion that was not put into the water; this heat moves along to the hard end and softens it while the operator rubs off the thin scale with a piece of grindstone, which allows the colour to appear; and as soon as a purple is seen at the cutting part, the entire taper portion is cooled in water. This mode of tempering allows only about half an inch of the taper part to remain hard, all the remainder being soft; if not, the vibration caused while hammering would break the tool in the midst of the taper portion.
Some sorts of steel require hardening at a very dull red, and tempering until a quarter of an inch at the end is blue.
Sharpening chisels ready for use is effected on ordinary grindstones. The cutting edge should be made convex, to obtain two results, one of which is rendering the tool less liable to break, and the other result is the greater ease of cutting while holding the tool to its work. Those chisels that are to cut brass or gun-metal have their long taper portions, and also their cutting parts, thinner than the taper portions of chisels for iron and steel, those for steel being thickest of all; but the angles of the taper parts are about the same for all chisels. When, however, a small difference is made in such angles, the smaller angle is given to those for cutting brass and gun-metal. The angle of a hand chisel's long taper portion is only about 6°, but that of the cutting end is about GO0. In Fig. 10G a narrow side of a chisel is shown, and a couple of lines are made that extend from the cutting end; two other lines are also shown, which extend from the long taper part, the difference between the two angles being indicated by such lines.
It is only during the mending of a chisel that the proper management can be exactly effected. After they have been in use, the workman can decide whether the metal he is cutting requires the chisels to be harder or softer than they were when first hardened, so that he instructs the tool maker to make them harder, if necessary, or to make them thicker at the cutting part, if steel or hard iron is being chipped. By using a chisel it is also discovered whether it were left too hard at its tempering, and needs different treatment.
To prevent the head of a chisel burring around the edges with hammering, and causing pieces to fly off, the head should be frequently curved with grinding, at the time the cutting part is sharpened; and when a head is mended at a forge, the end may be tapered, but none of the burr is to be hammered; all these should be cut off with a small trimmer, or ground off with a grindstone, previous to tapering on the anvil.