In a rod chisel for cutting hot metal, the angle of the long taper portion should be ten or twelve degrees, and the angle of the short bevelled part for cutting about forty or fifty degrees. Chisels for cold iron require the angle of the long taper part to be about thirty degrees, and the short bevel for cutting about eighty degrees.

All rod chisels are best when made entirely of a tough hard steel, without any iron being added to make the head. Such a chisel will sustain a severe hammering without much injury to the head and cutting part, if only about half an inch is hardened, and the head made as soft as possible. After the cutting end is become too thick with hammering during ordinary use, or is become otherways damaged, the lower part of the taper portion is thinned to a proper shape, and only a short length of the chisel hardened as at the first making. Every time a chisel is repaired by such thinning, it is advisable to cut off about a quarter of an inch of the thin ragged end, because this portion becomes burnt during heating, and also contains several cracks that are not visible to an unassisted eye.

An ordinary method of preventing a rod chisel being driven through its handle during the hammering consists in making a few ragged notches into that part of the chisel which is to be encircled by the handle; the sharp projections thus made stick into the wood handle at the time it is twisted around the tool, and it is thus firmly gripped until the wood is broken with repeated hammering. Another mode of fastening the tool consists in making around the chisel a groove having a half-round bottom, the width of the groove being about equal to the diameter of the chisel-rod, or wire handle, which is to be twisted around. Such grooves are made with a couple of top and bottom fullers, that may be narrow to suit a wire handle, or broad to fit a wood handle.

In chisels for steam hammers, the angles of the taper parts are about the same as those of small chisels; but the cutting parts are thicker, and of greater length. A very useful class of chisels is that named trimmers; these are represented by Fig. 246. A trimmer is used for cutting all sorts of thin bars, rods, and plates; also for cutting off all superfluous metal that may be attached to any forging which is being shaped to the finished dimensions. To permit the free use of a trimming chisel, its cutting edge is convex, because this form enables the smith to slide the chisel easily along a cut which may be of great length, although not very deep. A trimmer may have an iron handle as shown in the Figure (246), or a wood handle similar to that in Fig. 247. A wood handle is preferable for a heavy chisel, to obtain a proper amount of holding power, that the workman may keep the chisel as nearly upright as possible.

In addition to trimming chisels, another sort of hand chisels is employed, named arched or concave chisels. In such chisels, the cutting edges are concave, instead of straight or convex. Concave chisels are useful for making a square cut through an axle or shaft, while it is being rotated on a gap bearing-block, which maintains the shaft in a proper situation for cutting.

The concave chisel shown by Fig. 248 is a tool for two handles, that are welded to the two short ends extending from the chisel, and indicated in the Figure. These two ends are named tangs, and may in some cases be made for two wood handles that are fixed in the manner shown in Fig. 247 ; or, in other cases, the tangs may be bent to about forty-five degrees with the remainder of the chisel, and the wood handles fixed so that they also shall be at an angle of forty-five degrees with the broad side of the chisel. A concave chisel having two handles thus fixed is supported on a shaft beneath a hammer by means of two men, one at each handle; consequently one man is at each side of the shaft, and the shaft is between the two men. While the chisel is thus held, it is driven through, or partly through, the shaft to the depth required, while being rotated in the usual way.

To avoid the necessity of holding a chisel on the work, the chisel may be keyed to the hammer; and if the bottom chisel also is to be used, it may be keyed to the anvil-block. A couple of keyed chisels are shown by Fig.' 249 ; the upper one being keyed to a hammer-block, and the lower arched chisel being keyed to an anvil-block. When the hammer chisel is to be used without the bottom one, the shaft to be cut is supported on a proper number of bearing blocks, and also prevented from moving forwards or backwards by means of half-round clips or bands of some kind. By thus holding the shaft, it can only move in a direction of rotation around its own axis; and this is the only movement required for rotating the shaft with the rotators to make a cut entirely around the work, which shall be at right angles to its length.

When both hammer-chisel and anvil-chisel are used at one time, no bands or clips are required to keep the shaft in its place ; it is only necessary that the concave part of the bottom chisel be of greater curve than the curve of the shaft, and that the depth of the concave be nearly equal to half the shaft's diameter. Such a form will allow the work to be rotated without sticking on the chisel-edge, until a cut is made to a short depth entirely around the work. After the cut is thus commenced, the anvil-chisel and its block is taken away, and, if necessary, the work is again heated and the cutting off completed with the hammer-chisel only, the work being supported with the bearing blocks, but without the concave chisel.

While cutting any large shaft or axle, the suspension of the work with the cranes must be properly managed; and the supporting blocks also demand attention. The greater the length of the work to be cut, the greater should be the number of suspending chains. If the anvil is too small, which is often the fact, three or four chains are required for suspension, so that the piece to be cut off may not get out of position and break a chisel, or do other damage by falling; and that the work itself may be kept as nearly as convenient in the horizontal position. But if the anvil is long enough, four or five gap bearing-blocks, or supporters, as they are termed, may be used, because they are preferable to a number of suspending chains. Two of these supporters should be put as near as possible to the chisel, so that the cut may be between each supporter. Another supporter may be put near one end of the superfluous piece to be cut off, and a fourth supporter is required for supporting the opposite end of the shaft or axle. This outer supporter, being at a great distance from the chisel, may in many cases be on a gap pedestal similar to that in Fig. 201. In this Figure, it may be observed that supporters are used also for axle-twisting. For this purpose, two supporters are required, so that one may be at each side of the crank which is being hammered for twisting the axle.

The forging of chisels for keying to hammer-blocks and anvil-blocks consists in making them entirely of steel; and several of them should be made, and also fitted to each block, so that if a chisel breaks during the cutting of a shaft, another chisel can be immediately put inr and several hours of reheating the work will thus be avoided.