A broach-flute requires the same careful shaping and polishing of the cutting side, as a tap-flute ; but if a broach has only three flutes, a greater amount of bearing surface behind each cutting edge must be allowed than is allowed for a tap of similar diameter. Smooth round files, and other files having cranked tangs, are also necessary for long flutes, to keep the workman's hand above the work while filing.

The hardening of broaches requires rather more care than the hardening of taps, especially if the broaches are long and slender. If a long taper tap bends in hardening, the tool is no1 greatly injured, except it happens to bend in some part of the parallel portion of the screw; when this occurs, the tap will screw a nut so that its hole will be larger in diameter than the diametei of the tap, unless the nut is very short - short enough to allow the tap to sinuate as it progresses, while its thick part is screwing the nut. If the nut is long, the sinuation of the tap is prevented, and the two extremities of the bent portion are thus made to cut at one time, which enlarges the hole beyond its proper size, the amount of enlargement depending on the length of the nut and the amount of bend in the tap-screw. A broach is much more often used for long holes than a tap, so that the bending of a broach is a much more important consideration. This bending is of only a small degree, if the broach were softened two or three times as directed; and when bending does occur, the tool may sometimes be straightened with careful management, even while in its hardened state.

The heating of a long broach for hardening resembles that described for a drift having a handle, and consists in placing the broach in red-hot powdered charcoal, to prevent the cutting edges being injured during heating. After a uniform dull red is obtained along the cutting part, and also an inch or two of the stem, it is put into old water with the length of the tool in a vertical position, to prevent bending, and not to allow bending, as when hardening a tap ; the broach is next taken out, after a proper cooling, and tempered, while in the hole of a tube, in the same manner as for tempering a tap. During tempering, the stem is not allowed to remain as hard as the teeth, as for a tap, but is made quite soft; the hard junction of the stem is therefore held in the hole until thoroughly softened, so that the adjoining extremities of the teeth must also become soft, this softening of the teeth being of little consequence while the broach is in use. because that portion is seldom or never required to cut. The colour to which the fluted portion is tempered is a golden brown, but the extremities at the junction are black, through being soft. By this treatment no portion of the entire stem is harder than it was previous to hardening, which is a suitable condition to admit of being straightened at the junction, and also bent, if necessary, in any place along the length of the stem, to make the fluted part rotate truly while the broach is on the lathe-pivots.

After the tempering, straightening is effected. The portion which requires the most straightening is generally that which is easily straightened, being the junction of the stem with the thicker part; through this being soft it is easily rectified, without much risk of breaking by means of a half-round top-tool and sledge-hammer. The straightening of a hard broach or other tool of similar character should be effected on a heavy block of cast copper or of lead, several inches thick, and of proper length and width to suit the work usually allotted to it; such blocks are represented by Figs. 613 and 614, and should have several broad gaps formed across their upper surfaces, to provide spaces over which the bent portions of the work are to be placed to receive the hammering. No attempt should be made to straighten any sort of steel while cold, whether it is soft or hard ; and it is not only necessary to heat the bent part, but also the entire tool; consequently, to straighten a broach, it should be heated in a tube, or on a warm plate, until the whole tool is as hot as can be only just handled with a bare hand; it is then laid across the heavy lead block, and the bent part at the junction, or other portion of the stem, is placed over a gap with the convex side upwards; a top-tool is next applied, having a gap which is broader than the work, to admit a piece of sheet copper between the tool's gap and the part to be straightened. While such a tool is held firmly by the operator, and pressing tightly downwards on the broach, a sledge-hammer blow without recoil, if possible, is given by the hammerman. Such a dead blow, as it is named, is necessary to prevent vibration of the broach, which would tend to break it; the hammer is therefore held tight on the top-tool at the moment the blow is given, and all other blows that may be afterwards required are to be of the same character.

For delivering the blows, an ordinary steel sledge-hammer is suitable, if firmly held ; but a large copper or tin hammer is preferable, when one is accessible.

A broach that requires its fluted part to be straightened needs the same sort of blows, while the bent portion is across a gap in the lead block ; but the flute which happens to be in the convex side, and therefore upwards, must be occupied with a packing-piece or filler, to prevent the cutting edge being shattered by contact with the top-tool or flatter which is used to apply the blows. This filler is a piece of soft iron which fits loosely sideways in the flute and projects a short distance above the cutting edge, the length being about as long as the flute ; the piece is laid into its place with a strip of emery cloth in immediate contact with the work, and a sledgehammer blow without recoil, as directed, is given to the tool while it is tightly held on the filler. The top-tool employed should be either a flatter, or a half-round tool having a shallow gap with but little curve, to avoid possibibility of the tool's edge touching the broach while hammering.

An efficient mode of straightening hard broaches and other tools consists in applying a few proper blows to the work to be straightened while it is bolted to the cast copper block, instead of merely lying on it. This bolting is effected by placing a plate across the filler while in a flute, or across any other portion that may be bent, and fixing the plate with the screw-bolts, one through a hole in each end of the plate. These bolts are attached to another plate beneath the block, or may be tee-head bolts which are fixed in some of the slots in the upper portion of the straightening block, or of a large table, if such is being used for supporting the apparatus. When a tool is to be straightened by such means, it is properly heated and put across a gap, the plate and bolts applied and screwed tight until the work is straight; in this condition it should be allowed to remain a short time and kept hot, and the screwing will sometimes straighten it without hammering. To ascertain whether it is permanently straightened, the bolts are loosened, and a few minutes are allowed for the tool to resume its former bent shape, if it will do so; and in this case it is screwed down again, and again, if necessary, and also hammered while it is down, by striking the plate at that place immediately over the bent part of the tool. By means of a plate and bolts thus used much of the danger of breaking the work while hammering, is avoided.

There is less risk of breaking a thick broach while straightening than of breaking a thin one, because a thick one cannot be hardened at the centre, and is therefore more pliable and less brittle than a thin one, which is entirely hard. Although these methods for straightening are here recommended, it is frequently necessary to heat the work to redness if much bent during hardening, or bent in two or three places; after which it is straightened, while hot, with a wood hammer, while the tool is lying on a wood block, and when straight it is again hardened.