In addition to tapering, another trimming is given to the dies after they are completely shaped with a tap or hob, and this trimming consists in cutting off all thin edges of the thread at both ends of the die-screw, so that all the cutting surfaces shall be as broad as the thread will permit. At the tapered end, none of the teeth can be made to present a section of a complete thread, but at the parallel end, two or three teeth can be thus treated, and by a careful filing, the desired across section of a full thread is obtained.

To avoid some of the filing when tapering die-gaps, rosebits should be employed. These are fitted to a drilling machine and made to cone the dies after they are bored for the screw, but previous to commencing the screwing. Each rosebit should have a long conical end for the convenience of making one rosebit cone dies of several sizes. If each pair of dies are thus roughly coned, the filing of the conical part after serewing, is quickly effected. Dies with tapered screws, require the parallel end to be put first upon the rod to be screwed, when it is necessary to completely form the thread from one end of the screw to the other; but a thread is less liable to break if its junction with the adjoining part of the bolt is tapered, consequently, to make such a thread, the tapered end of the die-screw requires to be put on first when beginning the screwing of the bolt or rod.

The small gap or channel which exists at the bottom of a die-gap, is of no use for cutting unless it is of great width and depth ; if small, its proper use is to admit oil during screwing, and to diminish friction at the bottom of a broad die-gap intended for a bearer. The bottom of every such oil-channel, and also the bottom of every other gap or groove in dies, should be curved, which renders them less liable to break in hardening.

According to the processes now described concerning dies that are screwed with small hobs and taps, it will be seen that the principal cutting surfaces of dies in general, are those at the sides of the gaps, and not at the bottom; therefore the outer surfaces which adjoin the gap-sides, and are named the faces, require bevelling to make them cut easily, and are necessarily liable to become blunt during use and require sharpening. This is effected without softening the die, by merely gripping it in tongs and holding it to a grindstone. The bevelled faces are at about forty-five degrees to the length of the slot in the die-frame; both sides of the die require bevelling, because both sides should cut while screwing. The die termed the bearer, may remain with its faces at right angles to the length of the slot, because the bearer is not required to cut; but for uniformity, it should be bevelled to the same shape as the cutter. In a pair of dies that were entirely screwed with a taper tap or small hob, both the screwed gaps have similar good properties for cutting, so that both dies will cut if both are bevelled and sharpened by grinding when requisite.

The hardening of screw-dies is easily effected by heating them to a dull red in a charcoal fire, covering them with powdered prussiate of potash, and placing the screwed part of the die first into the water until about five-sixths of the die is in; it is then moved about, but not sufficient to suddenly cool the back of the die ; this may be quite soft, or merely toughened, and should therefore be slowly cooled. All large dies are thus easily managed, but small ones, for three-quarters or half-inch screws, must be entirely hardened, to ensure a proper hardness in the teeth.

The tempering of dies resembles that for screwed plates, and consists in heating the dies on a thick plate which is over the fire, or placed in some other suitable situation, if sufficient daylight cannot be obtained while the tempering plate is at the forge. Each die while being heated, is put upside-down a few times, and tempered until a light golden colour is seen at the screwed portion, when it is cooled in water.