Hobs and taps are made by four principal processes, including forging, turning, screwing, and fluting. The steel selected for taps from about an eighth to half an inch thick should be tough cast steel that was originally carbonised with charcoal; but the steel for large taps, two or three inches thick, should be bars or rods that were first carbonised with wood charcoal, but not afterwards cast; such metal is not so liable to break during the hardening process as steel which is cast, because casting destroys the fibrous shape of the metal, and makes it exceedingly good for all small or thin cutting tools, which require a close texture, but not suitable for large expensive implements which are to undergo the force exerted by the contraction white hardening. Steel which is made by the Bessemer process must, in all cases, be tested previous to selecting for taps; this testing consists in hardening in water a few pieces belonging to the same bars or rods of which the hobs or taps are to be made; and if the trial specimens will become very hard, without breaking or cracking in any part during cooling, the metal may be used, and only a very few of a large number will break at the final hardening. During the heating of these specimens some of them may exhibit a large number of small fissures whose lengths are parallel to the length of the rod or bar; if such marks are seen, it is an additional proof that the metal is suitable, and should in all cases be preferred if the steel is also capable of being made hard.

The only forging which is necessary for hand taps that are less than half an inch in thickness consists in making them of round steel, and cutting each tap or hob to its proper length while red hot, and afterwards trimming the rugged pieces from the two extremities, to make them flat and square to the tap's length. The small wire for taps only about a quarter thick, merely requires cutting partly through with a file and breaking while cold. A small piece thus prepared is shown by Fig. 536. Hand taps that are half an inch or three-quarters thick require the square heads to be forged; one of this class is shown by Fig. 537. Those that are to be about fiveeighths thick, and all larger sizes, require to be forged sufficient to produce the stems extending from the thicker portions, and also the square heads. When thus shaped they appear as denoted in Fig. 538.

Long taper taps for engineers are not made smaller than about a quarter of an inch in thickness, and those that are not to exceed three-eighths in thickness may be forged by merely cutting them to length and squaring the extremities. All that are to be larger than three-eighths should be forged to a form resembling Fig. 539 ; this figure denotes a forged tap which is about as conical as it will be when finished, and the stem of which is of the same shape, having a square head, which is comparatively longer than the head of a hob.

The mode of producing the square head of a small tap or hob consists in shaping it with a set-hammer. This shaping is effected by first ascertaining the forged length of the cylindrical part of the stem, and then driving in the set-hammer at the proper distance from the thickest portion of the tap; the set-hammer is driven in at this place to produce the four-sided shape, and is afterwards driven in to square the lump which remains at the end. After the head is properly shaped and flattened to the thickness, the total length of stem from the shoulder of the thickest part is marked upon the head, and the superfluous piece is cut off to complete the head.

To commence the shaping of a large square head a couple of narrow fullers are first driven in at a proper distance from the shoulder, and the squaring is afterwards completed with a sledge-hammering and smoothing with a flatter. The shaping of large square heads is also easily commenced with a couple of angular gap tools, the steel being held still while between the two tools, instead of being rotated as in a process for rounding. During the final forging of a tap-head the reducing and smoothing should be carefully managed to avoid all planing of the head, if the taps or hobs are only about an inch thick, and for all larger sizes, only about a tenth of an inch should be allowed for planing. By this means all the smaller taps' heads are finished with mere filing, and all the larger ones with as little planing as possible.

A long taper tap needs a careful straightening, while red hot, at the conclusion of forging, and this is effected by means of a straight-edge which is as long, or longer, than the tap; this is applied to two opposite sides of the taper portion, and also to two opposite sides of the parallel portion; and by thus placing the tool a few times to the work the operator can easily see if the conical portion is straight, and whether the stem is concentric and in line with the thicker part. In order to avoid unnecessary turning, each tap or hob requires to be reduced to a proper thickness and smoothly rounded with top and bottom tools. Taps not exceeding an inch in diameter should not be more than an eighth of an inch thicker when forged than they are to be when lathe-turned ; and those about two or three inches in diameter need not be more than a quarter of an inch thicker. The softening of taps after forging consists in heating each one to only just red throughout its entire length, and burying it in cinders or charcoal till cold, the charcoal being used if the tap is thought to be rather deficient of carbon. During this final heating the tap must be frequently rotated in the fire, and the air-blast gently administered to avoid bending the work.

Centring each tap or hob for lathe-turning is the next step, and is effected by two or three means, the precise amount of centring which is necessary for any one tap depending on its forging. The general rule for centring is to place the centre recesses in a straight line extending through the middle of that portion of the work which is nearest to the finished dimensions. By being thus centred, that part of the work which requires the least turning, is made to rotate truly in the lathe, and the entire surface of that portion is equally and smoothly turned by cutting of only a small quantity. Therefore if the stem of a tap is only an eighth of an inch larger in diameter than the finished diameter, and the thicker part a quarter larger, the stem is that which is made to rotate truly while on the conical pivots, because the stem may not be concentric, or in line, with the thicker part.