The shaping of the heads of taps and hobs consists in squaring the four sides and finishing each head to a proper length, the length of a tap-head being longer than the head of a hob, because a much greater amount of wear and tear attaches to a tap. The length of a head is marked upon the stem with a sharp vee-tool, and consists of a circular line made around the stem after it is turned and previous to commencing the screwing. The outer extremity of the head also is marked with a circular line which is scribed with a divider, the diameter of this circle being equal to the desired distance between two opposite flat sides of the head. The circular line on the extremity is indicated in Fig. 561.

If a tap-head were properly shaped and reduced to suitable dimensions during forging, as directed, the only finishing required is effected with filing. But great numbers of hobs and taps are forged without any attempt to produce the square shape required, so that the squaring is an important process. If the tap or hob has no square part, it is proper to make at least one flat side, previous to screwing, that the work may be securely held. The head-shaping may therefore be either partly or entirely effected after the stem is turned and previous to proceeding further with the work.

Those heads that are too large to be squared with mere filing are shaped with small shaping-machines, each hob or tap being fixed with its length at right angles to the direction in which the cutting-tool moves, the work remaining stationary and fixed between two poppets, that may be the same as those used for fluting. A hob thus situated on the table of a to-and-fro shaper is denoted by Fig. 563, the cutting-tool being a slightly bent corner-tool, for the convenience of neatly shaping the corners where the circular line indicates the termination of the head. After the work is properly fixed, so that its length is parallel to the across traverse, one of the head's four sides is planed until one extremity coincides with the circular line on the stem, and the other extremity coincides with the circular line on the outer extremity of the head ; the poppet-screws are then loosened, and the work moved a quarter of a complete rotation, which places the planed side at right angles to its first position; the hob or tap is then again fixed while being adjusted with an el-square whose blade is in contact with the planed side, and whose pedestal is in contact with the table of the machine. When the work is fixed, another side of the head is planed to the gauge-lines, and afterwards two other fixings are effected to complete the head. To prevent unintentioned shifting of the work while planing, the stem is gripped with a holdfast, or with a couple of small poppets which are fixed to the head, so that each screw-point may bite one side of the head.

When poppets are not accessible, a head may be squared while the tap is supported on a broad vee-block; this is provided with a broad bearing for the gap, that the tap-stem may be firmly gripped ; and the gripping is accomplished with a cap having a vee-gap, which is put over the stem and fastened to the vee-block and table by means of bolts and nuts. To avoid bruising the stem, soft iron packing-pieces are put into the vee-gaps, to prevent them being in immediate contact with the work.

It is necessary to carefully file and polish the cutting edges of all hobs and taps, whether they were fluted with round cutters or with straight grooving-tools. Much of the smoothing can be done with soapy water, while the work is being planed, and the polishing is completed with a flat smooth file and with emery cloth, the cloth being wrapped around the same smooth file, or wrapped around a wood polisher. The filing of long flutes is effected by using round files and flat ones whose tangs are cranked.