The smaller sizes of taps have square heads, which are as thick as the screw-portions ; some of them are made in sets of three short taps each, and are termed hand taps; others are long taper taps, one of which is sufficient for completely screwing a hole, instead of three; these long ones are also hand taps, if small, because they are used with small tap-spanners. The larger sizes of long taper taps, that are one, two, or three inches thick, are termed machine taps, because they are generally rotated in screwing machines, although sometimes with ordinary tap-spanners. Such taps are turned by commencing with a vee-point roughing tool, in a manner similar to that for turning hobs; but at the preliminary turning only the smallest quantity which is sufficient to make the tap circular, should be taken off, in order to avoid straightening the tap after it is bent with the first hardening. Through the comparative great length of such a tool, it may bend so much in hardening as to require an eighth of an inch to be turned off only one side of the mid-portion to make it rotate truly in the lathe ; therefore it is necessary to take off as little metal as possible at the first turning, that the greater portion may remain to be taken off at the second turning.

When the tap has undergone the first turning and hardening, it is softened for a final turning without being straightened. This turning commences by first reducing the thickest part of the tap to nearly its finished diameter, next reducing the stem entirely to its finished diameter, and also cutting off the metal at the shoulder of the thick part until the stem is of sufficient length. The stem is smoothed either with filing or with springy-tools, and made to fit a gauge-gap in a steel-plate gauge, which is made for the particular size of tap or taps being turned. The diameter to which a tap-stem is turned is rather less than the diameter of the tap measured between the bottoms of the thread-groove, the quantity less being about a fiftieth of an inch for taps whose screw-parts are under three-quarters of an inch in thickness, and about a thirtieth of an inch for taps between one inch and two inches; those which are two or three inches require the stems to be about a twentieth or a sixteenth of an inch smaller than the bottom diameter of the screw.

If a number of long taper taps are being turned, the turning of all their stems is finished previous to finishing the turning of any of their screw-portions and conical parts; but if only one tap is in progress, the gripper is taken from the taper end as soon as the stem is turned, and is fastened to the square head ; and while thus gripped, the turning progresses by first reducing the thickest portion to its finished diameter. The turning of this portion resembles the turning of a hob's screw-part, in being reduced to the finished diameter of the intended screw, if the work is smoothly filed; but if rough, the work must be of the proper diameter at the bottom of the rough projections; consequently, the outer diameter is larger.

After the stem and thick parallel portion is turned, the conical part should be turned. This must be entirely reduced to its finished diameter at the time of turning, to avoid all necessity of thinning with planing, shaping, filing, and other unnecessary processes after the tap is screwed. In order to avoid hand labour during this cone turning, either of two methods is adopted; one of these consists in making the upper slide of the rest turn the cone, and making the lathe work the screw of the slide; and the other method consists in adjusting the poppet-head, and making the ordinary long traverse of the lathe move the tool along to produce the required cone. When the upper slide of the rest is to be adjusted, the poppet-head remains in its usual position for parallel work, and the arranging of the rest commences by first roughly placing the slide to about the proper angle, and turning with a vee-tool a short portion of the tap's point to nearly its finished diameter; the tool is next brought out from the work to make a space of about an inch between the tool's point and the short part just turned; while at this distance, the space is measured with an inside calliper, and the tool is then wound along with the upper slide screw without moving the carriage, until the tool's point is opposite the beginning of the thick parallel portion of the tap; while thus situated, this space also is measured, and if the tool-point is at the same distance from the parallel part, as it was from the tap's point, the slide is near enough to the proper angle to commence the turning of the cone along its entire length. As soon as the slide is thus found to be nearly adjusted, the fixing bolts are tightened and the conical part is roughly reduced along its entire length, until near its finished dimensions, at which time the adjustment is completed by working the proper screw, if one belongs to the rest; but because many of the best lathes have not such a screw, the final adjustment of the slide must be accomplished by partly unscrewing the fixing bolts and giving the rest a few blows with a tin hammer. During cone turning by such means, the traverse of the tool is effected by the operator rotating the screw with his hand, or by putting a grooved pulley on to the square end of the screw, and working it with a cord that is actuated by the traverse bar of the lathe, or by an arrangement over the lathe, or by one of several other means usually adopted.

When the cone is to be turned by adjusting the poppet-head, the slide-rest needs no adjustment, but the poppet-head is pushed towards the lathe front by means of the adjusting screws belonging to the head ; or if the head is without such screws, it is driven forward and adjusted with a tin hammer. When the head is brought partly forward, a short portion of the tap's point is turned to near the finished diameter, as in the mode for adjusting the slide-rest; the tool is next brought out and the distance measured with a calliper; but instead of working the slide-screw to advance the tool along to the thick part of the tap, the carriage and slide-rest are moved along the bed with the carriage handle, and as soon as the tool-point is thus put opposite the beginning of the parallel part, the distance between the tool-point and tap is measured as before, and the poppet-head is shifted until the distance is the same at either end of the conical portion, at which time the turning of the cone along its entire length may be commenced. The turning next progresses by advancing the tool with the ordinary long traverse of the lathe until the conical portion is made circular and regular, when the poppet-head is finally adjusted, either with its screws, or with a tin hammer, according to the particular sort of head belonging to the lathe.

In order to properly turn the tap to its dimensions, a steel-plate gap-gauge is provided for each size of taps ; such a gauge resembles one for hob-turning, but has three gaps instead of only two. Fig. 549 represents a gap-gauge for taps, having one gap for the thickness of the stem, another gap for the thickness of the intended screw-part, and a third gap for the small end of the cone, which is termed the tap's point.

By means of either of the two processes just mentioned, the cones of all the taps in progress should be turned, and thus made ready for the screwing in the same lathe, or in a lathe specially appointed for screwing; and when taps of different lengths and cones are to be turned, the small quantity of shifting, to adjust the head to turn each cone, is conveniently effected without interfering with the slide-rest or cutting-tools.