The screwing of a long taper tap ought to commence by fixing a gripper to the square head, and beginning the screw at the thickest portion; this part is parallel, and is therefore screwed while the poppet-head is adjusted to produce parallel work; so that if the taper portion of a tap has been turned with the same lathe which is to make the screw, the first step is to put the head right, if not already in its proper position. When the head is adjusted, the parallel part of the tap is screwed with a one-point vee-tool whose angle is about sixty-five, which is used until the thread-groove is nearly equal in width to its intended finished width, when a one-point tool having an angle of fifty-three is put in and used until the groove is of the intended finished depth in the parallel portion. By the time this second tool has made the groove to its finished depth along the parallel part, a great portion of the cone is also screwed, but the extreme point of the tap is still without any screw, because the screw-tool has been travelling along in a direction which is parallel to the centre length of the tap, instead of in a direction parallel to its slant side.

When the parallel portion of the screw is thus roughly shaped with one-point tools, a vee-tool having two teeth is required to curve the summits of the screw, and smooth it to very near its finished dimensions. During the use of this tool it is necessary to adjust the poppet-head a small quantity, in order to slightly taper the bottom of the thread-groove along its entire length, and to cause a gauge-nut to fit loosely at the junction of the taper portion with the parallel part. The next step is to bring forward the poppet-pivot still further, to make the screw more taper, but with a one-tooth tool of fifty-five degrees, instead of one with two teeth; by this means the groove is gradually deepened at the small end of the cone, but nothing is taken from the large end, and this screwing is continued until the small end is screwed about half as much as the large end, which is termed screwing the small end with only half a thread. When this is done, and the work properly smoothed with light cuts, the thread-groove at the small end of the tap is widened, but not deepened; this is accomplished by using a one-point tool having a cutting end with an angle of about ninety degrees, and the widening is effected by bringing forward the poppet-pivot yet further, to make the screw more taper only at the point of the tap. The length along the tap to which this widening extends is about four-tenths of the entire length of screw, so that the cut becomes gradually thinner and thinner as the tool is moved along to about four-tenths of the length from the point, at which place the tool is entirely free from the thread. By means of this widening of the groove's mouth, the summits of the thread are sharpened, and the tap will be more efficient for its work than those which are not thus shaped.

After the taper part of the screw is thus screwed and nicely smoothed, the remaining operation is the final adjusting and polishing of the thickest part of the screw. The tools required for this purpose are two; one of these is the ordinary straight tool with two teeth that were shaped with a standard hob, and the other tool is a two-teeth springy tool, which also is shaped with a standard hob. These slide-rest tools are similar to those for finishing hob-screws, and are represented by Figs 442 and 443. A gauge-nut also is necessary, which fits a standard hob of the same diameter as the new tap being made. When these are to be used to adjust a long taper tap, the straight screw-tool having two teeth is first applied to the work while the poppet-head is adjusted to slightly taper the screw, so that a very thin shaving shall be taken from the parallel part of the screw and a portion of the taper part at one cut. The head being thus fixed, the screw-tool is carefully adjusted to make its teeth fit the thread, and a few light scrapings are given to the work; after which the tool is taken out of its holder, and the springy tool is put in to finally smooth the same portion of the screw. By now properly cleaning the thread and screwing on the gauge-nut, it will be easily and loosely screwed along the entire length of the conical part, but will stop at the beginning of the parallel portion; the poppet-head is then to be adjusted to parallelism for finally smoothing this thick portion until it fits the nut, the straight screw-tool being first employed, and afterwards the springy tool, which finally adjusts the thickest part to its finished diameter.

The taper and parallel portions being thus finished, another tapering is to be given, but at the end of the screw adjoining the tap-stem. To do this, the poppet-pivot is adjusted to taper the screw the opposite way to that for all the previous tapering, so that about three steps of the thread may be tapered until the extremity of the screw is about a fiftieth of the tap's greatest diameter smaller.

During the finishing of the parallel part of a tap-screw, a gauge-nut should be used whose screw exactly fits the hob, or other plug, to which the tap is being made. This nut is adjusted to the proper size by screwing the hob through the nut from both ends of the hole; and if the new tap is screwed to fit tightly in the gauge, the future hardening of the tap will enlarge it sufficient to screw the nuts so that they will fit loosely on the hob, while they are warm, through the tapping, and when cold, each nut will be about the intended size, and will fit the hob.

By means of the screwing process just mentioned, taps of all sizes are accurately finished without any hand screw-tools being used ; but the quicker method consists in finishing the tap or taps with hand-tools; these, in the hands of a careful man or boy, will adjust and polish the screws of taps and hobs quite as accurately as the best screwing-lathe or die-nuts ever made. But to avoid the laborious exertion necessary to finish with hand-tools a large number of long taps whose screws may be two or three inches thick, it is always advisable to make the slide-rest tools do the greater part of the smoothing, and this cannot be effected without great attention to the tool, so that a large number of thin cuts shall be taken off, instead of a few thicker ones. Smaller taps, that are only about an inch thick, should in all cases be partly screwed with hand-tools.

The length of the parallel part of a tap-screw is only about two-tenths of the entire length of the screw, and in order to obtain a proper idea of the shape and dimensions of such a screw, it is necessary to refer to Fig. 565; in this Figure the entire length of the screw is considered as ten equal lengths, and the number given to each length or portion is used to plainly point to any stated portion of the screw that may be spoken of. The mark No. 1 denotes that length of the tap which should enter the hole of the gauge-nut when the tap is newly made; consequently, at the time the tap's point is turned and smoothly filed, the gauge-nut should fit at one inch from the tap's extreme point, if the tap-screw is to be ten inches long. From the tap's point to the mark numbered 4, is the length of that portion of the screw-groove which is widened with the one-point tool of ninety degrees that was mentioned. From the mark No. 4 to No. 6, the thread is about three-quarters of the complete shape belonging to the thread of the parallel portion, and at about No. 7 the thread is completely formed, being in exactly the same shape as any portion of the parallel part, but smaller, the entire thickness of the tap at No. 7 being less than the extreme thickness at No. 8 or No. 9. The difference between the diameters at No. 7 and No. 8 is about a fiftieth of the tap's greatest thickness; consequently, in a two-inches tap, the difference is a twenty-fifth of an inch. The portion of the tap between the marks 8 and 10 is parallel, and the thickest part of the entire tool. By referring to the screw in the Figure it may be seen that the bottom of the thread-groove when finished is nearly parallel to the tap's slant side, and not parallel to the centre length.