A pair of dies finished with a master-tap are also much more liable to stretch the screw of a bolt while being screwed, than a pair screwed with a small hob, through the greater pressure and friction resulting from the greater bearing surface during the entire time of screwing; and this stretching is often sufficient to render a screw which is only two inches long, unfit for its nut, box, lever, rod, or other piece for which the screw is made. The stretching places the summits of the thread further apart than they should be, and the screw must therefore be reduced below its proper diameter to get it into its place. A pair of master-tap dies are also much weakened by the comparative large gaps for shavings, such gaps being necessary to cause the dies to cut at the bottoms of their screw-gaps.

Master-tap dies are represented in Figs. 607 and 608, by which it may be seen that all, or nearly all, of the die-screw must bear on the bolt during the entire time of screwing; Fig. 607 showing the situation of the dies at the beginning of a screwing process, and Fig. 608 showing their situation at the conclusion. In this Figure it may also be observed that if the dies were screwed with a master-tap of large diameter, the outer edges of the thread at the mouths of the gaps, cannot cut at all at the end of a screwing process, because they are then extended quite free from the bolt, as shown in the Figure.

To produce a couple of dies that shall cut well and also have a firm bearing while screwing, is very desirable, especially for large dies, to avoid a slow screwing of bolts, and to avoid the risk of spoiling a large screw through the dies getting out of a proper position during screwing. A couple of dies should therefore be made so that one shall have more bearing surface than the other, the one having the smaller surface being intended to cut, while the one with the larger bearing surface shall become a bearer to maintain the necessary parallelism of the die-screw with the screw being made. This difference of bearing surface is effected by using odd dies, and is obtained by either enlarging the mouth of one die-gap, by using both a large hob and a small one for screwing one pair of dies, or by making a very large gap for shavings in one die, that the bottom of this one die-gap shall not come into contact with a bolt during any part of its screwing.

A greater bearing surface in one die is obtained by screwing a pair while tight in their frame, by means of a small hob instead of a master-tap, and afterwards filing off the teeth at the two sides of the gap in one die, and allowing the sides of the gap in the other die to remain as when screwed; this die which is not filed consequently becomes the cutter, and the filed one the bearer.

To produce one couple of dies by screwing them with both a hob and a master-tap, it is necessary to drill them while tightly screwed together with a packing-piece between, as described, but to bore them so that the gap in one die shall be rather deeper than the gap in the other die. The diameter of the hole is equal to the diameter of the small hob at the bottom of its thread, and when screwing begins, this small hob is the one first put in, and the screwing progresses by gradually forcing the dies towards each other with the adjusting screw, and rotating the hob to and fro as for other die-screwing, without a packing-piece. When the complete thread is thus obtained, the die which has the smaller screw-gap is taken out of the die-frame and another blank die having a gap about the same size, but no thread, is put into the same place. After filing off a part of the thread at the edges of the large-gap die, the screwing again proceeds with the master-tap instead of the hob first used. This screwing widens the mouth of the gap in the die already screwed with a full thread, but does not make the gap any deeper, or if any, only a small quantity ; the widening therefore gives a greater angle to the gap's curve, and makes it suitable for a bearer, as required. When the thread of the bearer is completed, the blank die, or the one having only a very shallow thread-groove, is taken out, being of no further use at the present. It is generally advisable to commence screwing the auxiliary blank die with the same small hob at first used, in order to more easily commence the screwing with the larger hob - or master-tap, as it is termed - the only use of the auxiliary die consisting in providing a bearing for the hob while the die having the smaller gap is out of the die-frame. After the bearer is screwed with the master-tap, it is necessary to again screw the small hob through both the dies a few times while they are together in the frame in their positions for use. By means of these processes, the die having the small gap becomes the cutter, as intended, because it has not been in contact with the master-tap, and therefore is of smaller curve.

The making of sharp cutting edges in a die by means of a broad gap for shavings, is easily effected, and the die thus treated should be one of a couple whose screws were both finished with a master-tap, both gaps being of one shape and depth until the large channel or gap for shavings referred to, is made. This broad gap is shown in the die named the cutter, in Fig. 611, and greatly reduces the bearing surface while screwing, also causes the cutting die to cut at the bottom of its gap during the conclusion of a screwing process when the thread of the bolt is fully formed. The opposite die, which is the bearer, also has a gap for an oil-channel, which is but small, through not being intended for cutting edges.

The cutting properties of all dies, whether screwed with long taper taps, small hobs or large ones, may be greatly improved by tapering the die-screw at one end of the hob. By whatever tap or hob the dies are screwed, the entire cutting of the dies while in use, is effected by about two steps of the thread at one end of the die-screw, if this screw is entirely parallel. But if tapered at one end, the dies will cut in a manner similar to that of a good tap, and several steps of the die-screw will cut at one time, instead of only one or two, so that the dies will execute more work without getting blunt, and will require less grinding than dies having parallel screws. Such tapering is easily done with filing, after the screwing of the dies is completed, and after a rough and smooth fifing with small half-round files, the tapered surfaces are polished with emery cloth. It is also necessary to make the die teeth analogous to the teeth of a tap, by filing off a portion of the thread at the rear of each cutting tooth, which prevents the back of the tooth coming into contact with the bolt being screwed, in about the same way that the back of a tap-tooth is prevented touching the nut while being tapped.