The screw is converted into the tap, by the removal of parts of its circumference, in order to give to the exposed edges a cutting action; whilst the circular parts which remain, serve for the guidance of the instrument within the helical groove, or hollow thread, it is required to form.

In the most simple and primitive method, four planes were filed upon the screw as in fig. 537, but this exposes very obtuse edges which can hardly be said to cut, as they form the thread partly by indenting, and partly by raising or burring up the metal; and as such they scarcely produce any effect in cast iron or other crystalline materials. Conceiving, as in fig. 537, only a very small portion of the circle to remain, the working edges of squired taps, form angles of (90 + 45 or) 135 degrees with the circumference, and the angle is the greater, the more of the circle that remains. It is better to file only three planes as in fig. 538, but the angle is then as great as 120 degrees even under the most favourable circumstances.

In taps of the smallest size it is imperative to submit to these conditions, and to employ the above sections. Sometimes small intermediate facets or planes, are tipped off a little obliquely with the file, to relieve the surface friction; this gives the instrument partly the character of a six or eight-sided broach, and improves the cutting action.

Section III On Cutting Internal Screws With Screw  20085

There appears to be no doubt, but that for general purposes, the most favourable angle for the edges of screw taps and dies, is the radial line, or an angle of 90 degrees. This condition manifestly exists in the half-round tap fig. 539, which is advocated in the annexed quotation from Sir John Robison, who in speaking of the tap, says, "I propose that this should be made half-round, as it will be found that a tap formed in this way will cut a full clear thread (even if it may be of a sharp pitch), without making up any part of it by the burr, as is almost universally the case, when blunt-edged or grooved taps are used."

"It has sometimes been objected to me by persons who had not seen half-round taps in use, that, from their containing less substance than the common forms do, they must be very liable to be broken by the strain required to turn them in the work. It is proved, however, by experience, that the strain in their case is so much smaller than usual, that there is even less chance of breaking them than the stouter ones. Workmen are aware that a half-round opening bit makes a better hole and cuts faster than a five-sided one, and yet that it requires less force to use it."*

Fig. 540, in which two-thirds of the circle are allowed to remain, has been also employed for taps; this, although somewhat less penetrative than the last, is also less liable to displacement with the tap-wrench. It is much more usual to employ three radial cutting edges instead of one only; and, as in the best forms of taps, they arc only required to cut in the one direction, or when they are screwed into the nut, the other edges are then cham-fered to make room for the shawings; thereby giving the tap a section somewhat like that of a ratchet-wheel, with either three, four, or five teeth, as in figs. 541 and . 549.

* Select Papers read before the Soc. of Arts for Scotland, vol. i., page 41.

It is more common, however, either to file up the side of the tap, or to cut by machinery, three concave or elliptical flutes, as in 542; this form sufficiently approximates to the desideratum of the radial cutting edges, it allows plenty of room for the shavings, and is easily wiped out. What is of equal or greater importance, it presents a symmetrical figure, little liable to accident in the hardening, either of distortion from unequal section, as in figs. 539 and 540, or of cracking from internal angles, as in 540 and 541.*

Still, considering alone the transverse section of the tap, it will be conceived that before any of the substance can be removed from the hole that is being tapped, the circular part of the instrument must become embedded into the metal a quantity equal to the thickness of the shaving; and in this respect figs. 537 and 538, in which the circular parts are each only the tenth or twelfth of the circumference, appear to have the advantage over the modern taps 541 and 542, in which each arc is twice as long. Such, however, is not the case, as the first two act more in the manner of the broach, if we conceive that instrument to have serrated edges; but figs. 541 and 542 act nearly as turning-tools, as in general the outer or the circular surface is slightly relieved with a file, so as to leave the cutting edges a, somewhat in advance of the general periphery; which is equivalent to chamfering the lower plane of the turning tool some 3 degrees (see page 534), to produce that relief which has been appropriately named the angle of separation.

But in the tap fig. 543, patented by Mr. G. Bodmer of Manchester, this is still more effectually accomplished. The instrument, instead of being turned of the ordinary circular section in the lathe (or as the outer clotted line), is turned with three slight undulations, by means of an alternating radial motion given to the tool. From this it results, that when the summits of these hills are converted into the cutting edges, that not only are the extreme edges or points of the teeth made prominent, but the entire serrated surface becomes inclined at about the three degrees to the external circle, or the line of work, so as exactly to assimilate to the turning tool; and therefore there is little doubt but that, under equal circumstances, Mr. Bodmer's tap would work with less friction than any other.

* In fluting taps, as in cutting the teeth of wheels, the tap or wheel is frequently chucked in the lathe, just as in turning; but the mandrel is held at rest by the dividing plate, and the tool is a cutter, revolving horizontally, and traversed through the groove by the slide-rest screw. The round flutes are made with cutters having semicircular edges and placed centrally; the ratchet-form flutes are made with thick saws or square-edged cutters, the one edge of these is placed to intersect the center of the tap, and leave the radial edge.