The choice of section is collectively governed: First, by the facility of construction, in which the plain angular thread excels. Secondly, by the best resistance to strain, which is obtained in the square thread. Thirdly, by the near equality of strength in the internal and external screw. For similar materials the space and thread should be symmetrical, as in the square thread, and in figs. 626 to 630, which screws arc proper for metal works generally; whereeas in dissimilar materials, the harder of the two should have the slighter thread, as in the iron screws figs. 631 to 684, intended to be screwed into wood; the substance of the screw is supposed to be below the line, and the head to the right hand. Fourthly, by the resistance to accidental violence, either to the screws, or to the screwing tools, which is best obtained by the rejection of sharp angles or edges, as in the several rounded threads. This fourfold choice of section, like every other feature of the screw, is also mainly determined by experience alone.
Fig. 626, in which the angle is about 60 degrees, is used for most of the screws made in wood, whether in the screw-box or the turning lathe; and also for a very large proportion of the screw bolts of ordinary mechanism. Sometimes the points of the screw tool measure nearly 90 degrees, as in the shallow thread, fig. 627, used for the thin tubes of telescopes; or at other times they only measure 45 degrees as in the very deep thread, 628, used for some mathematical and other instruments; the angles represented may be considered as nearly the extremes.
In originating accurate screws, the angular thread is always selected, because the figure of the thread is still maintained, whether the tool cut on one or on both sides of the thread, in the course of the correctional process.
Fig. 629 is the angular thread in which the ridges are more or less truncated, to increase the strength of the bolt; it may be viewed as a compound of the square and angular thread.
Fig. 630 is the angular thread in which the tops and bottoms are rounded; it is much used in engineering works, and is frequently called a round thread.*
Sections derived from the Angular Thread.
Sections derived from the Square Thread.
* See foot-note on p. 670.
In 631 the thread is more acute, and truncated only at the bottom of the screw, this is used for joinery-work, and greatly increases the hold upon the wood; 632 is obviously derived from 631, and is used for the same purpose.
In 633, which is also a screw for wood, the face that sustains the hold is rectangular, as in the square thread, the other is bevelled. Fig. 634 is the form of the patent wood screw, sometimes called the German screw; it is hollowed to throw the advantage of bulk, in favour of the softer material, or the wood, the head of which is supposed to be on the right hand. In the last four figures, the substance of the screw is imagined to be situated below the line, and that of the wood above.
The screws which are inserted into wood are generally made taper, and not cylindrical, in order that they may cut their own nut or internal thread; some of them are pointed, so as to penetrate without any previous hole being made: they merely thrust the fibres of wood on one side. Screws hold the most strongly in wood, when inserted horizontally as compared with the position in which the tree grew, and least strongly in the vertical position.
Fig. 633 represents the ordinary square thread screw; the space and thread are mostly of equal width, and the depth is either equal to the width, or a trifle more, say one sixth.
Fig. 366 is a departure from 635, and has been made for presses: and 637 has obviously grown out of the last from the obliteration of the angles; various proportions intermed between 637 and 630 arc used for round threads.
In some cases where the screw is required to be rapid, one single shallow groove is made of angular, square, or circular section, leaving much of the original cylinder standing, as in fig. 6
For very slight purposes, a pin only is fitted to the groove, to serve as the nut; should the resistance be greater, many pins, or a comb may be employed, and this was the earliest form of nut; otherwise a screwed nut may be used with a single thread. But when the greatest resistance is required, the surface bearing of the nut is extended, by making the thread, double, triple, etc. by cutting or more intermediate grooves and a counterpart nut. The nuts or boxed of very coarse screws for presses are now mostly cut in the lathe, although, when the screwing tools were less perfectly understood, the nuts were frequently cast. Sometimes lead, or alloys of similar fusibility, were poured in betwixt the screw and the framework of the machinery (see note, p. 293, also 322-3, vol. 1); but for nuts of brass and gun-metal, sand moulds were formed. The screw was always warmed, to avoid chilling the metal; and for brass, it was sometimes heated to redness and allowed to cool, so as slightly to oxidize the surface, and lessen the disposition to a union or natural soldering of the screw and nut. It was commonly necessary to stretch the brass by an external hammering, to counteract the shrinkage of the metal in the act of cooling, and to assist in releasing from the screw, the nut cast upon it in this manner. The mode is by no means desirable, as the screw is exposed to being bent from the rough treatment, and to being ground by particles of sand adhering to the brass.
The tangent screws used for screw wheels, have mostly angular or truncated angular threads, fig. 629, as screws absolutely square cannot be fitted with good contact and freedom from shake between the thread and teeth; and probably the same rules by which the teeth of ordinary wheels and racks are reciprocally set out, should be also applied to the delineation of the teeth of worm-wheels, and the threads or teeth of their appropriate screws.