Screw

SCREW, a cylindrical or conical piece of wood or metal hav ing a helical groove running round it. The surface thus formed constitutes an external screw, while a similar groove cut round the interior of a cylindrical hole, as in a nut, constitutes an internal screw. The ridge between successive turns of the groove is the "thread." and the distance between successive turns of the thread is the "pitch." For screw as a mechanical power see MECHANICS for the screw used to propel steamships see SHIPBUILDING.

Standardization of Screws.

All screws made to-day are copies of pre-existing or master screws, which are made in nu merous forms. These are so standardized that a thread cut to a given standard in England fits its fellow thread cut to the same standard in America, Germany or elsewhere. The first attempt at securing uniformity in screw threads was made by Sir Joseph Whitworth about 1841. In the course of about twenty years the Whitworth system generally displaced the previous heterogeneous designs of threads, by the existence of which engineers' repairs had been rendered most inconvenient and costly, almost every establishment having its own "standard" set of screwing tackle.

When the Whitworth threads were accepted in England, Ger many and the United States, it appeared as though they were established for ever in an impregnable position, as a unifica tion evolved from chaos. Moreover, Great Britain at that time occupied a position of pre-eminence in manufacturing engineering, which was favourable to the establishment of an English system. But two things were wanting to permanence—the facts that the Whitworth threads were not based on the metric system, and that the United States was destined to come into rivalry with Great Britain. Metric systems became standardized on the continent of Europe and the Sellers thread in America over shadowed the Whitworth, though it is impossible to doubt. that the Sellers like the Whitworth must in time be swallowed up by some one metric system.

The threads now recognized as standard are included in about eight great systems, out of about sixty that have been advocated and systematized. Their elements are shown by the diagram, fig.

two rotate at the same speed, the thread cut on the spindle axis will be equal in pitch to that of the lead screw. If the spindle revolves more slowly than the lead screw, a thread coarser than that in the latter will result ; if it revolves more rapidly, one of finer pitch will be produced. The spindle is the first factor, being the driver,

and the lead screw is driven therefrom through the change wheels —the variables—which determine the number of revolutions of the I ; but tables of dimensions are omitted, since they would demand too much space.

There are four methods employed for the cutting of screw threads : one by means of a single-edged tool held in the saddle of the screw-cutting lathe, and traversed horizontally only, the cylin der which is to receive the thread revolving the while; another by means of short master screws, hobs or leaders, controlling chasers or comb tools ; the third by means of screw taps and dies, either the work or the tool being absolutely still. The fourth is by means of a milling cutter presented to the work in a special screw-milling machine, both the work and the cutter revolving.

In the Lathe.

The problem of screw-cutting in the lathe in the simplest form resolves itself into the relative number of revolu tions of the lathe spindle and of the lead screw (fig. 2). If the latter whether the same, or slower, or faster than the spindle. Screw-cutting in all its details is an extensive subject, including the cutting of what are termed odd or unequal pitches, that is, those which involve fractions, the catching of threads for succes sive traverses of the tool, the cutting of multiple threads and of right- and left-hand threads, which involve much practical detail.

The principle of screw-cutting may be stated briefly thus : the pitch of the guide screw is to that of the screw to be cut as the number of teeth on the mandrel or (headstock) wheel is to the number of teeth on the lead screw wheel. It is therefore simply a question of ratio. Hence for cutting threads finer than those of the lead screw, the guide screw must rotate more slowly than the lathe mandrel; and for cutting threads coarser than those of the guide screw, the lead screw must rotate faster than the lathe mandrel (fig. 2, C and D). When the ratios are ascertained, these facts indicate when the larger or the smaller wheels must be placed as drivers, or be driven. "Simple trains" are those which contain only one pair of change wheels; "compound trains" have two, three, four or more pairs (fig. 2), and are necessary when the ratio between the guide screw and the screw to be cut exceeds about six to one.