In respect to screws, the instrument, the size of which claims the most importance, is perhaps the plug-tap, or that which removes the last portion of the material, and therefore determines the diameter of the internal thread; but as the tap is continually, although slowly, wearing smaller, the first and last nut made with it unavoidably differ a little in size. It is on account of the wearing of the tap, amongst other circumstances, that when screws and nuts are made in large numbers, and are required to be capable of being interchanged, it becomes needful to make a small allowance for error, or to make the screws a trifle smaller than the nuts.
In order to retain the sizes of the taps used by Holtzapffel&Co. they some years ago made a set of original taps exactly of the size of the proposed screws, and to be called A; these, when two or three times used, to rub off the burrs, were employed for cutting regulating dies B, of the form of fig. 639, with two shoulders, so that the dies could be absolutely closed, and yet leave a space for the shavings or cuttings. In making all their plug-taps, they are first prepared with the ordinary shop tools, until the taps are so nearly completed, that, grasped between the regulating dies B, the latter close within the fortieth or fiftieth of an inch, therefore leaving the dies B next to nothing to perform in the way of cutting, but only the office of regulating the diameter of the working plug-taps. Should the diet B meet with any accident, the taps A, which have to this stage been only used for one pair of regulating dies, exist for making repetitions of B. This method has been found to fulfil its intended purpose very effectually for several years, but at the same time it is not proposed to apply this or any other system universally.
In conclusion, it may be said that by far the most import argument in favour of the adoption of screws of aliquot pitches applies to steam machinery and similar large works, and that, principally, because it brings all such screws within the province of the screw-lathe with change-wheels, which has become, in engineering establishments and some others, a very general tool. This valuable tool alone, renders each engineer in a great measure independent of his neighbour, as screws of 2, 2 1/4, 2 1/2, 3, 10, or 20 threads in the inch, are readily measured with the common rule, and copied with the screw-wheels, and a single- pointed tool, or an ordinary comb or chasing tool with many points.
And therefore, with the modern facility of work, were engineers severally to make their screw tackle from only the written measures of any conventional table, they would be at once abundantly within reach of the adjustment of the tools, and that without any standard gages; the strict introduction of which would almost demand that all the tools made in uniformity with them should emanate from one center, or be submitted to some office for inspection and sanction, - and this would be indeed to buy the occasional advantage at too dear a rate.
It must, however, be unhesitatingly granted, that the argument applies but little, if at all, to a variety of screws which from then smaller size are not made in the screw-lathe, but with die-stocks and the hand-chasing tools only; and which arc employed in branches of art that may be considered as almost isolated from one another, and therefore not to require uniformity.
For instance, the makers of astronomical, mathematical, and philosophical instruments, of clocks and watches, of guns, of locks and ironmongery, of lamps, and gas apparatus and a multitude of other works, possess, in each case an amount of skill which applies specifically to these several occupations; so that unless the works made by each arc returned to the absolute makers for reparation, they are at any rate sent to an individual engaged in the same line of business.
Under these circumstances, it is obvious that the gunmakers, watchmakers, and others would derive little or no advantage from one system of threads prevailing throughout all their trades; in many of which, as before noticed, partial systems respectively adapted to them already exist. The means employed by the generality of artizans, in matching strange threads, are, in addition, entirely independent of the screw lathe, and apply equally well to all threads, whether of aliquot measures or not; as it is usual to convert one of the given screws, if it be of steel, into a tap, or otherwise to file a screw tool to the same pitch by hand, wherewith to strike the thread of the screw or tap; and when several screws are wanted, a pair of dies is expressly made.
But at the same time that, from these manifold considerations, it appears to be quite unnecessary to interfere with so many existing arrangements and interests, it must be freely admitted that advantage would ultimately accrue from making all new screws of aliquot measures; and which, by gradually superseding the old irregular threads, would tend eventually, although slowly, to introduce a more defined and systematic arrangement in screw tackle, and also to improve their general character.
The author has now concluded the various remarks he proposes to offer on the formation of the screw for the general purposes of mechanism; on the modes pursued by various celebrated mechanicians for its improvement; and on various practical considerations which influence the choice of screws: but he is desirous briefly to advert to some few peculiar, interesting and practical methods of producing this important element of construction.
The threads of wrought-iron screws have been forged whilst red hot, between top and bottom swage tools, having helical surfaces like those of screw dies; screws have been twisted whilst red hot, out of rectangular bars, by means of the tail vice and hook wrench; as in making screw augers. Screws intended for ordinary vices, have been compressed whilst cold, somewhat as with die-stocks; the lever is in this case very long, and the die is a square block of hardened steel, with an internal square thread screw, left smooth or without notches. The thread is partly indented and partly squeezed up, the diameter of the iron cylinder being lets than that of the finished screw: this action severely tests the iron.*