Report of the committee, consisting of Sir Joseph Whitworth, Dr. C. W. Siemens, Sir Frederick Bramwell, Mr. A. Stroh, Mr. Beck, Mr. W. H. Preece, Mr. E. Crompton, Mr. E. Rigg, Mr. A. Le Neve Foster, Mr. Latimer Clark, Mr. Buckney, and Mr. H. Trueman Wood (Secretary), appointed for the purpose of determining a gauge for the manufacture of the various small screws used in telegraphic and electrical apparatus, in clockwork, and for other analogous purposes.
1. This committee was formed by the General Committee of the British Association assembled at York in August and September, 1881, for the purpose of determining a gauge for the manufacture of the various small screws used in telegraphic and electrical apparatus, in clockwork, and for other analogous purposes.
Unfortunately, though Preece's paper is noted in the Proceedings, his text was not printed.
2. At that meeting a paper was read by Mr. Preece, pointing out the desirability of establishing such a gauge. Although the Whitworth gauge is almost invariably adopted for the bolts and screws used in millwork and engineering in England, no general system has been hitherto applied to the smaller screws, used either in clockwork, philosophical instrument work, or in the numerous practical applications of electricity that are now rapidly becoming so important. In fact, at the present time, gauges and screw-plates almost equal in number the makers engaged in the trade. One instance was brought to the attention of the committee, by a manufacturer who had to execute an order for railway signal apparatus, in accordance with three sample instruments, containing among them twenty-one screws of different threads, not one of which happened to be in use in his shop. There is now no recognised form of thread, no specified number of threads per inch — in fact, no generally accepted gauge, based on practice and experience. Great inconvenience is felt in providing for repairs, which are, in consequence, more costly and less efficient.
The employment of some coherent and uniform system is manifestly required. It not only would render repairs easier, speedier, and cheaper, but it would introduce interchangeability of parts, and further the extension of piecework; and it would reduce the equipment of workshops with special and costly tools.
See Thury threads.
3. The subject of uniformity in screws has been very warmly taken up by the Société des Arts de Genève, which appointed a committee in December, 1876, who after assiduous labours issued a report in 1878. The system proposed by them has been very fully described by Professor Thury in two pamphlets published in Geneva.¹ The committee collected numerous screws of all sizes from many factories, measured them carefully, tabulated their several dimensions, and plotted the results by the ordinary method of linear co-ordinates. They determined the mathematical equations to curves that most closely corresponded with the ratios of diameter to pitch thus found to have been employed in practice, and adopted the one which most nearly represented the mean average proportions of the screws in use at various shops, and in different countries.
The Swiss Committee took 1 millimetre pitch as the basis of their system. It was agreed that such a pitch was best adapted to a screw having a diameter of 6 millimetres. The form of thread adopted was triangular. The angle made by producing the two sides being approximately 47½ deg.; the depth being 3/5 of the pitch, the top being rounded off by a radius 1/6, and the bottom by a radius 1/5 of the pitch.
The Committee has had an opportunity of examining screw-plates, and numerous packets of the corresponding screws manufactured on this system.
The following table gives the pitches and diameters in millimetres and 'mils'² to two significant figures, and the number of threads per inch of all the screws comprised in the small screw series, which happens to cover the exact ground to which the attention of the committee has been specially directed, namely diameters below the ¼ inch.
It is to be observed that the numbers by which the screws are designated, given in the first column, are not arbitrary. Each pitch of the series is 9/10ths of that which succeeds it in the table.
Thus the several pitches are:
1 mm.; 9/10 mm.; (9/10)² mm; (9/10)3 mm; (9/10)4 mm; … (9/10)n mm
This series may be expressed in the form:
0.90; 0.9¹ ; 0.9²; 0.93; . . . 0.9n; . . . (1)
whence it is at once evident that the designating number of the screw is the index of the power to which 0.9 must be raised in order to ascertain its exact pitch in millimetres.
The method by which the relation between pitch and diameter is arrived at will be gathered from the following explanation:—
Let D represent the diameter, and P the pitch. Then, generally,
D = f (P)
Evidently there can be no constant term, for when D = 0 P must also = 0. Moreover, D, practically cannot be a simple multiple of P, for experience has shown that small screws must have a less number of threads per diameter than large screws.
Hence the formula will be of the form
D = m Pk ...... (2)
where m and k are constants to be determined.
Since 1k is 1 whatever be the value of k, it follows that the coefficient m represents the value of D when P is 1. The Swiss Committee agreed that the unit pitch (1 millimetre) should be adopted for the screw having a diameter of 6 millimetres; in other words, they make m = 6.
The value of k must be ascertained by trial.
k = 1 would give a constant ratio, which we know is inadmissible.
k = 2 will be found on trial to give a far too rapid decrease in the ratio of diameter to pitch.
The several simple fractions between these limiting values were tried in succession, and the results obtained when using 6/5 were found to give results that best accord with practice and experience.
Substituting the values thus arrived at in (2), the formula becomes
D = 6P6/5 ......(3)
The Swiss system is thus very complete, but there are reasons which prevent this committee from recommending its adoption in its entirety.
4. No one has done more to establish gauges of all kinds in England than Sir Joseph Whitworth. His classical paper on "An Uniform System of Screw Threads" was communicated as far back as 1841 to the Institution of Civil Engineers. He had made an extensive collection of screw-bolts from the principal workshops throughout England, and the average thread was carefully measured for different diameters. The ¼, ½, 1, and 1½ inches were selected and taken as the fixed points of a scale by which the intermediate sizes were regulated. The result is an admirable thread for the large iron bolts and screws used in fitting up steam-engines and other machinery. The angle made by the sides of this thread is 55°. One-sixth of the depth of the thread is rounded off from the top, and one-sixth from the bottom. The actual depth is rather more than three-fifths, and less than two-thirds of the pitch.
The slow adoption of such an admirable system was perhaps due, in great measure, to the fact that it was put forward by an individual, and not by an Association. A single individual, however exalted his reputation, cannot secure that immediate and universal attention which is obtained by such an organisation as the British Association. The system of units of electrical measurements sanctioned by the Association obtained instant recognition, and has now, thanks to the Congress of Electricians, held in Paris in October, 1881, become universally accepted. It is hoped that the same result will follow the recommendations of this committee.
5. The question of the introduction of the metrical system occupied the serious consideration of the committee, but, considering the fact that it is not generally adopted in engineering or manufacture in England, and that it is as yet little understood by our workmen, it was thought better to suggest no change in this direction. The committee is not insensible to the simplicity of the metrical system and to its possible universality, nor to the fact of its gradual introduction in scientific circles, but while the manufacturing interests are still wedded to the British inch, and its multiples and sub-multiples, and while the British legal standard of length is still the yard, the Committee has felt it impossible to suggest a change which has little chance of adoption, and which might jeopardise the introduction of that with which they are more concerned—viz., a uniform screw-thread.
Hence it was determined that the unit of length taken should be the ‘mil,’ and that the decimal system should be adopted for expressing dimensions.
6. The use of a screw is to draw together and to unite certain parts of apparatus in firm and intimate contact. To attain these ends, a screw must facilitate the application of mechanical power to draw the parts together, and it must possess strength to hold them so; it must not interfere with the easy separation of these united parts when necessary; it must possess durability—that is, it must be capable of repeated use without undue friction and without wear, otherwise it will speedily become loose and dangerous when frequently removed and restored. There has to be considered the pitch of the screw, its relation to the diameter of the bolt on which it is cut, the depth of the cut, and the form of the thread. The pitch primarily determines the power of the screw, for it determines, for each diameter, the angle of the inclined plane; the depth determines the section of core left to resist shear or rupture; while the form of the thread determines the durability and efficiency, and determines also the surface of thread to bear endway strain.
7. The committee have devoted very considerable attention to the pitch, form, and depth of screws, and they have compared together a large number of different kinds, some of which are in actual use, while others have only been suggested. They have, moreover, decided on recommending the adoption of the Whitworth form of thread, not only because it is so well known, but because experience has proved it excellent, and unsurpassed when employed for engineers' bolts. The Committee, however, are not unanimous on all questions involved by this proposal, and as there are several points that require to be thoroughly sifted and tested, they ask to be re-constituted, and to be allowed a small grant to put their proposal to the test of practice, and to have a few gauges constructed for distribution or examination.
1. Systématique des vis Horlogères, Geneva, 1878. Notice sur le Système des vis de la Filière Suisse, Geneva, 1880.
2. The ‘mil’ is the thousandth part of the British inch.
To follow this thread, go on to the Second Report.
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Last revised: 3 January 2011.