1S46.] 



THE CIVIL ENGINEEERAND ARCHITECPS JOURNAL. 



373 



provements in the manufacture of the screws. The great difficulty is to 

 make the inclination of the thread uniformly regular, and the extraordinary 

 and microscopical accuracy which has been attained in this respect may 

 be considered among the most admirable triumphs of practical science. 

 The ingenuity, skill, and perseverance of the mechanicians engaged in this 

 pursuit can be but little known to general readers. 



" The late Mr. Henry Maudslay devoted an almost incredible amount 

 of labour and expense, to the amelioration of screws and screwing appa- 

 ratus ; which, as regarded the works of the millwright and engineer, 

 were up to that lime in a very imperfect state. Willi the view of pro- 

 ducing screws of exact values, he employed numerous modifications of 

 the chain or band of steel, the inclined knife, the inclined plane, and in- 

 deed each of the known methods, which however he remodelled as addi- 

 tions to Ihe ordinary turniuglaihe with a triangular bar ; a natural result, 

 »s he was then in the frequent habit of constructing that machine, and 

 which received great improvement at his hands. 



" It was noticed at page 5S1, that of all the methods he gave the prefer- 

 ence to the inclined knife, applied against a cylinder revolving in ihe laihe, 

 by means of a slide running upon the bar of Ihe lathe; which besides 

 being very rapid, reduced the mechanism to its utmost simplicity. This 

 made the process tu depend almost alone on the homogeneity of the mate- 

 rials, and on the relation between the diameter of the cylinder and the 

 inclmation of the knife ; whereas in a complex machine, every part con- 

 cerned in the transmission of motion, such as each axis, wheel and slide, 

 entails its risk of individual error, and may depreciate the accuracy of the 

 result; and to these sources of disturbance, must be added those due to 

 change of temperature, whelher arising from the atmosphere or from fric- 

 tion, especially when different metals are concerned The rela- 

 tions between the guide-screw and the copy were varied in all possible 

 ways: ihe guide was changed end for end, or difl'ertnt parts of it were 

 successively used ; sometimes also two guide-screws were yoked together 

 with three equal wheels, their nuts beii g connected by a bar joinitd to 

 each, and the center of ihis link (nhose motion thus became the mean of 

 that of the guides) was made to traverse the tool. Steel screws were also 

 cut, and converted into original taps, from which dies were made, to be 

 themselves used in correcting the minor errors, and render the screws in 

 all respects as equable as possible. In fact, every scheme that he could 

 devise, which appeared likely to benetit the result, was carefully tried, in 

 order to perfect to the utmost, the helical character and equaliiy of sub- 

 division of the screw. 



"Jlr. Maudslay succeeded by these means, after great perseverance, in 

 making a very excellent brass screw about seven feet long, and which, 

 compared with standard measure, was less than one-sixteenth of an inch 

 false of its nominal length. Taking the error as the one-thousandth part 

 of the total length of the screw, which was beyond its real quantity, to 

 make from it a corrected screw by the system of change wheels, would 

 have required one wheel of 1000 teeth, and another of one tooth less, or 

 999 ; but iu reality the error was much less, and perhaps nearer the two- 

 thousandlh of an inch ; then the wheels of 200O and 1999 teeth would 

 have been required ; consequently the system of change wheels is scarcely 

 applicable to the correction of very minute errors of length. 



"The change of the thousandth part of the total length, was therefore 

 given to the tool as a supplementary motion, which might be added to, or 

 subiracted from, the total traverse of the tool, in the mode explained by 

 the diagram, fig. 1, in which all details of construction are purposely 

 omitted. The copy C, and the guide-screw G, are supposed to be con- 

 nected by equal wheels in the usual manner ; the guide-screw carries the 

 axis of tlie bent lever, whose arms are as 10 to 1, and which moves in a 

 horizonlal plane ; the short arm carries the tool, the long arm is jointed to 

 a saddle which slides upon a triangular bar i i. 



Fig. 1. 



" In point of fact, the tool was mounted upon the upper of two longi- 

 tudinal and parallel slides, which were collectively traversed by the guide- 

 screw G. In Ihe lower slide was lixed the axis or fulcrum of the bent 

 lever, the short arm of which was connected by a link with the upper slide, 

 to tliat the compensating motion was given to the upper slide relatively to 

 the lower. 



" The triangular bar i t, when placed exactly parallel with the path of 

 the tool, would produce no movement on the same, and and G would be 

 exactly alike; but if i i, were placed out of parallelism one inch in the 

 %vhole length, the tool, during its traverse to the left by the guide-screw G, 

 would be moved to the right by the shifting of the bent level, one tenth of 

 the displacement of the bar, or one-tenth of an inch. 



" Therefore whilst the guide-screw G, from being coarser than required, 

 moved the principal slide the one-thousandth part of the total length in 

 excess ; the bent lever and inclined straight bar i i, pulled back the upper 

 or compeusating elide, the oae-thousandtb part, or ibe quantity in excess ; 



making the absolute traverse of the tool exactly seven feet, or the length 

 required for the new screw C, instead of seven feet and one-sixteenth of 

 an inch the length of G. To have lengthened the traverse of the tool, the 

 bar i i must have been inclined the reverse way ; in other words, the path 

 of the tool is in the diagram the liifftrence of the two motions; in the re- 

 verse inclination, its path would be Ihe sum of the two motions, and i i 

 being a straight line, the correction would be evenly distributed at every 

 part of the length.* 



"Whilst Mr. Maudslay's experiments in perfecting the screw were being 

 carried on, his friend Mr. Bartonf, paid frequent visits to his manufactory, 

 and also pursued a similar course. Mr. Barton preferred however, the 

 method of Ihe chain, or flexible band for traversing the tool the exact 

 quantity ; because the reduction of the diameter of the pulley or drum 

 afibrded a ready means of adjustment for total length ; and all the wheels 

 of the mechanism being individually as perfect as they could be made, a 

 near approach to general perfection was naturally anticipated on the first 

 trial. This mode, however, is subject to the error introdued by the elas- 

 ticity or elongaliou of the chain or band, and wliich is at the maximum 

 when the greatest length of chain is uncoiled from the barrel. 



"Ihese two individuals having therefore arrived, by different methods, as 

 near to perfection as they were then respectively capable of; each made a 

 screw of the same pitch, and 15 inches long, and the two when placed 

 side by side w ere found exactly to agree throughout their length, and were 

 considered perfect. The two screws were submitted iu 1810 to the scru- 

 tiny of that celebrated mathematical instrument maker, Ihe late Mr. Kdw. 

 Troughton, F.K.S., &:c., who examined them by means of two powerful 

 microscopes with cross wires, such as are used for reading oif the gra- 

 duations of astronomical instruments; applied like a pair of the most 

 refined compasses, to measure the equality of some 20, 50, or 100 threads, 

 taken indiscriminately at diiferent parts of the length of the screws.t 

 From this severe trial it resulted, that these screws, which to the 

 unassisted sight, and for almost every purpose of mechanism, were un- 

 exceptionable, were found to be full of all kinds of errors, being un- 

 equally coarse at diflerent parts, and even irregular in their angles, or 

 " drunk." This rigid scrutiny led both parties to fresh and ultimately 

 successful efforts, but of these our limits will only allow us to notice one, 

 apparently derived from the use of the tico microscopes. 



" Wr. Barion employed iiio pairs of dies upon the one screw ; the dies 

 were fixed at various distances asunder upon one frame or bar, and the 

 screw was passed through them. This was found to distribute the minute 

 errors so completely, that little remained to be desired ; as it is obvious 

 that at those parts where the screw was too coarse, the outer sides of the 

 threads were cut, and which tended to shorten the screw ; and where it 

 was too short, the inuer sides were cut, which tended to lengthen the 

 screw; in fact the two pans temporarily situated within the dies, were 

 continually endeavouring to approximate themselves to the fixed unvarying 

 distance, at which the dies were for the time placed. 



" Blr. Barton informed the author that he employed the screw corrected 

 in the above manner, in his engraving machine, employed for cutting with 

 the diamond, the lines as fine as 2000 in the inch, on the steel dies referred 

 to in the note on page 42, vol. i. ; and he said ' that such was the accu- 

 racy of the mechanism, that if a line were missed, the machine could be 

 set back for its insertion without any difference being perceptible.' The 

 author unintentionally ascribed the first application of the diamond to 

 turning steel, to Sir John Barton, whereas it had been used long before by 

 Kamsden in cutting the hardened-steel screw for his rectilinear dividing 

 engine." 



These adjustments and corrections, almost perfect as they would seem 

 to be, have been subjected to still further refinement. One of the most 

 beautiful mechanical devices which we have ever met with is described in 

 the subdivision relating to the application of the screw to the graduation of 

 mathematical scales. 



" One very important application of the screw, is to the graduation of 

 mathematical scales, ihe screw is then employed to move a platform, which 

 slides very freely, and carries the scale to be graduated ; and the swing 

 frame for the knife or diamond point is attached to some fixed part of the 

 framing of the machine. Supposing tlie screw to be absolutely perfect, 

 and to have fifty threads per inch, successive movements of fifty revolutions, 

 would move the platform and graduate Ihe scale exactly into true inches; 

 but on close examination, some of the graduations will be found to exceed, 

 and others to fall short of the true inch. 



" The scales assume, of course, the relative degree of accuracy of the 

 screw employed. No test is more severe ; and when these scales are ex- 

 amined by means of two microscopes under a magnifying power of ten or 

 twenty times, the most minute errors become abundantly obvious, from the 

 divisions of the scales failing to intersect the cross wires of the instrument ; 

 the result clearly indicates, corresponding irregularities in the coarseness 

 of the screw at the respective parts of its length. An accustomed eye 

 can thus detect, with the microscope, differences not exceeding the one 

 thirty-thousandth part of an inch, the tweuty-five-thousandth part being 

 comparatively of easy observation. 



* The apparatus was htted to the second screw-lathe of those described, and the in- 

 clined bar was placed on temporary wooden standards. 



t Subsequently Sir J. Barton, Comptroller ot the Mint, &c. 



i The niicrosi-Ope had been long ustd in the process of graduating instruments, but 

 this invaluable mode of employing two micwscopes in combination, was first Buccesaluky 

 practised by Mr. Troughton. 



