EMERY-WHEELS. 



249 



cure the best mechanical results, a very high 

 rate of speed was necessary, and immense diffi- 

 culties lay in the way of securing adequate 

 cohesion of the particles. Rubber, glue, 

 shellac, oxidized linseed-oil, and various me- 

 tallic and mineral cements were tried, with 

 varying degrees of success. Among the earlier 

 devices was a composition of emery with 

 melted brimstone ; but the friable character 

 of this was so effectually demonstrated by fre- 

 quent accidents, that its use was speedily dis- 

 continued, and years passed before the record 

 of precedence was forgotten. The soluble 

 silicates were tried at an early date, and with 

 promising, but not wholly satisfactory, results. 

 Some of the earlier experiments hinged upon 

 the chemical affinities and interchanges of the 

 oxides and chlorides of zinc, magnesium, and 

 other bases, and many failures resulted, owing 

 to the unknown and unexpected changes that 

 went on silently beneath the exterior surface 

 of the wheel, -and only made themselves mani- 

 fest after a lapse of time altogether uncertain 

 in duration. 



An instance is mentioned, on good authority, 

 where a manufacturer kept samples of all the 

 wheels sent out from his establishment, and 

 found after a time that they had fallen to pieces 

 on the shelf where they were stored. Another 

 difficulty encountered by the early manufact- 

 urers was the warping of wheels, particularly 

 those that were thin. It was found that this 

 was due to turning the sides first and the face 

 last. A reversal of the process corrected the 

 tendency. The main difficulty, however, lay 

 in the composition. Vitrifaction, it was found, 

 was inseparable from a degree of brittleness 

 incompatible with sufficient cohesion, and it 

 appears that upon the whole tanite and vul- 

 canite wheels give the best results. The secret 

 of the details of manufacture of all the best 

 makers is closely guarded to this day ; but it is 

 known that success was reached only through 

 a long series of careful experiments. The ter- 

 rific force of an exploding wheel is little ap- 

 preciated save by the workmen who are 

 obliged to stand over them, and by the em- 

 ployers who are, or ought to be, responsible 

 for their safety. The testing-room of one of 

 the largest manufactories of emery-wheels is 

 similar to a bomb-proof for strength. Its 

 walls are of stone, 3 to 4 feet thick, and the 

 roof is made of heavy oaken logs covered with 

 loose stones. The testing-machine is run by a 

 belt, which passes through a narrow slit in the 

 rear wall, and can be thrown into and out of 

 gear by a person outside of the building. The 

 test-speed applied to the wheels is largely in 

 excess of any to which they are likely to be 

 subjected in practice, and when they break in 

 the testing room they strike the walls and roof 

 with the force of artillery, splintering the 

 stone and penetrating the solid timbers. There 

 is, in fact, no safe-guard except the intrinsic 

 cohesiveness of the wheel. Hoods, cowls, 

 and screens can not be made heavy enough to 



withstand the shock, and side-flanges can not 

 be so tightly adjusted as to restrain the frag- 

 ments of a broken wheel, responding to a cen- 

 trifugal force of 1,500 or 1,600 revolutions a 

 minute. 



All this elaboration of experiment and pre- 

 caution presupposes an enormous demand for 

 a trustworthy manufactured article. In point 

 of fact, the solid emery-wheel has already 

 superseded the file, and all other methods of 

 grinding and polishing on a large scale. An 

 elaborate series of experiments have been in- 

 stituted to establish the comparative merits of 

 the old and new methods. Perhaps the most 

 striking of the results is found in a table com- 

 paring the cost of wearing away one pound of 

 brass, cast-iron, wrought-iron, and steel, re- 

 spectively, with a file and with an emery- 

 wheel. Skilled workmen were employed ac- 

 customed to the use of the file, and they 

 worked as rapidly as possible to wear away 

 the required pound of filings and chippings in 

 the shortest possible time, without regard to 

 the shape in which the metal was left. 



COST PER POUND OF REMOVING OR WEARING AWAY 

 METALS. 



A careful comparison of these results shows 

 very largely in favor of the wheels. While 

 every stroke of the file or chisel diminished the 

 cutting power of the instrument, the wheel re- 

 mained as good as new at the end of the trial. 

 The wheel is in fact a circular file, costing 

 2f cents a pound, while tiles cost about 16 

 cents apiece. The speed of the wheel during 

 the test-trials was 1,660 revolutions, or about 

 one mile a minute measured at the cutting- 

 edge. 



Emery-wheels are made of all sizes, from 

 the size of a shirt-button up to 36 inches in 

 diameter, and of various thicknesses, according 

 to the work required of them, which may be 

 simple saw-gumming, or grinding the treads 

 of car-wheels, evening plate-surfaces, or other 

 heavy work. The mounting of the wheels to 

 secure the utmost durability and immunity 

 from fracture is of the highest importance, and 

 has called for the exercise of much ingenuity 

 in bringing them to their present state of per- 

 fection. Vibration has been reduced to a 

 minimum, and the breaking of a good wheel 

 when properly mounted is now a rare occur- 

 rence. "Wheels when fresh from the factory 

 are, or should be, absolutely true, but the 

 wear of usage is apt to make them somewhat 

 uneven. This is easily corrected by the use of 

 a diamond tool, which is the only instrument 

 that can remove the inequalities and reduce 

 the cutting-face to its normal condition. 

 Wheels are made with edges of various shapes, 



