14 MEMOIRS NATIONAL ACADEMY OF SCIENCES, VOL. X, NO. 1. 



finished without in any way disturbing the adjustment of the ring on its .support; otherwise 

 errors of eccentricity, etc., would be likely to occur. 



Two forgings of '"Crescent special" tool steel were obtained from the Crucible Steel Company 

 of America and two of a special self-hardening steel from the Westmoreland Steel Company of 

 Pittsburg. A fifth forging of phosphor bronze (the hardest available nonmagnetic metal, unless, 

 possibly, some nickel steel) was also obtained, it being intended to vibrate this with a small agate 

 plate interposed between the ring and the knife-edge; but only the first rough work has been 

 done on this. With a view to getting greater homogeneity all of these blanks were made so 

 that more than one-half inch of material had to be cut away at the center and rim, and one-fourth 

 inch from the faces, this, of course, being done with the annealed ring before hardening. 



The hardening of such a large piece of metal is not a simple matter, and, indeed, one of 

 the "Westmoreland" rings cracked quite in pieces shortly after hardening; however, the other 

 Westmoreland and one "■Crescent" ring were successfully hardened. After a little preliminary 

 grinding to remove the "skin." the rings were artificially aged by several days' "tumbling" in 

 an ordinary foundry rattler, and being heated and cooled through about 100° C. A little further 

 grinding proved the mechanical arrangements then available to be inadequate and the rings were 

 laid aside for nearly a year while the grinding facilities were being improved: this gave them 

 time to assume a more permanent condition. 



The funds at command did not permit the construction of a special grinding machine — so a 

 small milling machine was rigged up for the purpose. The general result may be seen in plate 1. 

 Profiting by the first experience, a solid spindle head was built, having a heavy vertical axis run- 

 ning in conical bearings and carrying a large horizontal face plate, (^reat care was taken as 

 to the accuracy of the bearings, the proper support of end thrust and the relief from belt strain. 

 This self-contained spindle head was mounted on the milling machine carriage, and could be given 

 at will a vertical, in and out. or transverse horizontal motion with respect to the grinding wheel 

 which was rigidly fastened to the top of the milling machine. Projecting from the top of the 

 face plate were three hardened steel lugs (on which the ring rested), the tops of which were first 

 ground until coplanar. Around these lugs, between the ring and the face plate, was a flat zig- 

 zag of insulated wire, waxed to the plate below, and having on its upper surface small lumps of 

 wax which projected above the plane of the steel lugs. After the ring had been put roughly into 

 position the wire would be heated electrically, and the whole mass of the ring slowly warmed 

 until the lumps of wax softened and stuck to the steel; the ring would then be centered, tempo- 

 rarily held in position by clamps bearing immediately over the lugs, and left to cool. Mounted 

 in this way the ring was held very solidly and at the same time three surfaces were exposed for 

 grinding. Of course before all this the spindle carrying the ring had been adjusted with great 

 accuracy to be respectively parallel and at right angles to the two motions of translation which 

 were to be used in generating the cylindrical and plane surfaces; it having been found before by 

 test that these two motions were themselves near enough mutually rectangular. The degree to 

 which various "taper" effects could be avoided was not, however, limited to the accuracy of 

 these preliminary tests and adjustments, for by means of weights and springs the relation of 

 various parts of the machine to each other could be altered during the grinding and any tendency 

 to cut "taper" minimized in this way. It is true that by so doing irregularities and tremors 

 were doubtless introduced which would have been absent with a properly constructed machine, 

 but with the arrangements at hand this was the best that could be done. Of course the design 

 of the several linear constraints on a milling machine are not at all what would be chosen for 

 work such as this, but by careful manipulation very satisfactory results were obtained. 



In finishing a flat side, the surface was tested after every cut with a standard straightedge, 

 and the high spots were taken off on the next cut; these finishing cuts could not be measured, 

 but were probably about .1/'. In a similar way watch was kept for possible taper in the inside 

 hole, and no systematic variations in the inside diameter as great as 0.002 mm. were allowed. 

 After the surfaces were finished the ring would be turned over and again waxed down; failure 

 to be properly seated on the three steel points would be detected by the first cut or would be 



