GRINDING WHEELS 433 



not comparable because the size of wire employed in the sieves 

 used for the grading is not always the same. Since it is the 

 number of linear meshes tb the inch and not the diameter of 

 the opening that is recorded, the size of the wire greatly influ- 

 ences the screened product. 



Although for industrial purposes abrasive wheels may be said 

 to conform closely to the grade and grain indicated by the manu- 

 facturer, it will be found that in preparing specimens for micro- 

 scopic study, wheels are not easily duplicated and if we purchase 

 a wheel to replace one accidentally ruined we are apt to find 

 that it will not do just the work of the one lost. 



Wheels of softer grade and coarser grain (at high speeds) can 

 be used for roughing chilled iron and steels, hard and of high 

 tensile strength, than for material like brass soft and of 

 low tensile strength. 



In the preparation of minerals and ores for microscopic studies, 

 however, it has been found that a wheel of mixed grain size gives 

 better results than wheels of fairly uniform grain. Murdoch 1 

 has found that a carborundum wheel consisting of a mixture 

 of 40, 60, and 80 mesh grains, soft bonded yields the best results. 2 



No single type of wheel as to grade and grain will answer 

 for all purposes. A laboratory in which a great variety of 

 work is to be done will therefore require a series of wheels. 



A fairly satisfactory system of study with reference to the 

 selection of wheels for different materials and the proper speeds 

 for grinding consists in examining with the microscope the 

 roughed surface of the specimen as ground under different con- 

 ditions and also the dust or particles falling from the wheel. 

 These particles consist of material torn off the specimen and 

 particles of abrasive and binder. The character of the dust and 

 the furrows upon the specimen will, with a little experience, 

 indicate at once, to the worker, whether he is employing the 

 proper grade, grain and speed. It is strongly urged upon the 



1 Microscopical Determination of the Opaque Minerals. Prof. H. Ries of the 

 Cornell University Department of Mineralogy, has also found these wheels to be 

 more satisfactory than uniform grain wheels. 



2 The specifications for this wheel (Carborundum Co.) are: Grit 403; Grade M; 

 Bond 63. 



