432 
ELEMENTARY CHEMICAL MICROSCOPY 
The character of the binder and the degree of incipient fusion 
characterizes a wheel as hard or soft. The degree of hardness 
or softness is technically spoken of as the grade or hardness of 
the wheel. American manufacturers usually indicate the grades 
of their wheels by letters of the alphabet, but the scale of hardness 
as indicated by the letters is by no means uniform with different 
manufacturers.^ Consequently, a letter indicating a grade can¬ 
not be interpreted without reference to the scale of hardness of 
the particular firm from whom the wheel was obtained. For 
example, we find that a wheel marked U may be “hard” as 
supplied by one firm, but if we purchase a U grade from another 
firm we will obtain a “very soft” wheel. In selecting wheels 
for grinding specimens, it is safe to be guided by the general 
rule that a soft wheel will cut more rapidly and deeper than 
a hard one, will clear itself more readily, but is more easily 
worn away, and therefore more liable to be spoiled. The soft 
•wheels as a rule must be run at higher speeds. Hard wheels 
on the other hand tend to glaze over, cause more heating of the 
specimen and often yield aggravated cases of surface films or 
surface flow of soft components, but they cut slower, hence do 
not so deeply score or furrow the specimen through injudicious 
pressure and may be employed to better advantage when only 
low speeds are available. 
Besides the grade or hardness of grinding wheels as influencing 
their suitability for certain work, the diameter and the uniform¬ 
ity of the individual particles employed in building up a wheel 
must be taken into account. The size of the component par¬ 
ticles is called the grain or grit. Grain is obtained in manu¬ 
facturing by screening the abrasive powder. The number of 
linear meshes to the inch through which the powder will pass 
is the grain number of the wheel. For example, in a wheel 
marked 50, the component particles will pass through a sieve 
having fifty meshes to the inch. 
The grain numbers employed by different manufacturers are 
^ An instructive table of the comparative grading of scales of hardness employed 
by different manufacturers will be found in; Jacobs: Abrasives and Abrasive Wheels; 
page 93. The Norman W. Henley Publishing Co., New York, 1919. 
