Several national and international standards define particle size distributions of abrasive grains. All are based on sizing by sieving in the sizes typical of most regular grinding applications.
5.8.1 The ANSI Standard
In the case of the ANSI standard [B74.16 1995], mesh size is defined by a pair of numbers that corresponds to sieves with particular mesh sizes. The lower number gives the number of meshes per linear inch through which the grain can only just fall, while staying on the surface of the sieve with the next highest number of meshes which is the higher number.
5.8.2 The FEPA Standard
FEPA [ISO R 565-1990, also DIN 848-1988] gives the grit size in microns of the larger mesh hole size through which the grit will just pass.
5.8.3 Comparison of FEPA and ANSI Standards
The FEPA and ANSI sizing standards are closely related; FEPA has a tighter limit for oversize and undersize (5 to 12%) but no medium nominal particle size. ANSI has somewhat more open limits for oversize and undersize (8 to 15%) but a targeted midpoint grit dimension. FEPA is more attuned to the superabrasive industry, especially in Europe, and may be further size-controlled by the wheel maker; ANSI is more attuned to conventional wheels and, in many cases, may be further broadened by mixing two or three adjacent sizes. In tests, no discernable difference could be seen between wheels made using grain to the FEPA or ANSI size distribution [Hitchiner and McSpadden 2004]. A major attraction of working with the FEPA system is that it provides a measure of the actual size of the grain (in microns), whereas with the ANSI system the mesh size increases with the numbers of wires in the sieve mesh and, therefore, becomes larger as the grain size becomes smaller.
5.8.4 US Grit Size Number
There is also a system called US grit size number with a single number that does not quite correlate with either the upper or lower ANSI grit size number. This has created considerable confusion especially when using a single number in a specification. A FEPA grit size of 64 could be equivalent to a 280, 230, or 270 US grit size depending on the wheel manufacturer’s particular coding system. This can readily lead to error of one grit size when selecting wheel specifications unless the code system is well defined. Table 5.3 gives the nearest equivalents for each system.