2.8.3.1 Sieving
The size distribution of abrasive grits is adjusted by sieving for coarser grits and sedimentation for finer grits [KLOC05a, p. 27, MARI07]. Sieving is the simplest method to separate particles and works with sieves of defined mesh sizes [SALM07, p. 132]. Sieving can be performed manually or automatically on dry or wet batches.
Several national and international standards exist to define the size ranges (Tables 2.4 and 2.5). It has to be noted that most standards define the grit sizes as a range and not as single values. European standards are mainly based on DIN ISO 8486-1 and the so called FEPA standard from the European Federation of Abrasives Producers (FEPA) [KLOC05a, p. 28]. As example, the FEPA standard ranges from the largest grit size F4 (diameter 3 mm) to the finest F1200 (diameter 3 pm).
Most standards define conventional abrasives by a pair of numbers corresponding to the mesh size of two sieves [ANSI01]. The mesh size is given in number of meshes per linear inch. The lower number is the mesh number, through which the grit just passes, while staying on the sieve with the upper number mesh size (Eq. 2.18) [MARI07].
ANSI, FEPA conv. = [mesh per inch upper sieve/mesh per inch lower sieve]
(2.18)
Superabrasive grits are commonly defined by the mesh size, through which the grit will just pass, given in micrometer (Eq. 2.19) [MARI07, KLOC05a].
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Table 2.4 International standards for grit sizing for bonded tools [MENA00, BENE10, MARI07, ANSI01]
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Table 2.5 International standards for grit sizing for coated abrasives [MENA00, ANSI01]
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FEPA superabrasives = [size of mesh hole through which grit just passes in microns]
(2.19)
The size definition by mesh implies no absolute value, but rather a size band. The tolerances, i. e. the permitted particles with sizes above and below the defining band, are based on the total weight. Consequently, the proportion or number of finer particles to coarser ones within a given size is not fixed [ENGL03]. This leads to variations in number of particles per carat and average grit size within a specified size. Therefore, abrasive tool performance can fluctuate.
2.8.3.1.1 Sieving Procedure
Each grit size has a set of five sieves of metal wire cloth with decreasing mesh size, e. g. defined by ISO 3310 [DIN97]. The reason for the five sieve setup is to simulate the actual mass when using the sieves for test grading [ANSI01]. The sieves are arranged with the roughest sieve above and the finest below it, on a cup. The aperture size decreases technically by a factor of 72 in the stack of sieves [MALK08, p. 12]. A certain mass of a representative grit probe is put onto the top sieve. After a certain time on a test-sieving machine (defined by ISO 9284 or ANSI B74.12) the amount of particles on each sieve is weighted [ANSI01].
As example, the total probe volume of a F10 grit sample has to pass sieve 1 (mesh size 3.35 mm) (Fig. 2.24). Sieve 2 (mesh size 2.36 mm) can be passed fully, but a maximum of 20 % mass can be withheld. On sieve 3 (mesh size 2 mm) a minimum of 45 % probe mass has to retain, but 100 % may have passed sieve 2. The combination of sieve 3 and 4 (mesh size 1.7 mm) has to hold a minimum of 70 % of the total weight, so sieve 4 has to retain the difference, i. e. a maximum of 25 %. There are no definitions for the finest sieve 5 (mesh size 1.4 mm), but only 3 % of probe mass is allowed to pass sieve 5 and to be collected in the bottom cup.
Metrological variances are considered in percentage deviations of 3 to 4 % of retained probe mass [DIN97, ANSI01].