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|The end user of micro grits must not assume that materials having the same size designation from different manufacturers will be equivalent to each other or that they will produce the same finish.|
The U. S. Department of Commerce provides a standard specification for abrasive grit using "screened" sizes from No. 8 through No. 240 in its Commercial Standard CS 271-65. In essence, this standard defines a specific mesh size hole in each screen used to classify a material as a particular grit size. The grit size (a.k.a. the mesh number) comes from the number of standardized holes that fit within the standard dimensional sized screen, i.e., 200 holes equals 200 grit. As a result, as the number of holes increase in the stipulated dimensional sized screen, the smaller the holes are and the smaller the size of the particles that fall through the holes in the screen. Therefore, the larger designated grit sized number, the small the particle size. Please note, the opposite is true for micron designations because the average particle size is being measured, i.e., the smaller the micron number, the smaller the particle size.
Without government specifications, the standardization is not as well defined for grit sizes smaller than 280 grit. To a great extent it is left to each manufacturer to define the size range of the particles that make up each grit size designation based on the manufacturer's own definitions and sizing practices ,i.e., provided the manufacturer's designation comes within the government guidelines that may apply.
With respect to micro-grits, it is Micro Abrasives Corporation's opinion that screening method cannot be used effectively to separate particle by size for grit sizes above 300 mesh (50 microns). Usually, these finer particles are sized by use of either hydraulic or air separation technics. Once the particles are separated then the theoretical grit (mesh) sizes are designated (e.g. numbers 320, 400, 500, etc.) by the manufacturer to describe the particles size. In defining the theoretical grit (mesh) sizes, each manufacturer may rely on microscopic comparisons, Sedimentometer, and other test methods,but, in any event the manufacturer compares its particle sizes with those of the American National Standards Institute, the European Federation of Abrasive Producers, the Japanese Industrial Standards Organization and other organizations to develop its own designation.
In addition to the confusion that exists in defining micro-grit sizing, the user must also be alert to the fact that each manufacturer is describing a particular micro-grit size by an average particle size. As a result, each manufacturer may have a different variation around the average particle size, i.e., two manufacturers may designate their product No. 320 but the range for one manufacturer may be 300 to 340 grit while the range for the other manufacturer may be 280 to 360 grit.
As a last consideration, the end user can not assume that the same results will be achieved by substituting different material types designated with the same grit (mesh) number because each type of abrasive (e.g., diamond, silicon carbide, alumina, etc.) has its own breakdown rate and working characteristics.
The following example is a conversion chart of an abrasive company reflecting a conversion of its grit sizes into measurements in inches and /or microns. The company providing the example stated that the data displayed is only approximate and reflects the usual commercial variations for grinding and polishing abrasives. It also stated that the chart does not apply to every type of abrasive it makes and advises that sizing should be discussed with your abrasive supplier.
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