October 11, 2012
A variety of different methods and standards are used to describe the aggressiveness of commercial abrasives. Different standards are used for coated abrasives (sand papers) and bonded abrasives (grinding wheels and diamond floor polishing abrasives) as well as specific standards for some specialty abrasive particles like diamond. All of the standards use size as an indication of how aggressive the abrasive particle will be.
Where "Grit" comes from:
Before examining grit we need to understand some fundamentals of particle size grading. Historically a sequential series of screens was used to sort particle sizes. Each of these screens has a differing number of wires and openings per inch. As material passes through one screen and is retained on another we can sort them into size categories. ANSI B74.12 defines the size distribution of particles allowed to be contained in a certain "grit" of abrasive. Grit is therefore not a designation of specific particle size, but a description of a specific distribution of particle sizes.
A Closer Look at "Grit":
Conventional designation of abrasives by "grit" results in a wide particle size distribution for each grit. For example ANSI B74.12 specifies that to be considered a 100 grit abrasive all the material must pass through a standard 70 screen with a maximum of 20% of the material being retained on a standard 100 screen. This means that up to 20 percent of the material is larger (more coarse) than the standard 100 screen opening while, at least 80% is smaller (finer) than the standard 100 screen opening.
This wide distribution of particle size means that scratch patterns will be inconsistent, with larger particles leaving deeper scratches and finer particles leaving shallow scratches. This varied scratch pattern must be able to be removed by the next abrasive used. This has led to the convention in the stone polishing world that you should never more than double the grit of the previous step when moving on to the next. For example, if we began honing the floor with a 60 grit abrasive, a sample progression would be; 60, 120, 220, 400, 800, 1500, and 3500 abrasive steps.
A better way to think about abrasives:
The most important two factors to consider in an abrasive are the amount of work that will be performed (aggressiveness) and the resulting surface profile
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