Figure 5-4.--Standard marking system for grinding wheels. abrasives, you should learn this identification system. letter C in station 1) is harder but not as tough as. so you can identify the grinding wheel you need for a. 60; fine: 70, 80, 90, 100, 120, station in the following paragraphs: 150, 180; and very fine: 220, 240, 280, 320, 400, 500, 1.