This numbering refers to the shape specifications, and basically divided into: flat wheels, and depressed center. Additionally this numbering provides information regarding the type of use (cutting or roughing) as shown in the image.
In general terms, these are described as:
- 27: Roughing wheels of depressed center, diameters 76-230mm.
29: Depressed center flap wheels, 115mm & 125mm diameters.
41: Flat cutting wheels, diameters 40-400mm and 250-400mm.
42: Depressed center cutting discs, diameters 76-230mm.