Grain size range between 90 and 220 is considered as-

Explanation

Grain size or Grit size: It indicates the size of abrasive particles. i.e. Size of abrasives = 1/ Grain Size Number (GSN) σ = Eϵ When the GSN > 600, size of the abrasive particles becomes very very small and it cannot act like a cutting tool, therefore MRR is less. When GSN < 600, actual size of abrasive is increasing, the chip size is increasing and MRR is increasing. As the GSN is reducing or size of abrasive is increasing, the MRR is increasing first and then reducing. The grain size is selected based on the surface finish required on the workpiece i.e. for rough grinding, course or medium grain size is selected and for finished grinding fine or very fine grain size will be selected. 10 - 24 = coarse, 30 - 60 = Medium, 80 - 180 = Fine, 220 - 600 = very fine