this kind of cutting sand disc mainly use in air tools or power tools, for grinding metak. the material is aluminum oxide.  Sand disc Specifications

1) Sizes: 4", 4.5", 5", 6", 7" 2) Fiber thick: 0.6, 0.7, 0.8mm 3) Grain: alumina, silicon carbide, zirconia 4) Grit: 16-400

Features: 1) Sizes: 100X16mm, 115X22mm, 125X22mm, 150X22mm, 180X22mm

2) Coated grain: aluminium oxide, silicon carbide, zirconia oxide3) Usage: grinding weld seam, removing burr, sanding floor or furniture, polishing stainless steel fork or spoon. 

4) Grit: 16#, 24#, 36#, 40#, 60#, 80#, 100#, 120#, 220#...400#

5) Fiber thickness: 0.6mm, 0.7mm, 0.8mm

6) Center: flat, depressed

7) Hole: one hole, cross hole, multi holes

8) Advantage: sharp, wear-resistant, long life, stable performance

One hole depressed center aluminum oxide fiber disc for metal

sanding disc