Ok, education time! I'm aware of three different disk types...tell me where I'm wrong or doing things wrong, please:
1) Flap disk = a disk with a bunch of what appear to be cloth backed sand paper 'flaps' which overlap each other. My experience with these disks is that they do not remove significant amounts of material (relative to a grinding disk), but can remove material and 'polish' the metal. I typically use this disk at a ~30* angle to the surface being flapped.
2) Grinding disk = a firm/solid disk, about 1/4" thick, which remove significant amounts of material (relative to a flap disk) and leave the surface very rough. I typically use this disk at a ~45* angle to the surface being ground.
3) Cutoff disk = a firm/solid disk, about 1/16" thick, which is used to cut through material (cut off material). I typically use this disk at a 90* angle to the surface being cut-off.
I buy my disks from HD or Lowes...I think I buy Diablo and Dewalt branded disks.
Thanks.