The guru of anvilfire says that most beginners cheap out on grinding disks by buying the long lasting ones instead of the quick cutting (friable) ones. I saw this happen myself when I was helping another blacksmith radius an anvil made out of a piece of AR armor plate. He was buzzing away for 40 minutes without much progress, and declared the job impossible. I went to the welding store and picked up a premium Walter soft wheel and did the job in 5 minutes. It was better than a factor of 10, because I did more work than he did. I spent $5 for the wheel, but it was money well spent. I gave him the remainder of the wheel, but he said it was a little rich for his blood still. Running a wheel down that much is pretty hilarious, but I suspect that slow grinding at that lower surface speed could waste quite a bit of time.