1/4 install snafu

I don't know what kind of grinder you were or are using, but I have done this with minimal warping. I use the grinding disks for my 4 1/2 Dewalt that is the sand paper fanned disks. They work really fast and the fined pieces keeps the heat down lower than a hard composite disk. Using these and the long strokes that crackedback mentioned, should help lessen your problem.

Hope this is of some help.