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if i used one to sand down a metal model will it work? or would it mess up the drummel bit?
well it depends on the bit.
there are different ones available. Check the place that you got your Dremel from. They should have all the accessories, too. There are mill type bits for grinding down metal, too.
Since I don't know what model you have, why don't you check out their homepage:
Shut up and play.