You gotta' be careful buying the HF hole saws. Usually the cheap ones are carbon steel and good only for wood, sheet rock etc. If you are wanting to use them on sheet metal you need to use bi-metal hole saws. I bought one of the Drill Master angle grinders, the ones that look like they have cooling fins on the nose piece, the really cheap grinders and it lasted about a month before stripping gears. The orange ones with the smooth nose case are much better grinders, about double the price but worth it.