You can't successfully use self tappers in sheet metal because the drill head is removing the very limited thickness of material you are trying to hang the threads in. If you absolutely must use sheet metal screws bang a hole in the sheet metal with a very sharp scratch awl, which will add to the amount of material the screw threads have to hang in. The ultimate solution to the problem of screws in sheet metal is nutserts. You can but a nutsert setting tool on ebay quite reasonably, the nutserts are quite inexpensive when bought by the hundred. Something like 10/24 will do almost anything you will want to fasten. I have ended up buying 8/32, 10/32 and 1/4 20. Once you use these you will never want to be without them.