These cutters are really good - drilling big holes in mild steel is tedious work but fun to use. These aren't drills, they're more like milling cutters, but they're hollow. Large holes can be drilled very easily and quickly with a milling cutter or a drill. I believe the work must be securely fastened or there is a risk of it being damaged. I will never go back to regular bits larger than 3/8 inch.