We used to use those on our punch presses when they stripped out a clamp bolt hole. We would drill out hole in press and install a thread insert. Only problem was if you got in a hurry and pulled to hard on drill, magnet would pull loose and either break drill or spin whole thing around before you got switch turned off. Ours had a on-off handle on magnet like a surface grinder chuck. we had an electro magnet one too, but it would also pull loose if you pulled too hard on drill.