SuperMagnete documents a clever method for removing dents from brass instruments using powerful magnets. You insert a steel ball (smaller than the dent) in the instrument, and then use a padded magnet on the outside to “rub out” the dents. A more elaborate method uses a “reverse hammer” that works on harder surfaces.
To remove the dents on the hard parts of the instrument, Roberto developed a “reverse hammer”. It is made of a non-magnetic rod (e.g. copper) with a movable heavy metal block on it’s axis (e.g. the head of a normal hammer), a magnet on one end of the rod and a stopper on the other end (see drawing). When the hammerhead hits the stopper, a part of the resulting energy is transported through the magnet to the steel ball within the instrument, which “hammers” the dent from the inside.
Straightening out brass instruments
(via Red Ferret)