HOWTO make a magnetic "reverse hammer" to remove dents from brass instruments

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)


      1. Actually, it seems to me the two methods bear no resemblance to each other whatsoever.


        But later. I’ve got a candy-apple red pocket trumpet to fix.

        1. The “reverse hammer” contraption with the rod and sliding hammer head (as described in this picture: ) is called a slide hammer. The magnet-in-instrument-guy fixes dents by using a slide hammer with a magnet on the end to attract a steel ball while the car-fixer-guy uses a slide hammer with a suction cup on the end. Same principle, different methods of transferring the energy from slide hammer to metal.

  1. Without this I wouldn’t have guessed it’s so easy. Now I can fix my fugly old saxophone to look as good as it sounds!

    1. Just watch out, repair the dents and you change the internal volume of the instrument and the air flow through it… With some instruments the quality of the sound comes from the fact that it is dented.

  2. This very well may be the most personally relevant post I have ever seen on boingboing.  I have (literally) dozens of dented brass instruments that could use this kind of fixing.  You guys have no idea how amazing you are!  Thanks!

    (And to all the people who want to apply this to steel parts, keep in mind that a lot of the reason WHY this technique will work so well on brass instruments is that the magnet isn’t going to stick to the brass.  So it may not work as well on steel body panels.  I’m still going to try it, though)

  3. spot weld a washer edge on to the dint. Hook the slide hammer through the washer. Tap it out. Give the washer a twist to get it off. File it smooth.  It worked on my expansion chamber.

  4. Just a heads up on this. The magnets we use for this kind of work are incredibly strong. Getting your fingers caught between them and the steel ball, or even worse, another magnet can result in pinch skin and bruises at the least and broken bones at the worst. Anything in your workspace that can be pulled to the magnet is also a hazard. We won’t let anyone near our set in the shop that isn’t one of the repair techs. They are just too dangerous. Conn made a set of tools like this marketed towards band directors. I can’t imagine the potential liability of something like this in a school environment.

    Anyway, we’ve been using something like this in our repair shop for about 8 years now. While it can be a great tool, you can also do far more damage than good with them. I’d never use it on a trumpet, trombone, or horn as they are so soft you’re almost guaranteed to make things worse.  Some of those fancy euphoniums and tubas are really soft as well and you can easily pull a large crease into them or put more dents in trying to get the ball into place. This isn’t to say I have seen great results with this method of dent removal, but, it’s no magic cure all. It’s just another tool available for a repair tech when the situation is appropriate.

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