I made it at Techshop. http://www.techshop.ws/
I needed to remove small dents from metal tubes and curved surfaces which are hard to reach. Items like my horn collection, cans and occasional motorcycle body work. Well here is one set up of the Magnetic Ball Bearing Internal Curved Surface Dent Remover. I say one set up, because there are several variations of this to fit the specific requirements. The ball bearing is placed on the inner surface and is rolled back & forth manually over the dent, using the magnet to keep it intact. The curve of the ball bearing pushes the dents outward.
Items showed: Top cylindrical item is a rare earth magnet. Center item is a plastic milk cap; I choose this because of its smooth outer surface. Bottom item is a steel ball bearing.
Step 1: Set Up
A ball bearing is placed (sometimes dropped) inside the item that has a dent. I still have my boyhood ball bearings (we use to call them steelies). I select the largest ball that will fit inside the access hole. With tapers, I try and go with one size smaller so not to stick in the smaller diameter. I use hard drive magnets with a rag or a cylinder rare earth magnet with plastic milk cap. Having an item between the dented surfaced and the magnet, allows smooth movement without scratching the surface. The ball bearing is placed on the inner surface and is rolled back & forth manually over the dent.
Step 2: Removal of Dents on Brass Horn
Before and after dent removal, on one of my horns
Step 3: Dent Removal of Aluminum Can
These are not the best of pictures, but here is a before and after of an aluminum can. This technique works well (or complements) my snarling tools - see https://www.instructables.com/id/Snarling-iron-tool/
Please note; there is a correlation between the strength of the magnet and the wall thickness of the dented item and the severity of the dent. Bottom line is this technique works well but does not get all the dents out.