bending metal

He.y guys got a? . We learned how to shrink metal today I ask the teacher this ? And he just looked at me funny. So I turn to my peers to answer. When u heat up metal and just cool it down it shrinks. My? Is if u heat the metal up can u put a dolly behind it and beat it down easy.
seems like it would work a whole lot better than just heat cold heat cold till its gone. ......

Anytime you strike or bend metal you are moving it. Moving the metal weakens it. If it's a really thick piece and heat alone will not work then use a dolly. Use light blow instead of harder ones. If you can get by with one or the other then do it. Light strikes with just a body hammer will take care of most of the dent when using heat. Metal has like it's own life. When you figure out ways to make it do stuff it doesn't want to then you see it come to life. I have seen guys heat small quarter size deep dings with a torch and toss a cold rag over the dent and POW. It pops right out and you can hardly see it was ever there. I guess to answer you question just don't over work the metal. Too much work and you will get what they call a pop can effect. the metal will become too ?soft and it will pop like a pop can with the lightest preasure. The goal is to get the dent out with the least aggressive way of doing so.

After rereading your post I may have been looking at it as it you were removing a big or deep dent. Seems he's just showing you how to take out light spots or fatigued metal. As above just doing as he says will keep the metal as strong as possible. Using hammer and dolly may be more agressive and get the look you want faster but as I said it will make the metal less strong than doing it with the least amount of force.

dear hotrodaaron77;
By heating the sheetmetal then shaping, then re-heating, shaping, over and over, eventually the sheetmetal accquires a state known in the industry as *work hardening*. Basically, the sheetmetal becomes tempered through the working process. The only way to restore the sheetmetal to it's original state is to heat it then anneal it.

Theoretically, you can do this forever, however the reality is a different matter. Unless the sheet stock is heated and annealed under very precise temperatures, the various alloys in the stock tend to try a separate themselves out. This makes the material uneven in regards to hardness.

And of course, Rebel already mentioned the *oil can* or *soda can* effect. This effect occurs when the surrounding material is harder then the dented portion of the material. The best way to remove those types of dings is to heat the dented portion of the material and then cool it down quickly, such as by directing a stream of liquid nitrogen on it. The surrounding material will then need to be re-heated then cooled slowly.

To shrink sheet metal, a shrinking hammer and a backing dolly are generally used. It's a hammer with a very slightly concaved smooth face. When the material is struck, the material cups very slightly, causing the material to shrink. There are also serrated faced shrinking hammers but these are only used for roughing in.
Your friend;
LAMAR