Tanks and other sheet metal can be made from flat pieces of metal welded together but would lack a certain aesthetic quality. Hammering in some shape makes for an interesting appearance. Here is a quick overview of simple hammer forming using simple wooden or metal forms to put shape into sheet metal for tanks, fenders, battery covers or any other part that can be dreamed up.
Here is an end cap for a piece of aluminum tube that will eventually become an oil tank. This cap was hammered over a simple form from .125” aluminum sheet. This is pretty heavy metal for hammering by hand but when annealed properly, it works rather easily.
Here are some of the forms used to make this type of cap. The easier to make forms are just wood with a radius edged that can be done quickly with a router and radius bit or carved and sanded to shape by hand. In the far upper right is a wooden hammer form. Left of the wooden form is a steel hammer form, which will hold up to many parts. The screws in the form hold a block to the backside so the forms can be chucked in a lathe for shaping and later clamped in a vise when being used. The upper left corner of the picture has a couple of rough shaped caps. Bottom left is a finished cap. Bottom right is a blank piece of aluminum before shaping.
Here the blank piece of aluminum s clamped between a hammer form clamped in a vise and a piece of thick aluminum. This top piece keeps the flat surface from being distorted and can also be a piece of wood but must leave access for hammering the edges of the blank.
Shaping starts by hitting down the edge in four places alternating around the perimiter with a flat face mallet. This starts the forming process and helps keep the blank centered on the form. A plastic mallet is being used but a wooden or rawhide mallet will work as well. A large flat face is require as will be discussed later.
Again the edges of the blank are hit down alternating side to side between the first hits. Note the excess metal sticking out between this first set of hits. In order to be successful at metal forming a simple understanding needs to be reached. Basically two actions are required to form metal, shrinking or stretching. If a hard faced small hammer were used, it would compress the metal, making it thinner and bigger. In this case, there is excess metal that needs shrinking. This literally involves forcing the metal back inside itself to make the metal thicker in the areas that need to shrink.
Continuing to alternately drive down high spots causes the metal to flow together, thicken and shrink down to a smaller diameter than the original blank. This whole cap only takes a few minutes to hammer down.
When the top form is removed, it will be noticed that some of the excess metal has flowed into the center of the caps causing it to form a depression. This is fine if it is a desired shape.
Removing the depression is as simple as flipping the cap over onto a flat hard surface and again using a flat-faced mallet, work from the outside into the center to force the metal to shrink and flow towards the center of the cap. A little sanding to remove any scratches and the cap is ready to weld onto the end of the tank.