Question: What do pet food bowls, a wok and air cylinders for scuba diving all have in common?
Answer: They are all made by the same manufacturing process – metal spinning.
Metal spinning, or more precisely, spin forming, is a quick and economical way to produce complex forms that are symmetrical about a central axis. Unlike machining processes where metal is actually cut away to create the finished product, spinning is a cold plastic deformation process where an unheated metal blank is pushed into the final shape.
Spinning is performed on a machine that looks much like a lathe. A circular piece of sheet metal – the “blank” – is held in place against a form mounted where the chuck would be on a conventional lathe. The tailstock is then brought up to hold the sheet in place. The machine is then started up and the blank rotated at several hundred rpm. A form tool is then brought to bear on the metal blank, pushing it back against the form beneath and into the required shape.
An important design consideration is the need to remove the form from inside of the spun metal piece. This means that it is not generally possible to include reentrant features in the design, although this limitation can be avoided by using forms made from ice.
Perhaps the most significant advantage of spin forming is that unlike metal stamping processes, it does not need expensive tooling. This makes the process suitable for short production runs and even one-offs, which is why craftsmen and artisans looking to make decorative pieces such as candlesticks, like to use it. However, it is also a very common technique in manufacturing industry where large computer numerical control (CNC) machines are used to form pieces as big as six feet in diameter. Interestingly though, these industrial users still tend to do their process development work on manual machines, proving that the craftsman’s art has not been entirely supplanted by science.
Most metals can be spin-formed, including pewter, aluminum, stainless steel, copper, brass and even more exotic materials such as titanium. In general the process is used on thin sheet material but heavy-duty industrial machines using hydraulically assisted tools can form materials up to one inch in thickness. In extreme cases heat can be applied to improve the willingness of the metal to flow.
One negative aspect of spin forming is that some materials will work harden. Depending on the intended function of the finished piece, this can be advantageous as it improves tensile strength, but more often post-forming heat treatment is needed to provide stress relief.
For the product designer, spin forming is an often-overlooked way of producing parts economically in low volumes. The only constraint is to ensure that parts to be made this way are axially symmetric.