# Electromagnetic forming

1. ### chididdy

4
Hi,

Can someone explain to me how electromagnetic forming of sheet metal works? I understand how shrinking of cylinders works -- e.g. aluminum cans and tubes.

Let's use the coin-shrinking trick as an example; it can be viewed as a sheet of metal. A coin is placed in the center of a coil. A large transient discharge of electrical current is passed through the coil. The coil's magnetic field induces an eddy current in the coin. The eddy current creates an opposing magnetic field. The eddy current is rotating in the opposite direction as the coil current and thus the eddy current is forced radially inwards. Now, why would that make the coin deform -- i.e., shrink radially. Wouldn't the current density just increase towards the center of the coin? I understand how a tube or can would deform, as the charge carriers cannot leave the boundaries of the metal conductor, but a coin has no such boundary -- radially inward -- and the charge carriers are not bound in any way to the metal lattice.

Also, it has been said that the eddy current in the coin will tend to stay along the circumference of the coin due to the skin effect. So, there are two forces influencing the distribution of the eddy current -- i.e. the skin effect and the Lorentz force.

http://capturedlightning.com/frames/shrinker.html
http://en.wikipedia.org/wiki/Electromagnetic_forming

THanks!
Ken

2. ### chididdy

4
anyone?

OK, I was thinking about this a bit more. As the eddy current gets constricted to the center of the coin (i.e., due to lorentz force), the mobile charge carriers (i.e., electrons) concentrate in the center and creates a large electrostatic -- well not quite static -- gradient which attracts the stationary positive bound charges of the metal lattice towards the center of the coin.

does that make any sense at all? I don't claim to be a physics expert BTW. I'd love to hear some other explanations.

Thanks
Ken

3. ### chididdy

4
sorry, I don't mean to beat this horse to death. Does anyone know whom I may ask to get a correct answer to my question? Any electromagnetics experts and/or forums that you can suggest? Thanks!