Dedicated to Supplying Electromagnetic Metal Forming Equipment for The Manufacturing Industry

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Last Updated: 1301041430

The following discusses briefly the basic theory of the EMF process. It uses the compression method as a model. However, the same principles can be applied to expansion and flat sheet metal forming methods.

 

Think of something simple first, like an electric motor!

 

In an electric motor (Fig. 1) interaction of a time changing electric field, B, with a current carrying conductor (I), creates a force (F), on the conductor. If the conductor is imbedded in a rotor, and if the arrangement of the rotor and field is properly positioned, the force on the conductor will cause rotation of the rotor. The electromagnetic field density in a motor is somewhere around 15kG (fifteen thousand Gauss), and the current in an electric motor is several Amperes.

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In an electromagnetic forming device, we make use of the same principle except the generated force has to be sufficiently high enough to exceed the yield strength of the material to be formed. To generate such high forces, the required current needs to be several 100,000 Amperes compared to several Amperes for an electric motor; and the induced electromagnetic field needs to be as high as 300,000 Gauss compared to about 15,000 Gauss for an electric motor. The following shows how such high currents and field densities can be generated and how the device can be arranged to be useful as a manufacturing tool.

 

A basic electrical circuit of a MAGNEPULS machine is shown in Fig. 2. A high voltage power supply charges a capacitor bank over a period of several seconds. When a preset voltage level is reached, Switch-1 opens and Switch-2 closes and the stored energy discharges into the load L, which is the electromagnetic forming coil. The discharge current into the forming coil is a damped sinusoid as shown in Fig. 3.

Fig. 1: Schematic presentation of the electromagnetic field, the current, and the directions of the forces which cause the loop to rotate

 

Fig. 2: Basic EMF Circuit

For most industrial applications, the current is somewhere between 40kA and 500kA, and the period T about 100 micro-seconds. The current produces a uniform electromagnetic field in the coil as shown in Fig. 4. However, if the work piece is an electrical conductor, e.g. an aluminium or steel tube as shown in Fig. 5, then the electromagnetic field induces an azimuthal current in the work piece. This induced current prevents or reduces the field penetration through the work piece, increasing the field density between the coil and the work piece. The interaction between the high field density and the induced current generates an inward directed pressure pulse on the work piece. If the pressure pulse as shown in Fig. 6 exceeds the yield strength of the work piece material, the work piece will be permanently deformed.

Fig. 3: Discharge Current into an EMF Coil

Fig. 4: Forming coil without a work piece. The discharge current produces a uniform electromagnetic field in the coil

Fig. 5: EMF Forming Coil with an electrically conductive work piece inserted into the coil.

Fig. 6: EMF Pressure Pulse, typical T/2 is 25 to 50 micro-second

Note there is no physical contact between the coil and the work piece. There are no moving tools which push, press, or move the work piece material. The pressure pulse imparts kinetic energy into the work piece. Consequently, it begins to move until all the kinetic energy is dissipated. The forces which resist this movement are those due to inertial energy and the energy of deformation. The final shape of the deformed work piece depends on a combination of factors. These are the magnitude of the pressure pulse, the strength and geometry of the work piece, and the shape and geometry of a mating part such as a die or a mandrel, or a second work piece. Fig. 7 shows tube formed onto a fitting. Notice how the tube practically wraps itself around the contours of the fitting.

Fig.7: Tube formed onto a fitting

To adapt the coil to different work piece diameters and to concentrate the pressure pulse in particular areas, one makes use of a flux transfer member, also called a flux concentrator or a field shaper as shown in Fig. 8.

Fig. 8: This illustration shows a field shaper which concentrates the pressure in two areas, indicated as A.