|MadSci Network: Engineering|
Magnetic chucks hold down ferrous (iron containing) and other magnetic materials using powerful magnets. They cannot grip nonmagnetic materials such as austenitic stainless steels and aluminum. Gripping forces can be very high. The highest I saw while researching this response was 230 pounds per square inch.
There are three general classes of magnetic chucks: permanent magnet, electromagnetic and electro-permanent magnets. The third class is most common.
Permanent magnets are similar to the wide variety of magnets you may have seen around your home. The standard permanent magnets use cobalt-samarium alloys. Newer and much more powerful iron-neodymium alloys are now available as well. For further information on permanent magnet materials, consult Dexter Magnetic Technologies.
The problem with a permanent magnet is once you have a piece gripped with several hundred pounds of force, it is virtually impossible to move it. The solution is to have a sliding arrangement to move the magnetic poles relative to each other. The result is the ability to effectively turn the magnet on or off by adjusting the magnetic flux above the chuck.
To see the same effect, get two magnets, preferably with marked poles. If you align the north and south poles opposite each other, the magnets will strongly attract each other. If you have bar magnets, hold them in your hands and start sliding them relative to each other so the north (or south) poles come closer to each other. You will feel the amount of gripping force decrease and eventually turn into a repulsive force. If you have cylindrical magnets, the same effect can be obtained by rotating the magnets a half turn.
If you have ever seen a scrap yard where a car is picked up by a crane using a large metal disk, you have seen an electromagnet in operation. Electricity flowing through a coil of wire creates a strong magnetic field. The field can be intensified and focused using a core made of some magnetic material such as iron.
Other examples of electromagnets are speakers and microphones.
The disadvantage of a purely electromagnetic chuck is it requires power to be delivered continuously to grip the piece. If power is interrupted for any reason, the piece will likely go flying across the machine shop. This makes them a very real safety hazard. That is why electro-permanent magnetic chucks were developed.
By combining the permanent magnets with electromagnets, the best of both worlds can be obtained. The electromagnet produces most of the gripping force, but the permanent magnet keeps the piece in place if power fails.
Modern Machine Shop has a excellent article on magnetic chucks in general and electro-permanent magnetic chucks in particular.
The graphic below shows a typical arrangement for the electro-permanent chuck. As you can see, there are at least four types of materials used. The brass is nonmagnetic and acts as a spacer for the magnets and a work surface for the pieces. Permanent magnets within the table are made from the same materials as those for a permanent magnetic chuck. The electromagnets are made from copper wires and an iron (or other magnetic material) core.
As the electromagnets are switched on and off, they alter the magnetic flux or flow. They can fully demagnetize the table surface (lower illustration) or grip the part with full force (upper illustration). They also have the advantage that you can vary the clamping force by varying the electrical flow to the electromagnets. As explained in the article, there are many situations where less than full clamping force is required.
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