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A magnetic chuck is a device, or workholder, that uses magnetic force to hold a ferrous metal item during machining. It also refers to magnets used to secure a workholder, such as a vise or other fixture, to the work machine. An electromagnetic chuck is a type of magnetic chuck that produces an adjustable magnetic pull, allowing it to hold a variety of workpieces. Processes that use magnetic chucks may include drilling, grinding, and turning. They are not restricted to only industrial use; some hobbyists use magnetic chucks.
There are three basic types of magnetic chucks. The permanent magnetic chuck has a series of magnetic inserts that are constantly polarized, meaning they have permanent poles and constantly pull on the part. Another type is the electromagnetic chuck, which is engaged only while the electrical current is on. This chuck has the advantage of being able to turn the magnetic pull off, releasing the workpiece quickly and easily. The disadvantage of this chuck is that it releases the workpiece when the electrical current is stopped, often putting the worker at risk of injury if it happens during a work process.
The third type is the electro-permanent magnetic chuck, which is a hybrid of the permanent and electromagnetic chucks. This device locks the part with a jolt of electricity and then uses another jolt of electricity to unlock it. If there is a power outage while working a part, the chuck continues to hold tightly. Each magnet has an electrical coil around it that can reverse the polarity of the magnet extremely quickly. Another function of the coil is to control the amount of magnetic pull the magnetic chuck exerts.
Magnetic chucks hold only ferrous — iron-based — materials, limiting the type of workpiece that a machinist can work. A rare earth metal called neodymium iron boron, or neodymium, creates magnets that are at least five times as powerful as traditional magnets. Manufacturers often use it in industrial magnet applications. Machinists usually prefer to work with annealed metal workpieces because hardened materials sometimes retain the magnetism when the chuck or workholder is off.
The advantages of using a magnetic chuck usually include reduction of machine setup time, reduction of setup processes, and reduction in chuck damage to workpieces. Sometimes the magnetic force provides a stronger hold on the workpiece. Some magnetic chucks are strong enough to have a clamping force of 12 tons per square foot (10886 kg per 0.093 m2), making them useful for heavy stock removal.
Permanent magnet chucks are switched by moving the magnets or a perforated iron plate so that in one position the flux passes through the iron and in the second position it passes through a non-magnetic material. When the flux is "shorted" by the iron the workpiece can be removed, and when the flux is open it passes through the workpiece, holding it firmly.
I don't think that the part about electro-magnetic pulsing is correct - electro-permanent and pneumatic-permanent chucks use a motor or cylinder to move the shorting plate. You cannot reliably repolarise strong magnets with an electrical pulse, unless you heat the material to its Curie point, and even then the energy required is significant.
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