Evaluating Machining Applications for Magnetic Chuck Workholding

Evaluating Machining Applications for Magnetic Chuck Workholding

There a variety of factors to consider when selecting the type of magnetic chuck for a machining application.

As an alternative to mechanical clamping of work-pieces, magnetic chucks are ideal for holding ferromagnetic work-pieces together. They are commonly used in grinding, milling, spark erosion and pallet-loading applications.

Magnetic chucks consist of an accurately centered permanent magnet face. Fixed pole pieces are brought into contact with electromagnets or permanent magnets. The work-piece to be held closes the magnetic loop or path, onto those fixed plates, providing a secure anchor for the work-piece.

To reap the advantages, a number of factors must be considered to properly marry the magnetic chuck to the application.

Magnetic Chuck Application Considerations

There a variety of factors that need to be considered, including:

Type of Material: Magnetic chucks only work with materials that have an ability to be magnetized. Paramagnetic or diamagnetic materials do not magnetize well if at all and are not good candidates for magnetic chuck workholding. Material hardness, shape and flatness of the work-piece material are also part of the material evaluation process.

Type of Machining: Machines and type of machining are important considerations for the selection of a magnetic chuck. Factors such as the horsepower, spindle speed, and size of cutter or tool need to be considered.

Surface Area: Magnetic holding force is directly proportional to the surface area. Smaller parts that have less available contact area may require positive stops to overcome the forces of the machine. Irregularly shaped parts may require a magnetic fixture to securely hold the work-piece.

Chip Control: Chips are generally not a problem for magnetic workholding because the chuck's magnetic field does not cause chips to adhere to the work-piece. However, if chip problems occur, they can be alleviated by using variable control options to reduce the amount of force applied or by using riser blocks to keep the magnetic circuit away from the machining area. In other words, the chuck can be designed to limit the depth of the magnetic field, preventing the force from pulling chips into the work-piece.

Type of Magnet: There are three types of magnetic circuits used with magnetic chucks: permanent-magnetic, electromagnetic and electro-permanent. Permanent-magnetic chucks are mechanically actuated via a lever and have a non-variable clamping force. Electromagnetic chucks use DC voltage in a coil surrounding mild-steel pole pieces to generate the magnetic field; they need constant power application. But they can provide variable holding power, which greatly improves their ability to support a wide range of machining conditions. Electro-permanent chucks are a permanent/electromagnet hybrid. In this design, when DC voltage is applied to the coil that surrounds the permanent magnet, the material is charged and becomes magnetic. Electro-permanent magnets will not lose magnetic attraction if there is a power loss, which makes them portable.

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