Magnetic chucks use the magnetic force from a permanent magnet, electromagnet, or electro-permanent magnetic material to achieve chucking or holding action. In recent years, magnets have increased in popularity as an alternative to mechanically-holding workpieces for machining and holding. In general machine shops, are magnets reduce setup time and increase access to all sides of the workpiece. They are a valuable tool for workholding in surface grinding applications.

 

Magenetic chuck

Magnetic Chuck. Image Credit:JawChucks.com

There are many advantages to using magnetic chucks. Consistent clamping pressure ensures that there is no variation in how tightly or loosely the workpiece is held; full support of the workpiece surface decreases cycle time by securing its center area. Magnetic chucks have shorter setup and take down times because the workpiece can be placed right on the magnetic plate, and vices can be secured using the magnet.

Magnetic chucks make centering and working with small workpieces more difficult because they lack sufficient surface area. Vises can hold smaller parts as well as non-ferrous materials, however, and simply placing several vises on a magnetic chuck and pushing a button to secure all the vises at once is a time-saver.

 

How Magnetic Chucks Work

Magnetic chucks use magnetic force to hold a workpiece in place while it is being worked on. The magnets work by inducing polarity into the ferrous work material that connects across its north and south poles. When a workpiece is placed across the poles of the magnet, the flux flows into it. The ferrous components have poles that are opposite the polarity of the magnet so they attract each other. Amplifying and controlling this flux is the key to applying magnets in a metalworking operation.

 

Closing the gap between the workpiece and the magnet increases the attraction or "pull" of the magnet. Smooth surfaced workpieces are more tightly held than rough surfaced. The strength of the magnetic attraction is a function of how much magnetic flux can be induced into the workpiece.

Video of magnetic chuck in use.

 

Design Tip: Some Magnetic Fixturing Principles and Techniques

Tip and Image Credit: Modern Machine Shop

Here are some fundamental rules that shops looking to use magnetic workholding should understand.

  • Magnetic flux will always take the shortest route.
  • Magnetic flux is less effective through air gaps and other non-conductive circuits.
  • Magnetic flux from a north pole must have an equal area of south pole to return to.
  • Magnetic flux needs to be concentrated at the workpiece surface even at the expense of area.

Flux heightflux will try to escape if given a chance.

Selection Criteria for Magnetic Chucks

The GlobalSpec SpecSerarch database allows industrial buyers to search for magnetic chucks by type, pole technology, product specifications, material of plate and application.

 

Product Types

There are three magnetic chuck types.

  • Permanent magnetic chucks are manufactured from magnetized materials and exhibit a constant magnetic field without needing to introduce an electric current. Because these materials are highly magnetized materials, they do not lose their magnetic force. This also eliminates the need for electrical connections, switches, brushes, wires and auxiliary generators. The magnet is made of a special alloy and has high power and indefinite life. Most industrial permanent magnets sit within a carrier. Alignment of the poles determines the state of the magnet (whether it is on or off). In operation, these poles are physically moved to activate and de-activate the permanent magnet. These machines are best used for light duty machining because of their reliability and safety, but as the demands of the machining operations increase and the machines are scaled up, the mechanical effort required to overcome magnetic friction to turn the poles on or off becomes too great.
    • Advantages of permanent magnet chucks are their independence from electrical power and ease of installation for transfer from one machine to another. They do not generate heat, which could deform the pole plate or workpieces, and have a very robust design. Since they don't depend upon electrical current, there is no danger of the pieces being thrown off or released by power failure.
    • Disadvantages of permanent magnetic chucks are that the chuck dimensions are less adaptable for big workpieces, and are not as suitable for heavier workpieces. The mechanical switching system is not efficient for automated work, and it cannot be used with components at a temperature above 80°C.

square chuck

Permanet Square Magnetic Chuck. Image Credit: Techniks

  • Electromagnetic chucks generate a magnetic field when electrical current is introduced. The field is created with coils supplied by direct current (DC). These chucks are activated with a control unit which allows quick magnetizing and an easy demagnetizing of the workpiece. Electromagnetic chuck systems can be manual, automatic or combined.
    • Advantages of electromagnetic chucks are that the chucks can be operated with control units which can regulate the magnetic force and control the demagnetizing cycle, allowing for easy removal of the workpiece. Due to the coils, a strong magnetic force is generated to tightly clamp the workpiece. These devices can also reliably clamp rough workpieces and various chuck dimensions can be produced, enabling the user to meet the requirements of the workpiece size.
    • The disadvantage of this type of chuck is that it requires a power supply, and for safety reasons, often needs a back-up power supply as well. The system also produces heat, which can affect the accuracy of the machining. Electro magnets are not practical for machining operations because they require a continuous source of electric power to maintain magnetic attraction. Should power be interrupted, the electro-magnet loses its ability to hold the workpiece and creates a safety concern. 
  • Electro-permanent magnetic chucks are a hybrid of electromagnetic and permanent magnetic chucks, which allows these units to be larger and more powerful for metalworking applications. The fast action of strong direct current enables the system to take advantage of the ferromagnetic material to become a permanent magnet. In this state, they do not need an external power supply. An electrical current is applied to lock and unlock the chuck, but in the event of a loss of power, the chuck continues to hold the part tightly. An electro-permanent chuck contains a number of steel poles, each alternating between north and south polarity. The number of poles depends on the size of the chuck. Two magnets are sandwiched between the poles. One pole is an electronically switchable permanent set and the other is a permanent set. The permanent sets are constantly polarized in one direction and the switchable magnets have an electric coil around them so they can be polarized in the north or south direction by passing a current through the coil. When both sets of magnets have the same polarity, full magnetic power is achieved. When both sets have opposite polarity, they cancel each other out and the magnet surface is off.
    • Advantages of electo-permanent magnetic chucks are that operation from a control unit provides the possibility for automated use. There is minimal electricity consumed, and there's a greater safety factor since a loss of power won't cause the chuck to release the workpiece. The device can also handle heavy milling operations. Since its temperature doesn't fluctuate, there is no threat to machine accuracy.
    • Disadvantages include the higher cost of electro-permanent magnets than permanent magnets, and a higher sensitivity to air gaps between the chuck and component.

Magnetic Pole Technology

Magnetic chucks are constructed of a series of magnetic poles. In general the more of these poles that engage a given workpiece, the stronger the magnetic grip.

  • Concentric pole magnetic chucks are round chucks that are used when the workpiece needs to be rotated during machining. They have magnetic poles that run concentric to each other throughout the diameter of the chuck, and are separated from each other by a steel section.

Concentric poles

Concentric Poles. Image Credit:SAV

  • Parallel pole magnetic chucks have magnetic plates that run parallel to each other and are separated from each other by a steel section.

Parallel poles

Parallel Poles. Image Credit: Ningbo Realpower Magnetic Industry Co., Ltd.

  • Radial pole magnetic chucks are round chucks used when the workpiece needs to be rotated during machining. They have magnetic poles that are laid out in a radial pattern and are separated from each other by a steel section.

radial poles

Radial Poles. Image Credit: Alibaba.com

  • Square pole chucks use square pole magnets arranged in a checkerboard pattern, with alternating north and south poles.

Square poles

Square Poles. Image Credit:Techniks

Design Tip: Greater pole distances produce greater depth effect of the magnetic field and are better for thick workpieces.

To select a pole shape, consider the application (grinding, milling, drilling, EDM, etc.,) and the size, or range of sizes of the parts that will be held. Other variables, like the distance between the poles, the size of the part and amount of holding force required for the application are important to consider when selecting a magnetic chuck. Manufacturers often have information about which applications for which each of their chucks are suitable.

Product Specifications

Buyers should consider these product specifications when selecting magnetic chucks.

  • Chuck geometry determines how many workpieces will fit on the chuck.
    • Round chucks are used for applications that require the workpiece to be rotated during machining
    • Rectangular/square chucks are used for most applications due to their ability to hold many workpieces.
  • Holding force is the force which holds the workpiece to the chuck.

  • Compatibility of the device with existing equipment is important to consider when replacing or designing a new machine.

Plate Materials

Some materials are better conductors of magnetism than others. Annealed materials are best. Hardened materials don't absorb flux as easily and tend to retain some magnetism when the magnetic chuck is switched off.

 

Common materials used for magnetic chucks include

  • Steel
  • Aluminum
  • Brass

Recent advances in magnetic materials have increased the power of magnets while reducing the size of the magnetic workholding unit. Newer magnetic chucks are being made out of rare earth neodymium iron boron which is at least five times more powerful than other materials used.  


Applications

  • Grinding machines
  • Heavy cutting processes by lathe, milling machines and planers
  • Metallic mold machining

  • Electro discharge machining

References

Permanent magnets Walker Magnetics

Magnetic Chucks

Is Magnetic Workholding For You?

Bryan, Julien. "Adventures of a Roving Cameraman." Popular Mechanics June 1939: 840-45. Web. 02 Sept. 2011.

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