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Magnetic Workholding (Chucks) FAQS

 

What is an Electromagnetic Chuck?

An Electromagnetic Chuck involves the supply of a constant (direct) current to a coil surrounding a series of poles.  So long as the current is flowing a magnetic force is gQenerated.
Electromagnets can be very powerful – more current and/or more turns on the internal coils equals more magnetic force, but the chuck may be subject to increase in temperature.  Further, electromagnets are not very portable because they require a permanent hook up to a special controller.

What is a Permanent Magnetic Chuck?

A permanent magnetic chuck utilizes permanent magnets within an internal assembly.  The movement of this assembly under a series of poles will direct the magnetic flux externally (at ON) or internally (at OFF).
The switching process is usually undertaken by way of a lever that when connected to an eccentric shaft allows the magnet assembly to move.
As the magnetic chuck gets larger, then the friction between the internal magnets and poles increases.
Due to this friction, permanent magnetic chucks are not very strong and best served for grinding, light machining or EDM applications.

What is a Permanent-Electro (Electro-Permanent) magnetic chuck?

The latest of the three technologies, combines the advantages of both permanent magnets and electromagnets.
An electromagnetic force is required for just a couple of seconds to energize/orient the internal permanent magnets.  Once magnetized the product remains ON with permanent magnetic force.  Equally, an electromagnetic force (in reverse) is required to turn the product OFF.  Unlike electromagnets, the product is FAILSAFE and does not get hotter with time.  Unlike permanent magnetic products there are no moving parts and forces can be much greater.
They can be used for palletizing, because once energized, the cable can be removed.

What happens if I lose electrical power?

  • No electrical power is needed for a permanent magnetic chuck, so it will not be affected.
  • With a Permanent-Electro Magnet, the workpiece will stay clamped, but the magnet cannot be deactivated until the electrical power is restored.
  • An electromagnet will release the workpiece the moment the current is removed from the internal coils.

How much force can a Magnetic Chuck generate on a piece of steel?

The attractive force generated by a magnetic chuck is dependent upon the permeability of the whole circuit.  Like an electrical circuit, the ease in which the current can flow is dependent upon the whole resistance of the circuit.  In a magnetic circuit, the resistance is known as “reluctance” and is determined by:

  • All circuit Air gaps
  • Length of the parts within the circuit
  • Permeability of all circuit materials (particularly the workpiece) as determined by composition.
  • Volume of all materials to conduct the delivered quantity of magnetic flux.

It is theoretically possible to generate a maximum force on permeable steels (low carbon, mild), with adequate volumetric dimensions of 16kg/cm2 (225 LB/sq.in) over the pole areas.

Is the clamping force of a magnetic chuck calculable?

Yes, the key is to know the flux density at the pole face in contact with the workpiece.  Any qualified magnet designer can make that calculation based on the parameters referred previously.  Magnetic forces can then be compared with application forces and with the allowance of a suitable safety factor, feasibility can be established.
Some manufacturers will provide a basic calculator for users to follow.

Can a Magnetic Chuck affect people with pacemakers?

At all times, personnel with pacemakers should keep away from any high magnetic field.  With most Magnetic Chucks, the magnetic field is not deep and most if not all, is absorbed by the workpiece.  Also, a correctly designed magnet has the field emanating from the pole surface only.  But as a precaution, it is always prudent to post warnings.

How much electrical power do I need to run a Permanent-Electro Magnetic Chuck?

It depends on the size and type of magnet.  Permanent-Electro Magnetic Chucks are available from 110-480V, but there is a limitation on the size of chuck that can be actuated and the maximum current that the manufacturer’s controller is designed to work at.  Consult with the manufacturer and they should be able to furnish the information given the size and style of magnet required.  Actual current can be high, but since it is only for a very short time, fusing and cabling requirements can be lower.

What makes a good magnetic workholding application?

The primary advantages of clamping magnetically are:

  • Better tool accessibility to the workpiece
  • Speed of set up
  • Uniform clamping means better accuracy
  • Safety
  • Low maintenance

There are more application specific advantages which the manufacturer will provide at the time of inquiry.

What is the minimum thickness I can clamp securely with a magnetic chuck?

The pole design determines the magnetic field height and it is always best that the whole field is contained within the workpiece.  Ask the manufacturer for recommended minimum thickness

What is the minimum area I can clamp securely with a magnetic chuck?

The workpiece must straddle an equal amount of (opposite) poles, two being the minimum.  The application (cut forces etc.,) will determine whether the magnetic force is enough based on contact area.  The use of side stops is always recommended.  Some manufacturers offer free feasibility studies for their customers.

What happens to chips?

Sticking chips is always a problem for magnetic workholding – be wary of the supplier that is too casual when asked.  Sticking chips cannot be avoided when the parts are thinner than the magnetic field height and/or when there is through machining into areas where magnetic fields are present.  Some manufacturers have developed techniques, through the use of top tooling, for minimizing the affects of stray magnetism and sticking chips.

Will my machine control be affected by magnetism?

In most cases, the machine control is not affected by the utilization of a magnetic chuck.  Manufacturers design to ensure that the magnetic field is only present on the polar surface and that the depth of field is not capable of straying to other parts of the machine.

Will my cutting tool become magnetized?

A cutting tool is usually manufactured from a magnetically hard material which means that it is susceptible to magnetic influence.  If the tool is working within the magnetic field then it will likely become magnetized (albeit slightly).  Generally, a partially magnetized cutting tool does not perform any worse.

What pole design is best for my application (square, round or rectangular)?

There is no “perfect” pole design each has its limitations and benefits.  The workpiece followed by the application will determine which pole design is best.  Be wary of suppliers that only offer one pole design – they are more likely to suggest that it “fits all”.

What if my workpiece is not flat?

Most manufacturers offer some form of self-shimming capability.  Magnetic chucks offer the best solution for holding non-flat parts with practically distortion-free clamping.

Will the part retain magnetism?

The amount of magnetism retained by a ferromagnetic material is dependent on the characteristics of the material itself.  Generally, harder steels tend to retain magnetism more than softer materials.  Some magnetic chuck suppliers offer a partial demagnetizing facility to reduce residual magnetism