Basics of Magnetic Workholding

Why do permanent magnets behave the way they do? What properties do they possess that they can remain magnetized for a long time? Whether super powerful Neodymium magnets or ordinary Ferrite magnets, how do they seem to exert their influence through other medium like non-ferrous metals, wood, glass or even air?

Magnetic flux lines of force always exist between the North (N) and South (S) poles of a permanent magnet (Fig 1)

This magnetic flux can be used to attract and hold any ferrous material. Steel material placed in a flux field of a proximate permanent magnet are induced with opposite polarity than those of the permanent magnet and are thus attracted to the magnet (Fig 2)

The strength of the pull force depends on the flux density induced in the material and is proportional to the square of the flux density.

'Keeper steel plates' can sometimes be used to 'short-circuit' the magnetic flux in a permanent magnet and contain most of the flux within the keeper. In such a situation, the steel material will not be attracted to the permanent magnet (Fig 3). In fact, this is the property that is used, to contain a permanent magnet during transport through a sensitive zone, such as aircraft.

Magnets are usually packed in a box of steel sheets so that the flux lines do not penetrate the walls and potentially affect electronic sensors.