Understanding magnetic flux behaviour

Magnetic flux is a measurable quantity of a magnetic field over a given area. Flux lines are often used to depict the direction of flux as it flows from the north to the south pole of a magnet. The greater number of these flux lines the greater the density of the flux and ultimately, the greater the magnetic force.

Generally, magnetic flux will take the easiest route from pole to pole. A ferromagnetic material, like mild steel for example, is an excellent conductor of magnetic flux, conversely, air is a poor conductor of magnetic flux and paramagnetic materials such as aluminium and brass for example, can be regarded similarly to air.

Using air-gaps in designs and applications encourage magnetic flux to take a particular route that would benefit the holding of the workpiece or load. The use of air-gaps and pole pieces allows us to focus the magnetic flux where it is most effective and at the same time, remove or reduce flux from areas where it may become a nuisance.

The geometry of the pole and the amount of flux that it can carry to the workpiece has a measurable impact on the clamp force that can be generated. Stray flux is regarded as “useless” in the sense that its contribution to clamp force is negligible at best and as important, can become a potential burden.

The actual performance of magnetic chucks and magnetic lifters can vary depending on how they are applied. All users of these products must understand that the workpiece is an integral part of the overall circuit and magnetic flux will behave differently with dissimilar workpieces and how they are positioned over the poles.

Questions like “what happens to the chips?” and “will my cutting tool become magnetic?” are not uncommon when promoting a magnet as an alternative workholding device. These questions should not be underestimated.

As stated earlier, stray (or excessive) magnetic flux will not only impair cut performance but is also evidence of inefficient magnetic force. The target for any magnetic work holder is where the magnetic flux emanating from the poles is totally absorbed within the workpiece.

Fig. 4 shows a workpiece that is too thin to absorb all the magnetic flux made available by the chuck poles.  This results in excess flux to the top of the workpiece which may attract (ferromagnetic) debris (chips, for example). In addition, this inefficiency causes the density of the flux at the pole/workpiece interface to reduce which will affect clamp force.

Fig. 5 shows the positioning of a workpiece where there is an imbalance of pole areas. Accordingly, the flux (shown at the right of the workpiece) is not useful and contributes very little to holding force. In addition some of the stray flux will use the workpiece and possibly the cutting tool to find an alternative route. Simply sliding the workpiece to the right a little would improve holding force and minimise stray flux.