Magnetisation direction for NdFeB magnets

Magnetisation direction is the orientation of the magnet's easy axis and the arrangement of its poles. It is not a property of the grade. Two magnets made from the same N42 material can produce different useful flux in the same assembly if one is magnetised through its thickness and the other through its length.

Direction belongs on the drawing for the same reason dimensions do. A supplier cannot infer it from the shape, and a finished sintered magnet cannot be re-magnetised into a different easy axis after production.

What magnetisation direction controls

NdFeB powder is aligned in a magnetic field before pressing so that the grains share a common easy axis. That alignment, together with the tooling and the magnetising fixture, determines where the poles sit and how the flux leaves the part. The grade sets the material's potential energy product; the direction and the surrounding magnetic circuit determine how much of that potential becomes useful flux in the application.

This is why a grade alone cannot carry a performance guarantee. The same block can behave differently when it is magnetised through its thickness, through its length, or across a diagonal, even though the material and dimensions are unchanged.

Axial magnetisation

Axial magnetisation puts the poles on two opposite faces, with the easy axis running through the thickness or length of the part. It is the common default for discs, blocks and simple cylinders. The north and south faces are easy to identify, and the magnet can often be magnetised in a relatively simple fixture.

For a thin disc, axial magnetisation through the thickness is usually the natural choice. For a long block, the same phrase can mean through the length instead. That is why the drawing should show the direction arrow and state which face is north rather than relying on the word “axial” alone.

Stacked or mirrored assemblies need particular care. If one magnet is flipped during assembly, the opposing poles can cancel useful flux or create a force in the wrong direction. Mark the north face on the drawing and on the incoming inspection plan.

Diametrical and radial magnetisation

Diametrical magnetisation runs across a diameter. On a ring or cylinder, the poles sit on opposite sides of the circumference rather than on the two flat faces. It is used in sensors, couplings and some motor or actuator designs where the flux has to cross the part sideways.

Radial magnetisation runs from the centre outwards, so the flux radiates around the circumference. It is common for ring magnets and rotor parts, and it can be combined with a multipole pattern. Radial and diametrical parts are not interchangeable: the flux path, the fixture and the inspection method are different.

Not every shape and size can be magnetised in every direction. A very thin ring, a long cylinder or a part with a narrow web may limit the achievable pattern or the magnetising fixture. Confirm manufacturability with the manufacturer before the drawing is released, rather than after the tooling is made.

Multipole patterns

A multipole magnet carries several alternating north and south poles on one face or around one circumference. The pattern is defined by the pole count, the pole pitch, the pole width and the transition between poles. “Multipole” on its own is not a complete specification.

Multipole parts appear in encoders, magnetic rotors, couplings and position-sensing assemblies. The receiving sensor or the mating rotor sees a periodic field, so the pole geometry and the air gap matter as much as the grade. A back-iron path can shape the field, but it can also concentrate flux in places the drawing did not intend.

Agree the inspection method with the supplier. A single flux reading does not describe a multipole pattern; the measurement should define the points, the probe distance and the expected polarity around the part.

Magnetisation and the magnetic circuit

The useful flux in an assembly depends on the air gap, the return path, the steel around the magnet and the orientation of the magnet relative to the load. A direction that works in a free-standing test can perform poorly once the magnet is captured in a steel housing or placed next to an opposing field.

When the performance of the finished assembly matters, describe the circuit rather than only the magnet. A drawing of the mating parts, the air gap and the expected direction of force is more useful than a request for “the strongest grade”. Where the design is sensitive, a sample or a measured first article is the honest way to confirm the choice.

What to put on the drawing

• The magnetisation direction arrow, drawn on the part rather than described only in a note.
• Which face or region is north, especially for axial and multipole parts.
• The pole count and pattern for multipole parts, with the pole pitch or a reference view.
• Grade, dimensions and the tolerances that actually matter to the assembly.
• Coating and the environment it has to survive.
• Operating temperature and any heat exposure during assembly or curing.
• Quantity, application and whether a first article or flux measurement is required.

If the magnet is part of an assembly, show its orientation relative to the mating part. A direction that is correct on the magnet drawing can still be wrong once the part is installed.

Magnetisation quality checks

Useful checks include flux or field measurement at defined points, a polarity check around the part, dimensions and coating inspection, and a visual check for damage. For a new part, a first-article report is more informative than a certificate that only states the grade.

The measurement method should be agreed in advance: probe type, distance, points and the expected values. Without that agreement, two inspectors can measure the same magnet and report different numbers without either of them being wrong.

Common magnetisation mistakes

• Omitting the direction from the drawing and assuming the supplier will choose the obvious one.
• Using “axial” without saying whether it means through the thickness or through the length.
• Treating diametrical and radial as interchangeable.
• Not defining which face is north before the parts are assembled.
• Specifying a grade without describing the magnetic circuit it has to work in.
• Assuming a finished sintered magnet can be re-magnetised into a different axis.

Frequently asked questions

Discuss your magnetisation requirement