Custom neodymium (NdFeB) magnets

Sintered neodymium magnets are hard permanent magnets based on neodymium, iron and boron. The material is brittle, so a finished magnet needs to be treated differently from a ductile steel part. The geometry, material orientation, coating and pole direction belong in the same design review.

A grade designation summarizes material properties; it is not a direct rating of the finished assembly's pull force. Read the proposed specification revision and the conditions attached to each value.

Br — remanence

The residual magnetic flux density measured after saturation and return of the applied field to zero under the stated measurement conditions. It does not specify the field at an arbitrary point outside a finished part.

HcB and HcJ — coercivity

HcB relates to bringing magnetic flux density to zero under an opposing field. HcJ relates to bringing intrinsic polarization to zero. They describe different points on the demagnetization curves. Reaching HcB does not by itself mean that the material has lost its magnetization.

(BH)max — maximum energy product

A material figure of merit derived from the demagnetization curve. Geometry and the magnetic circuit still determine how the material works in a device.

Technical context: Arnold's NdFeB material reference. Its published grades and values describe Arnold materials, not a Magnetstek delivery specification. Arnold also explains the hysteresis-loop parameters (PDF).

The operating point is set by the magnet and the rest of its magnetic circuit. Changing the magnet's proportions, opening a working gap, adding a steel return path or applying an opposing field can change that operating point. This is why two parts of the same grade can behave differently.

Pull-off force also depends on the mating steel, its thickness and contact area, surface coatings, gap and direction of loading. A direct pull test against steel does not describe resistance to sliding or performance across a different gap. For a sensor, the useful quantity may instead be field strength and polarity at the sensing position.

Compare candidate parts using the same geometry, temperature and measurement setup. Document the target field or force, where it is measured and the allowed range. A material datasheet and an assembly test answer different questions.

Magnetic properties change with temperature. A reversible temperature coefficient describes a change over a stated temperature range that can recover when the magnet returns to the reference temperature. It is not a complete prediction of output across every temperature or magnetic circuit.

Irreversible demagnetization can leave a magnet weaker after it cools. The risk depends on the grade's temperature-dependent curves, the part's geometry and working point, and any opposing field. A catalogue operating-temperature value therefore needs to be read with its conditions. Exposure time and thermal cycling should also be included in the application review.

Curie temperature is the material's magnetic transition temperature; it is not a recommended operating limit. An NdFeB part can suffer irreversible loss below that point. The coating, adhesive, housing and other components may impose additional temperature limits.

Technical context: K&J's explanation of temperature, working point and magnetic loss. Request applicable curves for the proposed material rather than copying a generic maximum temperature.

A magnet drawing should distinguish the finished dimensions from any dimensions before coating. Identify locating surfaces, critical fits, holes, slots, chamfers and edge requirements. A single overall tolerance can leave the dimensions that control assembly performance unclear.

Thin sections, sharp corners and narrow material around holes need particular attention because sintered NdFeB is brittle. Do not assume that holes or edge features can be added to a coated, magnetized part without affecting the part. Review those features before the finished geometry and inspection method are agreed.

Use a drawing revision and identify dimensions that are essential versus those open to discussion. Refer to the drawing and RFQ checklist for the information needed to review a custom shape.

NdFeB can corrode when exposed to moisture or an unsuitable environment. A coating is part of the protection system, together with the part geometry, handling and any housing or encapsulation. Surface damage, exposed edges and the actual service exposure matter when comparing finishes.

Specify the required coating system, coverage and thickness where it affects fit. Describe moisture, chemicals, cleaning, wear and contact with other materials. Nickel-copper-nickel, zinc and epoxy are finish options to discuss against those conditions; naming a finish alone does not establish suitability for an application.

A corrosion-test duration needs its test method, specimen condition and pass/fail criterion. A salt spray result cannot by itself be converted into years of service life. The coating specification guide and ASTM B117 test practice provide context for defining the test.

For a disc or ring, axial magnetization places the opposing poles on the two flat faces. Diametric magnetization runs across the diameter, with opposing pole regions on opposite sides. A radially magnetized ring is a different arrangement, with an inner-to-outer direction; the terms are not interchangeable.

For a block or an arc, mark the required pole faces or axis on the drawing. An arc shape does not establish a radial magnetization pattern. Multipole designs additionally need a pole layout, pole count, orientation and a method for checking the pattern.

The drawing should also identify how the part sits in its assembly and where the working field is needed. Review the pole arrangement together with the dimensions and material before treating the magnetization as final. See the magnetization direction reference for illustrated examples.

Application names identify a starting point, rather than a qualification of a particular magnet. The useful specification describes the magnetic circuit and the output that must be produced in the device.

• Motors and generators: magnet layout, pole orientation, operating temperature, opposing fields, retention and the surrounding rotor or stator parts.
• Sensors and encoders: field and polarity at the sensing position, working gap, travel and the required repeatability.
• Speakers and actuators: available space, pole pieces, working gap and the required field or force over the intended motion.
• Holding and separation: mating material or material stream, contact or working gap, load direction, cleaning and the conditions used to measure performance.

For a safety-critical or regulated device, the component and assembled system need the applicable qualification and acceptance work. A general application description does not establish certification or suitability.

Material-property records, dimensional checks and magnetic-output tests provide different evidence. Agree which records apply to the proposed grade, which identify the delivered part, and how the finished geometry, coating and pole orientation will be checked.

A reproducible magnetic test states the fixture, position or working gap, temperature, direction and acceptance range. Establish the sample or first-article checks before comparing later deliveries. Use the first article inspection checklist to structure that review.

Packaging and handling also affect the accepted part. Keep magnets separated and protected against impacts and snapping together. Agree what edge damage or coating damage is unacceptable and how parts will be identified and packed. Store them in conditions consistent with the coating and packaging specification.