Sintered NdFeB Ring Magnets for an Encoder Application

The customer compared magnetization carried out using its own equipment, an external laboratory and our manufacturing partner's equipment. Remagnetizing returned parts did not resolve the reported problem.

According to the customer's test report, magnets that passed and failed the application test showed similar signal amplitudes. The reported difference was in the sine–cosine waveform shape and the resulting eccentricity error.

Four linear Hall sensors read the field as the magnet turns, producing one sine and one cosine signal, and the drive software derives rotor position from the two signals. That derivation is only as good as the relationship between the two signals: plotted one against the other, a clean pair traces a closed circle, and anything that deforms that circle — eccentricity among it — becomes a position error. The software then drives the motor from an angle it has read slightly wrong.

The failed batch varied part to part rather than shifting as a whole. Some magnets reconstructed a clean circle and passed the application test; others produced a distorted one and failed it. Same drawing, same material, same process route — and nothing in the specification told the production side which of the two outcomes it was shipping.

These findings shifted the investigation beyond magnetizing equipment. ProfurtherMag coordinated the exchange of test reports, sensor-layout information and production feedback to clarify what needed to be controlled.

The 2.0% was not on the original drawing. It came from the customer during the exchanges that followed the failed batch, as a goal for the application: “Our goal is to have Sine-Cosine signals without distortion, the maximum error in eccentricity should be possibly lower than 2.0%.” The clean-circle plot under Acceptance Comparison is the eccentricity limit the customer could accept in that application — a functional acceptance criterion for the sensor system, not a dimensional tolerance for the magnet.

The sensor arrangement also carries an angular requirement. Because the four sensing positions fix where the sine and cosine signals are sampled, a magnet's orientation relative to its own datum is part of the functional specification, not a detail left to production judgement. A ring can sit inside its dimensional tolerance and still shift the electrical angle the sensor reads.

The original drawing did not explicitly define the signal-quality requirement. During the investigation, the customer explained its measurement arrangement and provided examples of unacceptable and acceptable test results.

The orientation-deviation limit appeared only in a later drawing revision: Orientation deviation < 1.5°. Until it was written down, no delivered batch could be checked against it. The requirement existed only inside the customer's application test, so a batch that failed there could not be traced back to a stated drawing value — and a supplier working from a drawing without it has no reason to depart from its standard process for that geometry.

Once stated, the two requirements were treated as separate items: the orientation limit is measurable in production, while the signal-based acceptance criterion is confirmed by the customer's sensor test.

The production review focused on how the manufacturing process could better support the accuracy required by the application.

Cut-face perpendicularity was an important control requirement for this project.

Rather than continuing with the original direct-forming approach, the manufacturing partner adopted a different machining sequence: pressing block blanks, cutting them into rectangular bars, using centerless grinding to produce cylindrical blanks, and then slicing.

This change was intended to improve control of the cut faces relative to the cylinder axis. ProfurtherMag coordinated the revised samples and followed up with the customer's engineering team to obtain application-level feedback.

This sequence was settled between ProfurtherMag and the manufacturing partner, not with the customer. Once the signal requirement was on the table, meeting it left only this route instead of the conventional direct-forming process; the customer's part in the change was the application test that followed.

Production & Commercial Validation: After the manufacturing process was revised, the customer tested the 500-piece nickel-coated sample batch and confirmed that it met the required sine–cosine signal performance and acceptance criteria.

A subsequent 2,000-piece delivery was also tested, and the customer confirmed that all pieces passed.

These results validated the revised production approach for this application.