Neodymium (NdFeB) magnets are the strongest commercially available permanent magnets, but “strongest” is only useful if the magnet is specified correctly for your application. An under-specified magnet fails in the field; an over-specified one inflates your unit cost. This guide walks through the seven decisions that determine whether the magnet you receive performs the way your design assumes it will.

Choose the Right Grade

NdFeB grades are written as a letter-number code such as N42 or N52. The number is the maximum energy product (BHmax) in MGOe — higher means more magnetic energy per unit volume. A letter suffix indicates the maximum operating temperature class.

Sintered vs. Bonded NdFeB

Sintered NdFeB is pressed and fired to near-full density, delivering maximum strength — it’s the right choice for most motor, holding, and sensing applications. Bonded NdFeB mixes magnet powder with a polymer binder, trading roughly half the magnetic strength for injection-moldable complex geometries, tight net-shape tolerances, and crack resistance.

• Choose sintered when you need maximum flux, simple geometry, and high operating temperature.
• Choose bonded when you need complex shapes, integrated features, multipole magnetization patterns, or thin-walled rings that sintered material would crack in.

Specify Magnetization Direction

The same physical magnet performs completely differently depending on how it’s magnetized. Always call out the direction on your drawing — never assume the supplier’s default.

• Axial (through thickness): the default for discs and blocks; poles on the flat faces.
• Diametrical: poles on opposite sides of a cylinder’s curved surface.
• True radial: flux oriented along the radius of a ring, uniformly around 360°. Produced as a single ring — not segmented arcs glued together — eliminating flux dead zones and assembly failure points. Critical for rotor assemblies, torque couplings, encoders, and sensor targets.
• Multipole: alternating N/S poles around a ring’s circumference or face, common in encoders and BLDC rotors. Specify pole count and pole placement (OD, face, or ID).

Select a Coating

Bare NdFeB corrodes rapidly — a coating is not optional. Match the coating to your environment and assembly process:

Size, Tolerance & Pull Force

Standard NdFeB machining tolerance is ±0.05 mm (±0.002″); tighter tolerances are achievable but add grinding cost. Specify tolerances only where the fit actually requires them.

When sizing for holding force, remember that published pull force numbers assume ideal conditions: direct contact with thick, flat, clean mild steel. In real assemblies, derate for:

• Air gaps — even a coat of paint measurably reduces holding force; force falls off steeply with distance.
• Shear loading — a magnet resists sliding at only ~15–25% of its rated pull force.
• Thin or alloyed steel — sheet metal saturates and can’t return full flux.
• Elevated temperature — output declines as temperature rises, before any permanent loss occurs.

A practical safety factor of 2–3× rated pull force is typical for engineered holding applications.

Account for Temperature — Both Directions

Two distinct thermal effects matter:

• Reversible loss: NdFeB output drops roughly 0.11% per °C as temperature rises, and recovers on cooling. Your design must tolerate the hot-condition flux, not the datasheet room-temperature value.
• Irreversible loss: exceeding the grade’s maximum operating temperature (or operating near it under a demagnetizing field) causes permanent loss that only re-magnetization can restore.

Cold is rarely a problem for NdFeB down to about −40°C; for cryogenic service, discuss the application with your supplier, as behavior below −130°C changes.

Qualify Your Supplier

For prototype quantities, price and lead time may be all you need. For production programs — especially automotive, medical, and defense — supplier qualification is where sourcing risk actually lives. Ask for:

• Material certifications — mill certs and demagnetization (BH) curves for the actual lot, not generic datasheets.
• Quality system — ISO 9001 at minimum; IATF 16949 alignment and PPAP capability for automotive; ISO 13485 familiarity for medical devices.
• Regulatory compliance — RoHS and REACH documentation as standard; NDAA/DFARS specialty metals traceability for defense programs.
• Supply chain transparency — where the alloy is melted, where the magnets are sintered and finished, and what happens to your program if rare-earth export conditions change.
• Engineering support — a supplier who can review your drawing and flag a coating/adhesive conflict or a demagnetization risk before production is worth far more than one who just quotes the print.

Common Buying Mistakes to Avoid

• Specifying N52 by default. The highest grade is rarely the right one — check temperature class and demagnetization resistance first.
• Ignoring the air gap. Designs sized from ideal pull-force tables routinely under-deliver by 50% or more in the assembly.
• Assuming all “radial” rings are equal. Segmented arc assemblies and true radially-oriented rings are different products with different failure modes.
• Skipping incoming inspection criteria. Define flux measurement method and acceptance limits in your PO, not after the first discrepant lot.
• Buying on unit price alone. A cheaper magnet that arrives without lot traceability or BH curves can cost you an entire audit or field recall.