Magnet Design Guide
How to specify a permanent magnet that works in the real world — not just on the datasheet. Written by our applications engineers from thousands of customer designs.
Start from the Application, Not the Material
Most design mistakes come from picking a material first. Instead, define what the magnet must do:
- ✓Required force or flux — at the working gap, not at the magnet face
- ✓Operating temperature range — the real maximum, including self-heating and worst-case duty
- ✓Environment — humidity, salt, chemicals, vacuum, sterilization cycles
- ✓Space envelope — the volume you actually have, including tolerances and assembly clearance
- ✓Cost target — at your annual volume, not prototype quantity
Choose the Material Family
With the requirements defined, the material choice usually narrows itself:
| If you need… | Choose |
|---|---|
| Maximum force in minimum space | NdFeB |
| Stability above 150–200°C or in corrosive media | SmCo |
| Lowest cost, large sizes, corrosion immunity | Ferrite |
| High temperature with gentle handling | AlNiCo |
| Bendable, cuttable, print-ready sheets | Flexible |
Full property comparison: Material Specifications.
Respect the Temperature Limit — with Margin
The #1 cause of field magnet failures in the field.
Grade Suffix = Practical Limit
NdFeB grades carry a temperature suffix: standard N grades to ~80°C, M to 100°C, H to 120°C, SH to 150°C, UH to 180°C, EH to 200°C, TH to 220°C. Exceed it and irreversible flux loss begins.
Demagnetization Depends on Load Line
Thin magnets operating at a high permeance coefficient demagnetize at lower temperatures than the grade rating suggests. We calculate the knee point at your operating temperature and geometry — not just quote the datasheet.
Design with 20% Margin
Specify a grade whose maximum operating temperature sits at least 20°C above your worst case. The margin costs little now and prevents a field failure later.
Specify the Right Coating
NdFeB requires protection; other materials often don't. Match the coating to the environment:
| Coating | Thickness | Best For |
|---|---|---|
| Ni-Cu-Ni | 15–20 µm | General purpose, 48–72h salt spray |
| Epoxy | 15–25 µm | Outdoor, marine — 240h+ salt spray |
| Zinc | 8–12 µm | Cost-sensitive, dry environments |
| Parylene-C | 5–20 µm | Medical, thin and pinhole-free |
| Phosphate | ~2 µm | Vacuum, clean, short-term storage |
SmCo, ferrite, and AlNiCo generally need no coating.
Tolerances: Ask Only for What You Need
Sintered magnets shrink unpredictably during pressing and sintering — every dimension is finished by grinding, and tighter tolerances cost real money:
- ✓Standard ground tolerance — ±0.05 mm on ground faces, ±0.1 mm on as-sintered faces. This covers 90% of applications
- ✓Precision grinding — ±0.02 mm or better is available for sensor and instrument magnets
- ✓Put tolerances on functional faces only — leave non-critical faces at standard tolerance and cut cost
- ✓Bonded magnets — ±0.05–0.1 mm as-pressed with no grinding, ideal for complex shapes
Design for Magnetization and Assembly
The magnet is part of a system — design the system.
Magnetization Patterns
Axial, diametral, radial, and multipole patterns are fixed at the end of production by the magnetizing fixture. Define the pattern and pole accuracy in the drawing — retrofitting is impossible.
Never Machine a Sintered Magnet
NdFeB and SmCo cannot be drilled, tapped, or turned — only ground. Threads and press-fits are created with steel plates, housings, or adhesives. Bonded magnets are the exception and can be machined.
Charged vs Uncharged Shipping
Charged magnets simplify your line but complicate transport (IATA rules). We ship either way, with proper packaging and documentation. Tell us your assembly process and we recommend the best option.
Common Design Mistakes
- ✓Specifying Br when you need force — remanence is a material property; force depends on the whole magnetic circuit. Share the application and we engineer backward from the force you need
- ✓Thinner and thinner magnets — thin sections demagnetize easily and are fragile to handle. Below 0.5 mm, consider bonded or flexible materials
- ✓Ignoring the steel return path — magnet performance changes completely with and without a back-iron. Include it in the simulation
- ✓Chamfers skipped — unchamfered edges chip during handling and assembly. A 0.1–0.3 mm chamfer is nearly free and prevents cosmetic rejects
- ✓Over-specifying tolerance everywhere — precision on non-functional faces inflates cost 20–50% with zero benefit
- ✓No temperature test data — always request flux verification at operating temperature for anything above 120°C
Let Us Review Your Design — Free
Send your drawing or concept. Our engineers check the magnetic circuit, temperature margin, and manufacturability, then recommend the most cost-effective spec.