10 Questions Buyers Ask About NdFeB Magnets - Answered Without Hedging
These are the questions that show up repeatedly in technical inquiries, sample requests, and pre-order calls. Most supplier answers are vague because the real answer depends on application specifics. This article gives the engineering answer - the one that actually tells you what to specify and why - rather than the sales answer that defers every decision to "it depends on your requirements."
1. What Is the Difference Between N35, N42, and N52?
The number after N is the maximum energy product in MGOe (megagauss-oersteds), which is a measure of how much magnetic energy the material stores per unit volume. N35 has a maximum energy product of 33–36 MGOe; N42 is 40–43 MGOe; N52 is 50–53 MGOe. Higher number means stronger magnet for the same physical volume, or the same magnetic performance from a smaller magnet.
The practical implication: if you need a specific air gap flux density and are constrained by magnet volume (common in miniaturized motors and sensors), going from N35 to N42 lets you achieve the same performance with approximately 15% less magnet material. Going from N42 to N52 is another 15% reduction. But N52 costs approximately 20–30% more per kilogram than N42, and its Hcj (coercivity) is lower - N52 is more susceptible to demagnetization at elevated temperature than N42H even though N52 has a higher energy product at room temperature. For applications with operating temperatures above 60°C or peak demagnetizing fields, N45H often outperforms N52 in practice despite the lower room-temperature energy product.
2. What Does the Letter After the Grade Mean (H, SH, EH)?
The letter indicates the coercivity (Hcj) rating, which determines demagnetization resistance at elevated temperature. Standard grade (no letter): Hcj 12–15 kOe (955–1194 kA/m). H grade: Hcj 17–20 kOe (1353–1592 kA/m). SH grade: Hcj 20–25 kOe (1592–1989 kA/m). EH grade: Hcj 25–30+ kOe (1989–2387+ kA/m).
Higher coercivity means the magnet can withstand stronger demagnetizing fields at higher temperatures before losing magnetization. The tradeoff: each step up in coercivity requires adding more dysprosium (Dy) or terbium (Tb) to the alloy, which increases cost and slightly reduces Br (remanence). N42H has slightly lower Br than N42 (approximately 1.28–1.32 T vs 1.32–1.37 T) but much better resistance to demagnetization above 80°C. For motors, sensors, and actuators running above 80°C, specify H grade as the minimum. Standard grades in those applications are a common source of in-service demagnetization failures.
3. What Coating Should I Specify?
NdFeB corrodes rapidly without protection - uncoated NdFeB exposed to 95% RH at 50°C will show surface rust within 48 hours and structural degradation within weeks. The standard industrial coating is Ni-Cu-Ni (nickel-copper-nickel triple layer, total thickness 10–20 μm), which provides adequate protection for IP54–IP65 environments and handles humidity up to approximately 85% RH in normal temperature cycling. It's the default for most motor and industrial applications.
Epoxy coating (spray or dip, 15–25 μm) is better for complex geometries and for environments with mild chemical exposure, but provides less mechanical protection than Ni. Gold (Au) overcoat over Ni is used in medical devices and precision instruments where outgassing and biocompatibility matter - adds significant cost (10–20× Ni coating cost) but provides near-zero outgassing and clean-room compatibility. Parylene coating is used for aerospace applications requiring very low outgassing. Zinc (Zn) plating is cheap but provides inferior corrosion resistance compared to Ni and is not appropriate for anything other than cost-sensitive consumer products with sealed housings.
For outdoor industrial applications or marine environments: specify Ni-Cu-Ni plus an additional epoxy topcoat, or switch to SmCo which doesn't need coating at all.
4. What Is the Maximum Operating Temperature?
The number on the datasheet (80°C for standard, 120°C for H, 150°C for SH, 200°C for EH) is not the demagnetization temperature - it's the temperature at which the magnet's properties start to permanently change even without any external demagnetizing field. That's different from what engineers actually need to know.
What matters for motor and actuator design is the reversible temperature range (where Br decreases with temperature but recovers when cooled) and the irreversible demagnetization threshold (where Br permanently decreases due to the combination of temperature and external field). For a motor with armature reaction fields of 300 kA/m at peak current, an N45H magnet operating at 120°C may be operating near its irreversible demagnetization limit even though 120°C is within its rated range. The calculation method was covered in detail in the thermal demagnetization article in this series. The short version: always calculate the combined thermal + field demagnetization margin, not just the temperature.
5. How Much Does Magnet Strength Decrease With Temperature?
Br decreases by approximately 0.11–0.13% per °C for standard NdFeB grades. At 100°C above room temperature, a magnet with Br = 1.34 T at 20°C will have Br ≈ 1.20–1.21 T - approximately a 10% reduction. This is reversible: cool the magnet back to 20°C and Br returns to 1.34 T, assuming the magnet wasn't irreversibly demagnetized.
Hcj decreases much faster: 0.55–0.60% per °C. An N45 with Hcj = 1380 kA/m at 20°C has Hcj ≈ 620 kA/m at 120°C - a 55% reduction. This is why the grade number (energy product) matters less than the coercivity rating for elevated-temperature applications. Motor designers who optimize only for maximum Br at room temperature and then add a temperature margin often discover that the Hcj margin has already failed by the time the motor reaches operating temperature.
6. Can You Cut or Machine NdFeB After Magnetization?
No - and this is non-negotiable. Machining magnetized NdFeB is dangerous (the magnet will attract steel cutting tools, causing sudden tool capture and potential injury), damages the surface coating, and creates fine magnetic powder that is a fire and inhalation hazard. NdFeB is machined before magnetization, in the green (unsintered) or sintered but unmagnetized state.
If you need a custom shape, specify the geometry to your supplier and receive it machined but unmagnetized. If you receive magnetized magnets and discover the dimensions are wrong, the correct path is to return them for demagnetization, re-machining, and re-magnetization - not to attempt field machining. This adds lead time and cost, which is why dimensional tolerances should be confirmed and samples inspected before placing volume orders.
7. What Tolerances Are Standard?
Standard dimensional tolerances for sintered NdFeB are ±0.05 mm on length, width, and height for dimensions up to 20 mm, and ±0.1 mm for dimensions above 20 mm. Magnetic tolerances: Br ±3%, Hcj ±5% for standard grades. These are achievable without additional cost.
Tighter tolerances are available but cost more: ±0.02 mm dimensional for precision grinding (adds 15–30% to part cost), ±1.5% Br uniformity within a batch for applications requiring consistent back-EMF matching in motor assemblies (requires lot sorting and measurement). For sensor applications requiring matched magnetic field uniformity across multiple magnets, specify Br within ±1% and Bsurface uniformity via Hall scan on 100% of parts - this level of incoming inspection adds 20–40% to procurement cost but eliminates field failures from magnetic variation.
8. What Certifications Should I Ask For?
The certifications that matter depend on the application. For most industrial motor and machinery applications: ISO 9001 (quality management system - the baseline, every serious supplier has this) and material test reports (MTR) showing magnetic properties and coating thickness on the production lot. For food processing or medical-adjacent equipment: RoHS compliance certificate, material safety data sheet (MSDS), and surface coating biocompatibility report if magnets contact food or tissue. For automotive supply chain (Tier 1/2 parts): IATF 16949 is the relevant standard, though most magnet suppliers are not certified to it - verify that your motor manufacturer's quality plan covers the incoming material inspection. For medical devices (implantable or body-adjacent): ISO 13485 and USP Class VI biocompatibility testing for the coating material, plus outgassing data for enclosed device applications.
Do not over-specify certifications. Asking a standard industrial magnet supplier for ISO 13485 certification wastes time on both sides. Be specific about the regulatory environment your product operates in, and ask for the documentation that addresses it.
9. What Is Typical Lead Time?
Standard sintered NdFeB in common grades (N35–N52, H through SH, standard Ni-Cu-Ni coating) and common shapes (block, disc, ring, arc segments for standard motor pole counts): 2–4 weeks from order confirmation for quantities up to 10,000 pieces. Custom shapes with standard grades and coating: 3–5 weeks including tooling. Non-standard grades (EH, AH), non-standard coatings (Au, Parylene), or tight tolerances requiring sorting: 4–8 weeks.
Large quantities (>50,000 pieces) with specific lot and batch requirements: schedule accordingly - rare earth supply chain has volatility. SmCo: 4–8 weeks for standard grades, 8–12 weeks for large or complex custom orders. These are working estimates, not guarantees - confirm with your supplier based on current material and capacity availability, especially for dysprosium-heavy grades (H, SH, EH) where Dy price and availability affect schedule.
10. How Do I Verify That the Magnets I Receive Are What I Ordered?
For critical applications, don't rely on supplier certificates alone. Incoming inspection should include: Gauss meter measurement of surface field at a consistent measurement distance (compare to the expected value from the supplier's datasheet and your own FEA model), dimensional verification on a sample (at minimum first article, for critical applications 100% for key dimensions), visual inspection for coating defects, chips, and delamination, and weight check (mass is easy to verify and correlates with material density - NdFeB density is approximately 7.5–7.6 g/cm³; lower density may indicate voids or substituted material).
For volume production with tight magnetic requirements: invest in a magnetometer or B-H curve analyzer for incoming lot qualification. Surface Gauss check catches gross failures; B-H curve measurement catches grade substitution and Hcj shortfalls that may not be obvious from surface field alone. This equipment pays for itself quickly in production lines where a batch of wrong-grade magnets causes motor assembly failures downstream.
