A grade such as N35, N42 or N52 describes the potential energy density of the material. It does not describe the force a finished assembly will produce. Two magnets of the same grade can behave differently once geometry, temperature and the surrounding circuit are taken into account, so the grade is the beginning of a specification rather than the whole of it.
What the grade number means
The number in N42 is the maximum energy product the class is specified against, expressed in megagauss-oersteds. A higher number means more magnetic energy per unit of volume, which is what allows a smaller magnet to do the same job. The trade is not price alone: higher grades are harder to produce consistently, they are more sensitive to heat, and they are offered in fewer sizes, shapes and coatings.
The letter N identifies the material family, neodymium-iron-boron. Letter suffixes such as M, H, SH, UH and EH mark the temperature classes published for the same energy product, where each step raises the maximum operating temperature the class is rated for. The exact limits sit in the manufacturer datasheet. What matters for a buyer is that a grade code carries two decisions at once: energy density and temperature class.
Temperature usually decides the grade
Strength is the requirement buyers notice first, but heat is usually what forces the specification. Above a certain temperature a magnet loses part of its field permanently rather than temporarily, and the temperature at which that happens depends on the grade class, the shape of the magnet and the way the magnetic circuit loads it. A thin disc in an open circuit is far more exposed than a tall block captured between steel poles.
The practical rule is to leave margin. Choose the temperature class for the hottest condition the magnet will meet in service, including a stalled motor, a parked vehicle in the sun, a soldering step or a curing oven, not for the comfortable case on the bench. If the application runs above the rated class, the answer is usually a higher class, a different geometry or a different circuit rather than a larger magnet.
Where a part has already failed in the field, describe what changed: a new batch, a hotter enclosure, a thinner wall, a stronger opposing field. That description is often more useful than a grade number.
Geometry, and the difference between reference and assembly values
Every published figure for a grade is measured under reference conditions. In an assembly, the useful flux depends on the air gap, the length-to-diameter ratio, the return path and the magnetisation direction. A block magnetised through its thickness behaves differently from the same block magnetised through its length, even though both carry the same grade.
That is why a grade alone cannot carry a performance guarantee. When the performance of the finished assembly matters, the conversation has to include the geometry of the magnetic circuit, or a sample that can be measured.
Coating and environment
NdFeB corrodes quickly when it is left bare, so the surface finish belongs to the specification rather than to the packaging. Nickel-copper-nickel is the common default for dry indoor use, zinc suits some cost-sensitive applications, and epoxy or another organic coating is chosen where humidity, salt or chemical exposure matters. The useful question is not which coating is best in general, but which environment the finished part will live in.
Tolerances and magnetisation direction
Grade and coating say nothing about how closely the part has to match its drawing. Sintered magnets are hard and brittle, so machining adds cost; standard tolerances are looser than most buyers expect, and tightening them has a price. The same applies to the magnetisation direction and the pole pattern, which belong on the drawing with the same weight as the dimensions.
A sequence that keeps the specification honest
Start with the job: the force or flux the assembly needs, the air gap it works across, how often it operates and the hottest condition it will see. Then set the temperature class, because it constrains everything that follows. Then choose the energy density, meaning the lowest grade that does the work rather than the highest on the list. Then fix the geometry and the magnetisation direction, and state the tolerance the assembly genuinely needs. Finally, choose the coating against the environment and record all of it in one place.
What to send with the enquiry
A drawing or sketch with dimensions, the quantity needed now and the volume expected per year, the application, the operating temperature, the grade or performance target if there is one, the coating or the environment it will meet, and the magnetisation direction. If a part has failed, add photographs and the conditions at failure.
With those answers a manufacturer can propose a grade that fits. Without them, any reply is a guess, and the guess usually lands on the safest and most expensive option.
Where a grade number misleads
Grade labels are sometimes read as a performance promise. They are not one. The same grade produced on two lines can differ in coercivity, density and tolerance, and the assembly decides the working point. Treat the grade as one line of the specification, and confirm the final performance with a sample, an inspection report or a test on the assembled part.
Choosing between two close grades
When two grades both look sufficient, compare them on the conditions that are hardest to change later. A higher class costs more and is usually offered in fewer shapes, so it is worth checking whether the geometry or the circuit can be adjusted instead. If the assembly is already fixed, the more predictable option is normally the higher temperature class at the same energy density, because heat is the failure mode that is hardest to correct in the field. Record the comparison in the drawing notes so the reasoning survives the next revision.
This guide summarises published industry conventions and general engineering practice. It carries no supplier-specific performance claims; confirm final values against the manufacturer datasheet and first-article inspection for the parts you buy.