No two magnets come out exactly the same size — and that is completely normal. Every manufacturing process has natural variation: pressing, sintering and grinding all add tiny differences. What matters is how much variation is allowed. That allowance is called the tolerance.
What tolerance means
Every dimension on a drawing has a nominal value (say 10 mm) and a tolerance (say ±0.05 mm). Any part landing inside that band is accepted. Tolerances come in two flavours: size tolerance, how far the length, width or diameter may deviate, and geometric tolerance, how far the shape may deviate — flatness of the faces, squareness, roundness. A part can pass a size check yet still fail to fit because its geometry is off, so for parts that press into pockets or stack together, ask for gauge inspection, not just a micrometer.
What machining can actually hold
NdFeB is hard and brittle — it is ground, sliced and wire-cut, not machined like steel. Realistic tolerances: as-sintered surfaces about ±0.25 mm, ground faces about ±0.05 mm, wire cutting about ±0.05 mm. Precision grinding reaches ±0.02 mm and lapping goes tighter still, but every extra step adds cost.
Tight tolerance is expensive — three ways
- Higher-grade machining, like grinding instead of slicing, costs more per part.
- Tighter limits lower yield — more parts fall outside the band and are scrapped.
- Geometry checks need extra setup and measurement time.
Which dimensions deserve tight tolerances
Usually just a few: the height in the magnetization direction, which sets the magnetic strength, and any face that mates against another part. Put the tolerance on the drawing and confirm it with the supplier before ordering — the drawing is the law.
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