A magnet that measures right but does not fit is scrap. Dimensions decide whether your part drops into the assembly cleanly and whether every magnet in a batch behaves the same — which is why buyers watch size as closely as magnetic strength.
What does a tolerance mean?
No manufacturing process is perfect. If a drawing says 10 ±0.05 mm, it means the part is acceptable anywhere between 9.95 and 10.05 mm. That little band is the tolerance. The tighter the band, the more careful the machining — and the higher the price. As-sintered surfaces get loose tolerances; ground surfaces get tight ones.
The toolbox
- Caliper — the everyday tool. Good for quick length, width and thickness checks, resolving about 0.01–0.02 mm.
- Micrometer — more precise (0.001 mm). The workhorse for ground diameters and thicknesses.
- Profile projector — magnifies the part's silhouette onto a screen for fast checks of contours, chamfers and slots.
- Go/no-go gauges — the part must slip into (or past) the gauge. This checks size and assembly fit at the same time.
Two practical rules
- Match the tool to the tolerance. Your measuring instrument should be roughly ten times more precise than the tolerance band. A ±0.05 mm tolerance calls for a micrometer; a caliper is borderline.
- Size is not everything. A micrometer checks size only. A part can pass every size check and still jam the assembly line because of flatness or perpendicularity problems. That is the classic trap — and why go/no-go gauges, which test the real fit, catch parts the micrometer misses.
For critical features, check form as well as size — and agree the measurement method with the supplier before production starts.
Read on: dimensions and tolerances in detail → · how sintered magnets are machined →