Specifying Magnet Tolerances on Drawings: Dimensional, Magnetic and Geometric

A magnet drawing that says ±0.05 mm on every dimension and nothing else looks complete and is not. Sintered NdFeB is a hard, brittle material that is ground to size, its magnetic output varies from piece to piece, and the coating you specify uses up part of the dimensional budget. Tolerances that ignore those facts either cost more than they should or fail to protect the function that matters.

This guide covers the four kinds of tolerance a magnet drawing needs — dimensional, geometric, magnetic and cosmetic — with typical ranges, the mistakes we see most often, and a checklist of what to put on the drawing.

Why Magnets Aren’t Machined Like Metal

Sintered NdFeB starts as powder pressed in an aligning field and sintered into a blank. The blank shrinks during sintering, and it shrinks by different amounts along and across the alignment direction, so it cannot be made to final size directly. Blanks are made oversize and then diamond-ground, sliced or wire-cut to finished dimensions. The material is hard and brittle: it chips rather than deforms, so edges, corners and thin sections limit what is practical.

Two consequences follow. First, every step tighter than the standard ground tolerance is another operation or a slower one, which is why cost rises quickly at the tight end of the range. Second, the tolerance has to hold on the finished, coated part, so the whole process chain, not just the grinder, has to deliver the number.

Dimensional Tolerances: Typical Ranges

Feature	Standard	Tight (added cost, longer lead time)
Disc / cylinder diameter	±0.05–0.10 mm	±0.02–0.03 mm
Thickness / height	±0.05–0.10 mm	±0.02–0.05 mm
Block length and width	±0.05–0.10 mm	±0.02–0.05 mm
Ring outer diameter	±0.05–0.10 mm	±0.03 mm
Ring inner diameter	±0.05–0.10 mm	±0.03–0.05 mm
Arc segment radius / chord	±0.10 mm	±0.05 mm
Chamfer / corner radius	±0.10 mm	—

*Typical industry ranges for ground sintered NdFeB. Achievable values depend on part size, shape, grade and quantity — a ±0.05 mm tolerance on a 5 mm disc is a different job from ±0.05 mm on a 50 mm one. Confirm the figure for your part before it goes on a drawing.

Practical rules

Tolerance what the function needs. A magnet in a pocket cares about diameter and thickness; it rarely cares equally about everything else. Tight tolerances on non-functional features add cost and nothing else.
Scale with size. Large parts hold proportionally looser absolute tolerances. Matching a small part’s tolerance on a large one is a common reason for a quote that comes back high.
State that dimensions apply after coating. This is the usual intent and it should be written down (see below).
Inspect on the right parts. Magnetized parts attract ferrous gauges and each other, so dimensional inspection is normally done on unmagnetized samples or with non-magnetic or optical methods.

Geometric Tolerances

Size tolerances alone do not control form. Where function depends on it, add geometric controls per ASME Y14.5 or ISO 1101:

Parallelism of pole faces. Affects air-gap consistency in sensors and holding assemblies.
Perpendicularity of faces to the axis on cylinders and rings.
Concentricity or runout of OD to ID on rings. Important in motors and encoders, where radial variation turns into vibration or signal error. See our guide to ring magnet magnetization for how magnetization pattern interacts with ring geometry.
Flatness of pole faces for magnets that mate to steel or sit against a sensor.

Choose datums that reflect how the magnet is actually used, usually a pole face and the axis. A blanket general-tolerance note (such as ISO 2768) is a reasonable default for non-critical features, but it should not be the only control on a feature the assembly depends on.

Magnetic Tolerances

This is the section most drawings skip, and it is where sourcing disputes start.

Grade and minimum properties. Name the grade and let its published minimums cover coercivity. Remember that Br in a grade table is a typical value, so finished-part output varies with it. Our datasheet guide explains what each property means.
Field or flux at a defined point. If function depends on field strength, specify a measured value: gauss at a stated distance, with the probe type, fixture and temperature. Give a window with both a minimum and a maximum. A minimum alone invites the opposite failure, such as a Hall sensor driven into saturation. Production windows of roughly ±5–10% are common; tighter windows usually require measurement correlation between supplier and customer and often sorting. See how gauss measurement works for why the method matters.
Total flux for rings and assemblies, measured with a Helmholtz coil and fluxmeter, again as a window.
Magnetization direction. State the direction relative to a reference axis or face, and an angular tolerance if the application is sensitive to it. Typical sintered NdFeB orientation is within a few degrees; tighter requirements should be agreed in advance.
Pole identification. State which face is north and how it is marked. Reversed parts are one of the most common and most avoidable assembly errors.

Coating and the Tolerance Budget

Coating thickness is added to every surface. At 15–20 µm per side, Ni-Cu-Ni adds roughly 0.03–0.04 mm to an outside diameter. Epoxy at 20–25 µm per side adds closer to 0.05 mm and varies more at edges where it builds up. On a part toleranced at ±0.05 mm, that is a large fraction of the entire budget; on an inner diameter the coating shrinks the bore by the same amount.

Specify the coating stack, state whether dimensions apply before or after coating, and make sure the base-magnet tolerance leaves room for the coating you chose. Our coatings comparison covers typical thicknesses and how to call them out.

Chips, Cracks and Cosmetic Criteria

Brittle edges chip, and without a written criterion every chipped edge becomes a debate. Define what is acceptable: no cracks; chips no larger than a stated length and depth; no more than a stated number per part; none on functional faces such as sensing or mating surfaces; and an allowance for plating contact marks if appearance matters. If a part will be inspected visually, say at what magnification and distance.

Designing the Fit: Tolerance Stack-Up

Tolerances only matter in relation to the part the magnet goes into. For a magnet in a pocket or bore, calculate worst-case clearance as the pocket minimum minus the magnet maximum plus coating, and check that it stays positive.

Do not press-fit sintered NdFeB. Interference stresses can crack a brittle magnet immediately or later. Design for clearance and retain the magnet with adhesive, potting or a mechanical feature.
Leave room for the bond line. If the magnet is glued, the clearance has to accommodate the adhesive layer as well as the tolerances — commonly on the order of a tenth of a millimeter per side, depending on the adhesive.
Run both worst-case and statistical stack-ups. Worst-case guarantees assembly; a statistical (RSS) view shows how much tolerance you can relax and still assemble nearly every part.

Magnet Drawing Checklist

Material and grade, including temperature class (see NdFeB Temperature Classes).
Shape, nominal dimensions and tolerances on the features that matter, with units and a general-tolerance note for the rest.
Statement that dimensions apply after coating.
Coating stack and thickness range.
Geometric controls (parallelism, perpendicularity, concentricity or runout, flatness) with sensible datums.
Magnetization direction, pole identification and angular tolerance.
Field or flux requirement with measurement method and a min-max window.
Chip, crack and cosmetic acceptance criteria.
Edge chamfer or radius.
Sampling plan and documentation requirements, such as certificates or PPAP level.

Frequently Asked Questions

What is the standard tolerance for neodymium magnets?

For ground sintered NdFeB, ±0.05 to ±0.10 mm on the main dimensions is a common standard range, with tighter values available at added cost. The right number depends on part size and shape, so treat any single figure as a starting point rather than a rule.

Can I specify ±0.01 mm?

Sometimes, on small simple shapes, but it is expensive and hard to measure reliably, and the coating alone can consume it. Ask whether the function truly requires it, or whether controlling the mating part or adding clearance would do the job.

Are dimensions measured before or after coating?

Specify it. The usual intent is that dimensions apply to the finished, coated part, and a drawing that does not say so leaves it open to interpretation.

Does a tighter tolerance cost more?

Yes. Tighter tolerances need more grinding or finishing, more inspection and usually lower yield. Tolerance only the features that need it.

Why do I need a magnetic tolerance as well as a dimensional one?

Two parts can both be in dimensional tolerance and still produce noticeably different fields. If your application depends on field or flux, only a measured magnetic requirement, with a stated method and window, protects it.

Related Specification Resources

Radial Magnets supplies stock and custom NdFeB magnets built to your drawing, including true radial magnetization and PPAP documentation for automotive and medical programs. Send us your drawing through the Magnet RFQ Builder or contact us and we will confirm which tolerances are achievable and which will drive cost.