The tolerances that matter most are usually the stator bore and rotor outside diameter, their concentricity and runout, slot geometry, stack height, and lamination burr condition. These features affect the air gap, shaft and housing fit, winding space, core alignment, and the repeatability of motor performance. The right limits depend on the motor design, manufacturing process, and inspection method, so there is no single tolerance set that suits every custom core. We recommend prioritizing dimensions by their effect on assembly and function, then agreeing on measurable acceptance criteria with your supplier before production.
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A stator and rotor core are assemblies of thin electrical-steel laminations, and small variations can accumulate across the stack. Dimensional errors may affect air-gap uniformity, shaft fit, housing fit, winding insertion, or the way laminations seat together. The consequences depend on the motor architecture and operating conditions, so a tolerance that is acceptable in one design may be unsuitable in another. For sourcing, the practical goal is to make each critical feature clear, measurable, and linked to its function.
The stator bore and rotor outside diameter help define the radial air gap, making them primary dimensions for many motor designs. A bore that is too small or a rotor that is too large can create assembly problems or reduce the designed clearance. If these dimensions vary, the air gap may also vary around the circumference, depending on how the parts are located and assembled. State the measurement condition and relevant datum so the supplier and buyer are evaluating the same feature.
Consider a simplified example with a nominal radial air gap of 0.50 mm. If the combined radial offset in an assembly reaches 0.10 mm, the local gap could be reduced to about 0.40 mm on one side in that simplified geometry. This is an illustration, not a recommended design limit; actual clearance must account for the complete motor, operating conditions, and assembly tolerances.
Concentricity and runout describe how accurately functional features relate to an axis or datum. They matter when the stator bore, rotor OD, shaft interface, or mounting features must align in the finished motor. A drawing should identify which axis or surface is the reference and whether the requirement applies to the stamped part, the stacked core, or a later machined feature. Avoid relying on terms such as “concentric” without a defined geometric tolerance and inspection approach.
Slot width, depth, opening, and position influence available winding space and the relationship between the winding and the core. Variation can affect winding insertion or create interference with a specified wire or insulation system. The critical dimensions depend on the winding process, so provide the relevant design constraints rather than asking a supplier to infer them from a general profile. For complex slot shapes, identify key dimensions and use a controlled drawing or CAD model as the governing reference.
Stack height affects the active core length and the fit of the core into its housing or assembly. Flatness and parallelism can matter where end faces seat against other components or where the stack is clamped. The finished stack may behave differently from a single lamination because stacking, interlocking, bonding, or welding can influence its geometry. As an illustrative calculation, a 100 mm stack with a ±0.10 mm height tolerance has a dimensional allowance equal to 0.1% of its nominal height; whether that allowance is appropriate depends on the design.
Lamination thickness is a material and process specification as well as a dimensional consideration. For example, a design may call for nominal 0.35 mm electrical steel, but the suitable grade, coating, thickness tolerance, and availability must be confirmed for the application. Burrs at cut edges can interfere with stacking, insulation between laminations, or handling, so buyers should state any required burr limit and how it will be checked. Do not assume a supplier’s general burr-control practice meets a project-specific requirement unless the acceptance criteria are documented.
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Some cores are supplied as laminations or stacks, while others include machined or assembled interfaces. If the rotor core is fitted to a shaft, or the stator stack is fitted into a housing, specify the relevant fits and identify which component controls each dimension. Include keyways, press-fit areas, pilot diameters, and mounting features where applicable. Confirm whether dimensions are measured before or after any machining, joining, or finishing operation that could change them.
Not every dimension needs the same tolerance or inspection effort. A useful RFQ separates critical-to-function characteristics from reference dimensions and general tolerances, then explains the reason for each critical requirement. This helps suppliers assess process feasibility and quote against a consistent specification. It also reduces the risk of paying for tight limits on features that do not affect assembly or performance.
A frequent mistake is applying very tight tolerances to every feature without connecting them to motor function. This can raise tooling, inspection, and production demands without improving the finished assembly. Another is providing only a 2D outline or a 3D model without identifying the governing dimensions, datums, and revision status. Buyers should also avoid treating a single lamination inspection as proof that the completed stack meets its height, alignment, or flatness requirements.
Be cautious when a specification uses tolerance terms that are not tied to a measurement method. For example, runout results can depend on how a part is supported and rotated during inspection. If the supplier and buyer use different setups, they may report different results even when examining the same part. Agreeing on the inspection condition before production helps make acceptance decisions more repeatable.
When we review a custom motor-core inquiry at Onlink, we recommend sharing the motor application, annual or project quantity, material requirements, drawing revision, and the features that are functionally critical. Include whether you need loose laminations, a stacked core, or additional processing, because the inspection plan should match the supplied condition. If a limit is not yet finalized, identify it as a design target for feasibility review rather than presenting it as an approved acceptance requirement. This gives both sides a clearer basis for discussing tooling, quality controls, and quotation scope.
Ask the supplier to confirm which dimensions can be measured directly, which require a fixture or defined setup, and what inspection documentation is available for the order. You can also request feedback on tolerance interactions, such as how stack height and end-face alignment relate to your assembly interface. Any proposed alternative should be reviewed by your engineering team before the drawing or acceptance criteria change. This keeps sourcing decisions connected to the motor’s actual design requirements.
For most custom stator and rotor core sourcing projects, begin with the stator bore, rotor OD, concentricity or runout, slot geometry, stack height, and burr condition. Prioritize the features that control the air gap, winding process, and assembly interfaces, then define how each will be measured and accepted. The exact limits must come from the motor design and be confirmed with the manufacturing process rather than copied from a generic tolerance list. To prepare an RFQ, send your current drawing or model, identify critical features, and ask Onlink to review the specification and inspection expectations before production is quoted.
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