A dimension can be accurate and still be irrelevant to assembly. This happens when an inspector measures from a convenient cut edge while the winding fixture locates the core from its support face. Both measurements may be repeatable; they describe different coordinate systems.
A useful datum hierarchy starts with the functions that locate the core. Identify the feature that establishes its seated pose, the feature that fixes lateral orientation, and the feature that sets the remaining position. Then check whether those features remain accessible after stacking and clamping.
Account for the remaining freedom
For an ideal rigid object, a plane-line-point arrangement illustrates the six degrees of freedom. The primary plane constrains movement normal to the plane and rotation about two axes. A secondary reference then removes two remaining freedoms; a tertiary reference removes the last. Other locating arrangements are possible, but their constraints must be equally explicit.
| Assembly function | Candidate drawing reference | Question before selecting it |
|---|---|---|
| Seating height and tilt | Defined support contact region | Does the measured face reproduce the seated condition? |
| Lateral orientation | Defined side region or derived center plane | Can individual sheet displacement change its apparent position? |
| Longitudinal position | Accessible stop feature or reference target | Will it survive later assembly operations? |
| Winding alignment | Limb axis derived from specified sections | Are the section heights and fit method defined? |
The physical datum feature and the datum derived from it are different. A rough stack face contains many local points. A simulator contacting its highest points may locate it differently from a least-squares fitted plane. The drawing and inspection method must resolve that choice rather than treating every extracted plane as equivalent.
Keep magnetic edges out of unrelated chains
Joint geometry needs its own functional references. A miter edge can define joint closure while being unsuitable for locating the complete assembly. Similarly, the winding axis can control insertion without being the reference for a sheet’s cut length.
Trace each critical dimension back to its function. If one length is measured from datum A, a second from datum B, and the distance between A and B is inferred through those same lengths, the chain can become circular. An inspector then has to choose an unstated reference to complete it.
Make one independent relationship define the reference system. Express the remaining dimensions within that system, including any permitted movement or adjustment. Avoid closing every chain with tight tolerances; that can create conflicting requirements without improving fit.
Finally, test the datum scheme using two inspectors or two measurement methods. Ask both to establish the coordinates from the drawing alone and report the same selected features. A disagreement about the origin or axis is a drawing-definition problem. Resolve it before debating whether a particular core is a few hundredths of a millimetre outside tolerance.
Test whether the chosen datum consumes the fit margin
23ZH90 nominal thickness is 0.23 mm [1]. Assume an unintended projecting layer raises one seating contact by one nominal thickness across a 500 mm support baseline; the controlled limb point is 1,000 mm above that plane. Small-angle tilt is 0.23/500 = 0.00046 rad, giving 0.460 mm lateral shift. Combined with an assumed 0.10 mm stop-location error in the same direction, total shift is 0.560 mm. An assumed 0.50 mm alignment budget is exceeded. Redefine or verify the seating contact before tightening the limb-axis tolerance, and assess the complete datum-establishment uncertainty [2]. The model assumes a rigid assembly and actual contact on the high layer.
Manufacturing controls. Packing sequence, sheet seating, clamp load and support-face preparation establish the physical datum. Choose stable functional contacts and specify how their simulator differs from a least-squares fitted surface.
References
- Nippon Steel Corporation, Grain-Oriented Electrical Steel Sheets, catalogue D004je_03_202004, 2020, Specifications; Size and Form Tolerances.
- JCGM, JCGM 100:2008, Evaluation of measurement data — Guide to the expression of uncertainty in measurement, 2008.