A core brought from production may remain warmer than a gauge stored in inspection. The room thermometer describes neither object completely. If the fit allowance is small compared with thermal movement, a passing or failing result can depend on when the measurement was taken.
Treat the part and the gauge separately. Record their temperatures at locations relevant to the dimension, their materials and the reference temperature used by the drawing or calibration. Allow time for thermal equilibration when the measurement method requires it.
Two expansions affect the observed clearance
For uniform temperature and small expansion, a dimension D changes approximately as D(T) = Dref[1 + α(T − Tref)]. Here D is in millimetres, α is the relevant linear expansion coefficient per kelvin, and T − Tref is in kelvins. The coefficient must match the material, direction and condition; it should not be guessed from a generic metal value.
For a circular bore gauge and a core envelope, the observed diametral clearance is Cobs = Gref[1 + αgΔTg] − Dref[1 + αpΔTp]. The subscripts g and p denote gauge and part. This expression assumes both geometries expand freely and uniformly.
It shows why a single ambient correction is inadequate. A warm core tends to reduce clearance. A warm expanding gauge tends to increase it. Their coefficients and temperature changes may differ, so the two effects need not cancel.
| Information | Why it belongs in the record |
|---|---|
| Part temperature near controlled sections | Shows whether the measured region is thermally representative |
| Gauge temperature | Supports correction of the reference object |
| Reference temperature and coefficients | Makes the calculation reproducible |
| Time since movement between environments | Helps interpret an unsettled thermal state |
| Measured temperature range across the part | Reveals when uniform expansion is a poor assumption |
A gradient is a shape problem
If one side of the assembly is warmer, the dimension may vary along its length or the assembly may distort under restraint. Correcting a single number with the average temperature can miss that change in shape. Measure multiple relevant temperatures and compare dimensions after stabilization.
Clamps and fixtures can restrict thermal movement. A coefficient-based correction for a freely expanding specimen then describes a different mechanical condition. The inspection plan should state whether the core is measured clamped, supported or free, and whether that condition reproduces the intended fit.
Include temperature measurement uncertainty and coefficient uncertainty in the dimensional assessment when their contributions are material. A correction that is numerically large but poorly known does not create a precise reference dimension.
If the acceptance decision changes within the plausible thermal correction range, repeat under a better-controlled state or use a qualified task-specific method. Keep the original measurement and its temperature record. Rewriting it as a nominal-temperature value without those inputs removes the evidence needed to determine whether the problem was geometry or an unsettled part.
Keep thermal motion below the stock-width budget
The catalogue width tolerance for 300 mm wide, 0.23 mm stock is 0/−0.3 mm [1]. Assume a 300.0 mm reference core envelope governed by that cut width, a 300.5 mm gauge bore, reference temperature 20 °C, core at 40 °C and gauge at 25 °C. For sensitivity only, assume αp = 12 × 10⁻⁶ K⁻¹ and αg = 11 × 10⁻⁶ K⁻¹; these are not certified CRGO coefficients. Core growth is 0.0720 mm and gauge growth 0.0165 mm. Observed diametral clearance becomes 0.4445 mm instead of 0.5000 mm. The 0.0555 mm closure is 18.5% of the catalogue 0.30 mm stock-width range; that comparison does not set an assembly limit. If the assumed minimum is 0.46 mm, the warm-state fit misses it. Stabilize both objects or obtain qualified directional coefficients and correction uncertainty [2].
Manufacturing controls. Clamping, support restraint and the direction of the controlled dimension determine whether free expansion is a valid model. Thickness and slitting records do not establish the assembled core thermal expansion coefficient.
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.