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ISO 25178-73: Terms and Definitions for Surface Defects on Material Measures
Geometrical Product Specifications — Surface Texture: Areal
1. Introduction to Surface Defects on Material Measures
ISO 25178-73:2019 defines terminology and classification for geometrical defects that may appear on the surfaces of material measures and calibration specimens used in surface texture metrology. Unlike functional surfaces where imperfections may be tolerable, material measures are physical representations of mathematically simple shapes, and ANY deviation from the ideal shape is considered a defect that can compromise calibration accuracy.
This standard fills a critical gap by providing unambiguous, geometry-based defect terminology that replaces earlier cause-based descriptions from ISO 8785. By focusing on observable geometry rather than presumed causes, the standard enables clearer communication between manufacturers, calibration laboratories, and end users of measurement standards.
When inspecting measurement standards for acceptance, use the defect classification system in ISO 25178-73 to document every deviation. This systematic approach, required by ISO/IEC 17025, ensures that all parties have a shared understanding of the standard’s condition and its fitness for use.
2. Defect Classification
The standard defines three primary classes of geometrical defects based on their shape relative to the nominal surface:
| Defect Type | Definition | Examples |
|---|---|---|
| Outward defect | Deviation upward from the nominal surface | Pips, ridges, burrs, raised particles |
| Inward defect | Deviation downward into the surface | Pits, scratches, dents, gouges |
| Neutral defect | Deviation neither clearly upward nor downward | Stains, discoloration, embedded foreign material |
A special category, the negative defect, is defined for replicated material measures. This defect appears on a replica as the result of a corresponding defect on the mother mould surface. Understanding negative defects is essential for users of replica-based measurement standards commonly used in field applications where transporting the original standard is impractical.
2.1 General Terms
The standard introduces several foundational terms:
Measuring surface: The part of the calibration specimen surface over which measurements will be made. This is the region of interest for defect assessment.
Defect: A portion of the measurement standard’s geometrical feature where the real surface deviates from the ideal nominal surface. Crucially, this is defined as a portion of the surface (a localized feature), not a property of the entire surface.
A common mistake is treating surface roughness as a defect. Roughness is a distributed property of the entire surface, while defects are localized features with natural boundaries. ISO 25178-73 deals exclusively with localized defects, while roughness parameters are covered by ISO 25178-2.
3. Responses to Defects
The standard defines six possible responses to the presence of defects, providing a precise vocabulary for describing defect management strategies:
1. Remove the defect: Physically cut off the defective portion or discard corresponding data points. Only possible for material measures where the measuring area can be reduced.
2. Avoid the defect: Redefine the limits of the measuring area to exclude the defect. This preserves the standard’s physical integrity.
3. Repair the defect: Rework the surface, clean it, or retouch data points in software. Note that repair is distinct from removal.
4. Ignore the defect: Proceed with measurement without taking any action regarding the defect.
5. Measure by chance: Proceed without any plan regarding defects; measurements may or may not encounter them.
6. Tolerate the defect: The defect is deemed acceptable within specified limits for the intended application.
4. Engineering Design Insights
For calibration laboratory managers and quality engineers, implementing ISO 25178-73 requires establishing clear policies for each material measure in use. These policies should specify the maximum allowable defect size, the response protocol when defects are detected, and the documentation requirements per ISO/IEC 17025 clauses 7.2.1.1 and 7.8.5.
When designing measurement standards, manufacturers should consider that different applications have different sensitivity to defects. A sinusoidal roughness specimen used for Ra parameter verification is more sensitive to amplitude uniformity defects than to wavelength uniformity defects, while the opposite is true for RSm parameter verification.
5. FAQs
Q: How does ISO 25178-73 differ from ISO 8785?
A: ISO 8785 uses cause-based descriptions of imperfections. ISO 25178-73 defines defects purely by geometrical shape (outward, inward, neutral), avoiding ambiguity about causes and improving translatability.
A: ISO 8785 uses cause-based descriptions of imperfections. ISO 25178-73 defines defects purely by geometrical shape (outward, inward, neutral), avoiding ambiguity about causes and improving translatability.
Q: Can the defect definitions in this standard be applied to functional surfaces?
A: No. The standard explicitly states that its definitions apply only to material measures and calibration specimens. Functional surfaces have different considerations — removing or avoiding defects is generally not possible on functional parts like lenses or propellers.
A: No. The standard explicitly states that its definitions apply only to material measures and calibration specimens. Functional surfaces have different considerations — removing or avoiding defects is generally not possible on functional parts like lenses or propellers.
Q: What is a negative defect in replicated measures?
A: A negative defect appears on a replica as a result of a corresponding defect on the mother mould surface. It is defined separately because it propagates from the mould rather than originating on the replica itself.
A: A negative defect appears on a replica as a result of a corresponding defect on the mother mould surface. It is defined separately because it propagates from the mould rather than originating on the replica itself.
Q: Is it necessary to fix every defect on a measurement standard?
A: No. The standard defines “tolerate” and “ignore” as valid responses. The appropriate response depends on the defect’s size, location, and impact on the intended measurement uncertainty.
A: No. The standard defines “tolerate” and “ignore” as valid responses. The appropriate response depends on the defect’s size, location, and impact on the intended measurement uncertainty.
