Key decisions
- Do not convert Ra to Rz using a universal multiplier.
- Specify the governing standard and measurement conditions for the feature.
- Treat surface texture, visible defects and part geometry as distinct acceptance questions.
Start with what the surface must do
A surface-finish requirement should explain an interface, not merely request a smooth appearance. A sliding contact, sealing land, adhesive interface and decorative cover may need different characteristics. The lowest possible roughness value is not automatically the best choice for every function, and a roughness measurement does not establish flatness or dimensional fit.
Identify the affected surface on the drawing and its final condition. If blasting, polishing, anodizing or another treatment follows machining, state when acceptance applies. A good result on the machined substrate may not describe the delivered surface.
Also separate texture from damage and contamination. A part can meet a selected roughness parameter and still have a scratch across a sealing path, an unacceptable burr or residue. Define the relevant defect and cleanliness criteria independently so a single numerical finish result cannot be mistaken for complete acceptance.
Compare averages and height-sensitive parameters
Ra describes the arithmetic mean of absolute profile-height deviations over the evaluation used by the governing method. Rz is a height-based roughness parameter whose exact calculation and evaluation must follow the specified standard. The legacy Mitutoyo guide illustrates Rz through section peak-to-valley values; those historical conventions should not be silently substituted into a current drawing.
| Item | Question it helps answer | Important limitation |
|---|---|---|
| Ra | How large are the profile deviations on average? | Different peak and valley distributions can produce the same average |
| Rz | What height-based roughness result follows the specified method? | The standard and evaluation conditions must accompany the symbol |
| Rt in the legacy profile framework | What is the total peak-to-valley height across the evaluated roughness profile? | It is not interchangeable with an average parameter |
| Lay or texture direction | In which direction are the surface features organized? | A scalar roughness value alone does not describe direction |
| Visual defect criteria | Are scratches, dents or other imperfections acceptable? | These require separate limits and an inspection method |
Worked example: the same average can hide different extremes
A small numerical model makes the limitation of an average easier to see. It is a mathematical illustration, not a profilometer procedure or a calculation of standards-compliant Rz.
For an actual sealing land, the designer might need a height-sensitive parameter, a direction requirement or a separate scratch criterion alongside an average limit. The correct selection should follow the interface requirements and validation evidence. Adding every available parameter without a functional reason can create unnecessary inspection work and conflicting acceptance expectations.
Two idealized sets of surface heights
Consider four equally weighted heights in micrometers: profile A has values -1, -1, +1 and +1; profile B has -2, 0, 0 and +2. Both have zero mean height and a mean absolute deviation of 1 micrometer. Their simple peak-to-valley ranges differ: 2 micrometers for A and 4 for B. Real roughness measurement includes continuous sampling, filtering and prescribed evaluation rules, but the example demonstrates why an average alone cannot determine peak-to-valley behavior. Consequently, a fixed Ra-to-Rz multiplier cannot prove compliance for an arbitrary surface.
Match the instrument setup to the drawing
The reported value depends on what is measured and how. Mitutoyo explains the roles of profile filtering, stylus geometry and measuring direction. For a supplier comparison, ask for enough information to reproduce the intended evaluation: parameter, standard, units, filter settings, evaluation length, trace location and direction.
Limited access should be resolved before production. A narrow annular land may not provide the traverse length assumed by a default instrument setting. Quietly shortening the trace can change the meaning of the result. The drawing authority and inspection provider should agree on an appropriate method and document any departure from the specified conditions.
A disagreement between two instruments is not automatically evidence of bad machining or a failed gauge. First check whether the same surface area, alignment, filtering and evaluation were used. Then examine the instrument condition and measurement uncertainty. Comparing only the displayed number removes the information needed to investigate the difference.
Keep legacy drawings and current standards distinct
ISO 21920-2:2021 defines terms and parameters for profile surface texture, and ISO lists earlier ISO 4287 documents as withdrawn. Existing contractual drawings may still cite an earlier edition. Resolve that requirement explicitly instead of changing instrument defaults and assuming the same acceptance interpretation.
The Mitutoyo bulletin cited here is a legacy guide based on earlier standards. Its specific sampling and evaluation rules are useful for understanding those drawings, but they are not presented here as universal rules under ISO 21920. A standard reference should include its edition where needed to avoid ambiguity.
The production route also affects texture. Sandvik links turned surface quality to tool geometry, cutting conditions and vibration among other factors. That means an Ra target cannot be guaranteed simply by selecting a nominal feed value from a chart. The supplier must establish a suitable process for the actual material, geometry and final treatment.
Prepare a finish requirement that can be quoted
Give Beigain a marked drawing showing functional lands and cosmetic zones separately. Describe the mating surface or sealing element where relevant, and state whether a representative sample is for visual approval, tactile comparison or formal measurement correlation. Those are different uses of a sample.
- Identify each controlled surface and its final treatment state.
- Specify parameter, value, units and governing standard or edition.
- Define measurement location, direction and any necessary nondefault conditions.
- Resolve probe access and available trace length for small features.
- List defect, burr and cleanliness criteria separately from roughness.
- Agree on the number of parts, traces and records required.
- Avoid substituting a visual finish name or an Ra-to-Rz conversion for measurement.
Questions about this topic
Can Ra be converted to Rz by multiplying by four or seven?
No universal multiplier establishes compliance. The relationship depends on the profile and evaluation method. Measure the required parameter under the drawing conditions or approve a formally defined alternative.
Does a shiny surface necessarily have a lower Ra?
No. Appearance depends on lighting, texture direction and treatment as well as topography. A photograph or visual comparison cannot replace a required quantitative measurement.
Can a roughness value prove that a sealing face is acceptable?
It answers only the specified texture question. Geometry, scratches, material, contamination and the mating seal may also matter, so acceptance must reflect the complete interface.
Should a legacy ISO 4287 drawing automatically be evaluated under ISO 21920?
No. Clarify the governing requirement with the drawing authority. A change of standard or evaluation settings should be deliberate and documented, particularly for repeat parts.
Technical references
- Mitutoyo: Surface Roughness Measurement, Bulletin 1984 — Legacy metrology guide for profile parameters, filters and measurement setup under earlier ISO standards; its historical rules are not represented as current universal defaults.
- ISO 21920-2:2021 — Surface texture: Profile, Part 2 — Official scope, edition and predecessor-status information for the profile surface-texture standard.
- Sandvik Coromant: How to achieve good component quality in turning — Primary tooling guidance on factors affecting turned surface quality. The four-height example is an original mathematical illustration.
Continue reading
- Surface Finishing Services
- Quality Control & Inspection
- Defining CNC inspection requirements before production
- Surface finishing and critical dimensions: a specification guide
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