Why “Ra 1.6” can still leave a machined surface open to dispute
Agree the functional face, measurement path and evaluation conditions before comparing roughness reports.
Consider a hypothetical machined cover with a sealing face marked “Ra 1.6.” The supplier reports a pass, but incoming inspection obtains a different result. One measurement followed the machining marks near the edge; the other crossed them nearer the centre, using different filter settings. The useful first step is to compare the drawing revision, measured locations, direction and instrument conditions—not to demand a smoother finish immediately. A roughness number becomes usable for acceptance when both sides can reproduce what it describes.
Start with the face and the requirement behind the number
Mark the actual sealing or mating face on the drawing. Establish which portion is subject to the roughness requirement and whether the note applies before or after a specified finishing or surface-treatment operation. A general drawing note, a local symbol and a supplier’s process assumption can otherwise describe different surfaces. Retain the unit and the meaning of the limit under the applicable drawing convention; do not infer the complete acceptance rule from “1.6” in an email.
Ask what the face must do in the assembly. For this cover, roughness conformity and sealing performance are separate questions. The mating part, seal, flatness, damage and assembly conditions may also need consideration. If a leak test is required, define it separately with the responsible designer. Making the Ra limit tighter without understanding the failure could add finishing work while leaving the cause of leakage unresolved.
Ra and Rz answer different questions about a profile
Ra summarizes the magnitude of profile deviations; it does not retain the complete shape of the measured line. A broadly textured surface and a surface containing a localized groove can therefore need different examination even when their Ra results are similar. Rz concerns profile height extremes, but its precise definition and evaluation must be tied to the stated standard. Mitutoyo’s explanation also warns that older uses of Rz can refer to a different parameter. [1]
Do not turn an Ra requirement into an Rz limit using a universal multiplier. If a supplier offers only Rz data against an Ra drawing, ask whether Ra can be measured under the agreed method. If an additional parameter or a scratch criterion is needed for function, the design authority should define and approve that requirement. Adding Rz retrospectively to reject a delivered batch changes the acceptance basis; it does not clarify the original Ra result.
A measurement line is only a small part of the face
Place the measurement track on a marked drawing or photograph and show its direction relative to the machining marks. A trace along a groove and a trace across it need not describe the same texture. Choose locations and directions that fit the applicable specification and the functional concern, then document them. Do not let each inspector choose whichever accessible line gives the preferred result.
On the hypothetical cover, an edge, a hole or a narrow sealing land may prevent a planned traverse. Ask the supplier to show the available track and explain the limitation before production acceptance. An accessible adjacent face is not automatically an acceptable substitute. If the intended measurement cannot be made, agree an appropriate alternative and its limitations with the technical authority rather than silently changing the location.
- 1Functional sealing or mating face
- 2Measurement track and traverse direction
- 3Profile and recorded evaluation conditions
Schematic only. The arrow identifies a measurement direction, not a universal required orientation. The profile is illustrative; it supplies no roughness value or acceptance setting.
Compare the conditions before comparing the result
For a disputed report, request the selected parameter and unit, standard and edition, filter settings, relevant length settings, probe or sensor details, measurement locations and direction, and the stage at which the part was measured. Keep individual traces or results where available. “Factory default” is not a sufficiently clear explanation when two instruments use different software or standards settings.
These fields are a practical way to make the comparison reproducible; this article does not prescribe numerical settings. The appropriate choices and defaults depend on the applicable specification and method. Probe access, surface cleanliness and instrument verification also belong in the investigation. If contact and optical measurements are compared, establish whether the methods are suitable for this surface and requirement before treating their numbers as interchangeable.
| Finding | What it leaves unresolved | Useful next evidence |
|---|---|---|
| Only “Ra: pass” is recorded | What it leaves unresolvedThe measured line and evaluation conditions are unknown. | Useful next evidenceA marked location/direction and a report with parameter, unit and conditions. |
| The same face was measured in different directions | What it leaves unresolvedThe measurements may describe different cross-sections of the texture. | Useful next evidencePaired results under an agreed direction and location plan. |
| Rz is supplied for an Ra drawing | What it leaves unresolvedThe specified parameter has not been demonstrated. | Useful next evidenceRa results under the agreed method, or an approved specification change. |
| Ra conforms but leakage occurs | What it leaves unresolvedSurface-texture conformity alone does not establish sealing function. | Useful next evidenceRoughness records alongside separately agreed functional and geometric checks. |
Read an older drawing under its own version context
ISO’s public catalogs identify ISO 21920-2:2021 as covering profile terms and parameters, and ISO 21920-3:2021 as covering specification operators. Their life-cycle entries link the earlier ISO 4287 and ISO 4288 documents respectively. [2] [3] This establishes the relationship between documents, not all the detailed rules needed to interpret a drawing.
An older contract or drawing may explicitly reference an earlier ISO or JIS edition. Do not replace that reference merely because the measuring instrument offers a newer standard menu. Check the drawing’s general notes, referenced specifications and agreed revisions. If the parties decide to migrate to ISO 21920, identify the affected indications and evaluation conditions, compare the consequences, and record the approved revision before using it for acceptance. A change of menu is not evidence that old and new results are equivalent.
Resolve the disagreement without polishing away the evidence
Retain the disputed parts and their identifiers, drawing revision and original reports. Before reworking the face, compare the two measurement methods on identified tracks and keep the conditions unchanged for the comparison. Record which difference is being investigated. If the reports still disagree near the limit, address measurement uncertainty and the agreed conformity decision rule with the quality and technical teams; the smaller displayed number is not automatically the correct one.
Once the cause is understood, settle a practical inspection instruction: marked face and locations, parameter and limit, applicable standard, method and settings, part condition, reporting requirements and the route for exceptions. Include any agreed sampling and functional tests separately. If rework is proposed, assess its effect on dimensions and function, authorize it, and inspect the resulting condition. The goal is a shared acceptance basis that the next batch can follow, not a one-off favourable reading.
Supplier inquiry to adapt
Open supplier questions
Please review the roughness requirement for [part number / drawing revision] on the marked [sealing or mating face]. The drawing states [parameter, value and unit]; our intended acceptance stage is [stage]. Please confirm the standard and edition used, interpretation of the limit, measured locations and traverse directions, filter and length settings, probe/sensor and instrument details, and the condition of the part during measurement. Please provide the marked measurement tracks and individual results or profile records where available. Our concern is [report disagreement / groove / functional issue]. If access or the specified method is unsuitable, please describe the limitation and propose an alternative for approval before changing the method or reworking the part. Please keep any proposed drawing change and functional test separate from the current roughness conformity result.
Sources and scope
- Mitutoyo — 表面粗さの基礎知識
Mitutoyo official educational page. Parameter and legacy-notation explanations are presented within the JIS/ISO versions shown on that page; its tables are not universal settings for this example.
- ISO 21920-2:2021 — official public catalog
ISO 21920-2:2021 official public catalog: scope and document life cycle only. The full standard was not reviewed.
- ISO 21920-3:2021 — official public catalog
ISO 21920-3:2021 official public catalog: scope and document life cycle only. Detailed defaults and acceptance rules are not reproduced or verified here.