A short CNC land should not receive the same roughness conformity claim when its available straight surface cannot support the specified measurement traverse. Treat this as a drawing-and-metrology issue, not a reason to shorten a cutoff until a result passes. Confirm the intended parameter, standard, filtering, trace length, direction, and edge treatment, then obtain a documented engineering-approved measurement exception if needed.
For broader background on symbols and parameter selection, see surface roughness parameters and drawing symbols. This article focuses on the narrower approval problem: a specified line-profile roughness trace that will not physically fit on a short CNC land.
Why a short land creates a conformity problem
A CNC land is a deliberately flat, cylindrical, or otherwise functional surface band between features. Common examples include a shoulder beside a bore, a narrow seating face, a locating step, a small annular contact area, or a ground transition surface. Its usable measurement length may be much shorter than its visible width once chamfers, blend radii, burr-removal zones, reliefs, and excluded edges are removed.
A stylus does not evaluate a roughness number in isolation. It travels along a line, records a profile, and processing separates portions of that profile for the selected parameter. The physical traverse can also extend beyond the data ultimately reported. That extra path may be needed for instrument motion and processing at the beginning and end of the usable profile. Digital Metrology describes this practical distinction between evaluation data and a longer actual tracing requirement, and cautions that a shorter cutoff changes what is being defined as roughness. Digital Metrology
This produces a familiar dispute. The supplier sees a visually smooth shoulder and reports an Ra value from a short local trace. The customer expects a drawing callout to be verified with a different cutoff, filtering setup, evaluation length, or traverse. Both reports may show a number in the same units, but they may not describe the same measurand.
The core question is therefore not, “Can any instrument produce a value on this land?” It is, “Can this feature support the specified measurement definition and physical measurement path without a documented departure?”
Separate the profile, filter, lengths, and traverse
Clear terminology prevents an approval note from hiding an important change.
Measured or traced profile
The measured profile, also called the traced profile in line-profile terminology, is the raw path acquired along the selected direction on the part. It depends on where the trace begins and ends, how the part is aligned, the probe or optical method, local geometry, and whether edges or adjacent surfaces enter the acquisition.
A trace parallel to machining lay can give a different result from one across it. That is not automatically an error; it means the trace direction must be defined when direction is functionally important. Surface texture lay is separate from filter selection.
Filtering separation
Filtering separation is the processing step that separates profile content into categories relevant to the selected analysis, such as roughness and longer-wavelength form or waviness content. It is not a cosmetic cleanup step. Changing the separation settings can change the reported parameter, even when the physical surface is unchanged.
Cutoff or nesting index
A cutoff or nesting index identifies a scale used in the filtering and evaluation convention. Terminology and notation depend on the governing standard and instrument workflow. It must be written in the measurement plan rather than inferred from an Ra value alone. The Keyence terminology page explains, in a JIS B 0601 context, that sampling length for a roughness profile is associated with its roughness cutoff. That explanation is useful vocabulary, not a universal ISO 21920 prescription. KEYENCE America
Evaluation length and physical traverse
Evaluation length is the section of processed profile used to calculate and report the parameter. Physical traverse is the actual travel needed to acquire valid data. They are distinct. A feature might appear long enough for an evaluation region but still be too short once run-in, run-out, edge exclusion, and instrument behavior are considered.
This is why an inspection report should never merely say “Ra measured.” It should show enough information for a qualified reviewer to understand what was measured and whether the measurement matches the drawing intent.
The feature-length review should start at the drawing
Before a shop selects a roughness instrument, the drawing owner should identify the exact controlled region. A surface symbol pointed generally at a shoulder can leave open whether the requirement applies across the whole face, only on a contact band, or up to an edge break. For a short land, that ambiguity becomes a measurement risk.
Ask these questions during drawing review:
- Is the controlled area a complete surface, a defined band, or a local functional zone?
- Are chamfers, fillets, reliefs, burr-removal areas, and intersection lines excluded?
- Is the callout Ra, Rz, another profile parameter, or an appearance requirement mistakenly expressed as roughness?
- Which standard and edition govern parameter definition and verification?
- Does the drawing specify a filter, cutoff or nesting index, evaluation or section length, and trace direction?
- Does the functional requirement actually require line-profile roughness, or is another verified surface-characterization approach intended?
Do not answer missing information by mixing legacy ISO 4287/4288 or JIS conventions with ISO 21920 defaults. These frameworks may use related language, but a buyer should not assume their default rules are interchangeable. The design authority needs to state the governing basis, particularly where the usable feature length is constrained.
What to send the design authority when the trace will not fit
A useful escalation is a compact engineering package, not a vague request for permission to “inspect differently.” It should make the limitation visible and preserve the decision trail for future production and incoming inspection.
Include the drawing revision and the exact surface identifier. Provide a marked-up view showing the available straight, uninterrupted functional surface; identify excluded radii, chamfers, edges, and neighboring faces. State the proposed trace direction relative to the lay and datum scheme.
Then state the measurement requirement exactly as received: parameter and units, standard and edition if given, filtering settings, cutoff or nesting indices, evaluation or section length, and any stated trace-location rule. State the planned method too: stylus or optical extraction, tip or method details, reference artifact and calibration information, and the instrument’s relevant setup. Do not imply that an optical result and a stylus result are automatically interchangeable merely because each produces an Ra value.
Finally, document the gap: the available usable surface length versus the physical traverse required by the specified setup. Avoid turning that comparison into an unsupported universal numerical rule. Its purpose is to show why the original method cannot be applied on this geometry, not to create a new default acceptance threshold.
A short, hypothetical example illustrates the review. A drawing calls out Ra on a narrow milled shoulder but does not identify a trace direction or filtering condition. The manufacturing team determines that a conventional line trace extending only across the unobstructed shoulder would run into both edge-break zones. The correct escalation is to identify the surface limits and request a controlled plan. It is not to choose a shorter cutoff because that produces a more convenient trace or a passing value.
Options need approval, not automatic equivalence
Once the design authority sees the constraint, it may retain the original requirement by changing the part design or defining a larger accessible witness area. It may define a reduced or otherwise altered measurement plan for that particular feature. It may also choose another validated characterization method if the surface function and governing specification support it.
Each option has a different consequence:
- Redesign or accessible witness geometry preserves the intended measurement definition most directly, but can affect the part and assembly.
- A feature-specific measurement exception may be practical when the functional region is genuinely short, but it must state exactly what differs from the original verification requirement.
- A different method or extracted profile approach can answer a relevant engineering question, but does not automatically prove equivalence to the original line-profile requirement.
- A changed acceptance requirement belongs to the design authority. It should be revised in the drawing, specification, deviation record, or approved inspection plan.
Digital Metrology discusses software-oriented alternatives for short surfaces, including shorter analyses and different processing choices. Those examples demonstrate why method details matter; they are not universal current ISO 21920 recipes. In particular, do not assume spline filtering, a primary-profile Pa substitution, or reduced evaluation length is automatically equivalent to the called-out roughness requirement. Digital Metrology
The same caution applies to a report based on all available length. It may be technically informative, but it is a different decision from claiming unmodified conformity. Surface geometry, parameter definition, and the governing standard require qualified metrology review.
A numbered approval workflow for a documented exception
- Freeze the interpretation. Record the drawing revision, surface callout, parameter, units, and the stated standard or notes. If essential conditions are absent, label them as unresolved rather than silently supplying defaults.
- Map the usable surface. Mark the functional land, excluded transitions, nominal trace direction, and the true uninterrupted region available after edge exclusions.
- Assess physical feasibility. Compare the available region with the required acquisition path for the proposed method, including the physical traverse rather than only the reported evaluation length.
- Prepare the measurement brief. Specify filter settings, cutoff or nesting indices, evaluation or section length, tip or optical method, trace location and direction, reference/calibration data, and treatment of edges.
- Escalate to the design authority. Present the shortfall, feasible alternatives, and their limitations. Do not present a changed cutoff as a pass-making adjustment.
- Obtain written technical approval. The approved deviation should identify whether it changes the method, analyzed length, parameter, location, acceptance criterion, or drawing requirement. It should also say whether the result is an informational characterization or a conformity decision.
- Lock the report template. Link the approved plan to the part revision and inspection record so later lots do not revert to an undocumented method.
- Reopen review on change. Reassess if the toolpath, finishing process, material condition, feature geometry, revision, or functional use changes.
Compact buyer checklist for the RFQ or deviation request
- Identify the land on the drawing and show its usable straight region.
- State Ra, Rz, or the required parameter; do not use the terms interchangeably.
- Name the governing standard and edition.
- Specify units, filter settings, cutoff or nesting indices, and evaluation or section length.
- Define trace direction, trace location, and edge inclusion or exclusion.
- State stylus or optical method, relevant tip/method details, and reference/calibration expectation.
- Ask for a documented exception before production if the specified physical traverse does not fit.
- Preserve the approved plan with the revision-controlled inspection requirements.
For drawing-defined parts, SUUXIANG can use these inputs during DFM and inspection planning. The related surface roughness inspection sourcing guide can help organize the requirement before quotation.
FAQ
What are the different methods for measuring surface roughness?
Common approaches include contact stylus measurement and non-contact optical profile extraction. The suitable choice depends on the parameter, geometry, access, material response, traceability needs, and governing specification. A method change requires review; outputs with the same parameter name are not automatically interchangeable.
Does an Ra limit define a complete short-land inspection plan?
No. The limit and units do not establish filtering, trace direction, evaluation length, or edge exclusions. Resolve those conditions before deciding whether the physical trace fits.
Can an optical measurement replace an inaccessible stylus trace?
Only under a technically reviewed and approved plan. The measurement principle, acquired profile, and processing may differ; the same parameter label does not establish equivalence.
What is the difference between Ra and Rz?
Ra is an average-based profile parameter; Rz is a different parameter with a different response to profile features. They should not be substituted for one another unless the design authority revises or explicitly approves the requirement.
Can a supplier shorten the cutoff to measure a small shoulder?
Not simply to obtain a pass. Shortening the cutoff changes the filtering definition and can change the result. If a modified plan is appropriate, it needs qualified engineering approval and a documented deviation from the original requirement. Digital Metrology
If a roughness trace will not fit your short land, discuss your project requirements with the drawing revision, controlled surface markup, and intended inspection conditions before releasing the part.