Surface roughness is not a visual adjective such as “smooth.” It is a measured description of surface texture under stated conditions. A useful CNC drawing identifies the parameter, limit, units, controlled surface and applicable standard. When function depends on the result, it should also define where and how acceptance will be checked.
For quotation, a practical roughness requirement answers six questions:
Which surface or zone is controlled?
Which parameter is specified—Ra, Rz or another defined parameter?
What limit and units apply?
What process state applies: as-machined, before coating or after finishing?
What measurement direction, cutoff or specification operator applies?
What inspection evidence is required and on how many parts?
This guide explains those questions. It does not publish a universal machining-capability chart or guarantee a roughness value for an unreviewed part.
Ra and Rz answer different questions
ISO 21920-2 defines surface-texture profile terms and parameters. NIST calibration work separately identifies Ra, Rq, Rz, Rt, Rp, Rv and RSm, which is a useful reminder that “roughness” is not one interchangeable number.
Parameter | Plain-language role | Buyer caution |
|---|---|---|
Ra | Arithmetic mean of the absolute profile-height deviations over the evaluated roughness profile | An average can hide isolated peaks or valleys; Ra alone may not describe the functional risk |
Rz | A profile-height parameter that gives greater attention to peak-to-valley behavior under the governing definition | Rz definitions have changed across standards and editions; state the governing standard rather than relying on the letters alone |
Rq | Root-mean-square profile height | Do not treat it as a renamed Ra value |
Rt | Total height over the evaluation length | It is sensitive to the highest peak and deepest valley in the evaluated profile |
RSm | Mean spacing of profile elements | Spacing can matter when texture direction or periodic tool marks affect function |
Ra and Rz are not safely interchangeable through a fixed conversion factor. Two surfaces can have a similar Ra and different peak structure, lay or isolated defects. For acceptance, measure the parameter the drawing specifies under the agreed method.
A useful “surface finish chart” is a specification chart
Many online charts associate a roughness number with a machining process. Those ranges can be useful discussion prompts, but they are not universal supplier guarantees. Material, tool geometry, feed, speed, tool condition, machine dynamics, workholding, feature access and measurement settings all affect the result.
For procurement, this chart is safer:
Drawing decision | What to state | Why it matters to quotation |
|---|---|---|
Controlled area | Exact face, bore, sealing land, track or bounded zone | Prevents a local functional requirement from being priced across the whole part |
Parameter | Ra, Rz or another parameter defined by the named standard | Avoids comparing different mathematical descriptions |
Limit and units | Numerical limit plus µm or µin | Prevents a unit conversion from changing the requirement |
Process condition | As-machined, before finish, or after finish | Coating, blasting, polishing and handling can change the measured surface |
Lay or direction | Required direction when function depends on texture orientation | Sealing, sliding and cosmetic behavior can depend on direction |
Measurement conditions | Governing standard and any required cutoff, filter, evaluation length or location | Different settings can produce non-comparable results |
Evidence | Sample size, recorded values, report format and any traceability requirement | Lets inspection time and reporting scope enter the quote |
Converting micrometres and microinches
The unit conversion is exact enough for drawing preparation:
1 µm = 39.3701 µin
1 µin = 0.0254 µm
Conversion changes the unit, not the parameter or the measurement method. Writing Ra 0.8 µm as approximately Ra 32 µin does not authorize conversion to Rz, change the cutoff or establish a process capability.
Measurement conditions are part of the result
A stylus or optical system does not report a context-free truth. The measured value can depend on profile filtering, sampling length, evaluation length, stylus geometry, measurement direction, surface location and the condition of the part.
The measurement path should normally cross the dominant lay when the governing method requires it. Small lands, curved surfaces, holes and interrupted features may not provide enough travel for a default evaluation length. If access is limited, agree the method before production instead of measuring wherever the probe happens to fit.
For a functional surface, ask:
Where will the trace be taken?
Is the texture periodic or non-periodic?
Which cutoff and evaluation length apply?
Is the result an average of several traces or a maximum rule?
Will the surface be clean, coated and in its delivered state?
What uncertainty or decision rule applies near the limit?
NIST’s calibration documentation explicitly treats measurement conditions and uncertainty as part of surface-roughness metrology. A reported number without the required context may not be comparable with the drawing requirement.
Surface roughness is not the same as a finish treatment
Roughness describes texture. Anodizing, plating, polishing, passivation and blasting are processes or treatments. One part may require both a roughness value on a sealing face and a coating specification on another zone.
The sequence matters. A drawing should state whether the roughness requirement applies before or after the treatment. It should also identify masking, coating thickness, post-finish dimensions and protected functional surfaces.
Use the CNC machining surface finishes guide to compare anodizing, plating, polishing and blasting as purchasing decisions. That page remains the service owner for treatment selection; this guide is limited to drawing and measurement preparation.
Do not apply the tightest value everywhere
A blanket roughness requirement can add finishing and inspection work to faces that do not need it. Start from function:
sealing faces may need a defined texture and lay;
sliding or bearing interfaces may need texture compatible with lubrication and wear;
cosmetic surfaces need a visual acceptance rule as well as any measurable requirement;
adhesive or coating preparation may depend on the specified process, not only Ra;
non-functional clearance faces may not justify the same control.
Identify the functional surface and the failure mode the requirement is intended to prevent. The supplier can then review whether the proposed machining and finishing route is practical without silently relaxing the design.
How roughness affects a CNC quotation
A controlled roughness requirement can change tooling, finishing passes, access strategy, setup stability and inspection time. The impact is usually greater when the surface is deep, interrupted, curved, very small, difficult to probe or controlled after coating.
Include these items in the RFQ:
controlled drawing revision and matching model;
exact material grade and condition;
roughness parameter, value, units and controlled surfaces;
governing standard and measurement conditions;
finish or coating sequence;
critical dimensions that apply after finishing;
sample quantity and report expectations;
approved appearance reference when cosmetic consistency matters.
The CNC machining tolerance guide explains the related dimensional-planning questions. The quality and inspection overview helps distinguish a requested result from the evidence needed to accept it.
Surface roughness RFQ checklist
[ ] The exact surface or zone is identified.
[ ] The parameter and governing standard are named.
[ ] The numerical limit and units are unambiguous.
[ ] The requirement’s before/after-finish condition is stated.
[ ] Lay or measurement direction is defined when functional.
[ ] Cutoff, filter or evaluation rules are defined when required.
[ ] Access and trace length are practical for the feature.
[ ] Sample size and recorded-value requirements are stated.
[ ] Cosmetic approval is not being substituted for metrology.
[ ] The requirement is limited to surfaces whose function justifies it.
When these decisions are ready, send the drawing, model and inspection requirements for quotation review. The accepted quotation should record any assumptions about process state, measurement access and reporting scope.
FAQ
Is Ra the same as surface finish?
No. Ra is one surface-texture parameter. “Surface finish” is often used more broadly for texture, appearance or post-machining treatments. A drawing should name the measurable parameter or the exact treatment instead of relying on the broad phrase.
Can Rz be converted to Ra?
Not as an acceptance rule. Empirical ratios vary with the process and surface profile. Specify and measure the parameter required by the drawing.
Does a lower Ra always mean a better part?
No. The correct texture depends on function. An unnecessarily low value can add cost and may not improve sealing, lubrication, coating adhesion or appearance.
Should roughness be measured before or after anodizing or plating?
The drawing and purchase requirement should state the required condition. Finishing can alter texture and dimensions, so the process and inspection sequence must be agreed before quotation.
Editorial sources
These references support terminology and measurement context. They do not establish VOGUC’s capability for a particular roughness, process, feature, material, inspection result, price or lead time.