Specifying Surface Roughness on Machined Parts
For design and quality engineers who need a roughness requirement that a machinist can make and an inspector can verify.
It covers what Ra and Rz describe, where the callout belongs and which conditions to state. It also explains why quotes and reports for the same callout can differ.
What a Roughness Value Describes and What It Leaves Out
A roughness parameter summarises a measured profile. A stylus or optical sensor traces the surface, a filter separates roughness from waviness and form, and a value is calculated over a defined length. Ra is the arithmetic mean of the absolute deviations of the profile from its mean line. Because it averages, Ra hides isolated scratches and peaks, and two surfaces with clearly different textures can report the same Ra.
Rz, in the ISO sense, is the height from the highest peak to the lowest valley within each sampling length, averaged over the evaluation length. It reacts to the features that cut a seal or concentrate wear. Older national standards used the name Rz for different calculations, and US drawings often state values in microinches rather than micrometres, so the parameter name and the unit are part of the requirement.
Related: Materials and surface finishing
Roughness Parameters and When Engineers Use Them
Choose the parameter from the job the surface does. The table lists commonly used profile and areal parameters, what each one responds to, and the kind of surface where a designer typically calls it out. The limit itself comes from your design analysis, the mating part and the applicable standard, not from this table.
| Parameter | What it responds to | Typical use on a drawing |
|---|---|---|
| Ra | Average deviation; insensitive to occasional peaks and scratches | General machined surfaces and many fits, where process control is the aim |
| Rz | Peak-to-valley height in each sampling length | Sealing faces, gasket lands and surfaces where a scratch or burr causes failure |
| Rq | Root-mean-square deviation; weights larger deviations more than Ra | Surfaces assessed with RMS-based optical, RF or electrical models |
| Rmr (material ratio) | How much material carries load at a given depth | Bearing, sliding and running surfaces |
| Sa, Sz (areal) | Comparable ideas measured over an area instead of one trace | Small, textured or delicate surfaces measured optically |
| Lay | Direction of the dominant tool marks | Seals and sliding contacts where mark direction affects leakage or wear |
RF surfaces
conductor-loss models commonly compare RMS roughness with skin depth, so an RF designer may prefer Rq on internal passages and contact faces.
Reading and Writing the Surface Texture Symbol
ISO 1302, now succeeded by ISO 21920-1, and ASME Y14.36 define a symbol placed on the surface, an extension line or a leader.
Parameter and limit, filter, method, lay and allowance positions around the symbol — concept illustration
The basic tick says a texture requirement applies and any process is permitted; a bar across it means material removal is required, and a circle means removal is not permitted. Fixed positions around it carry the parameter and limit, the filter or evaluation length when not the default, the production method, the lay direction and any machining allowance.
Write the parameter name explicitly. Under older conventions a bare number was read as Ra; current standards expect the designation, and an inspector should not have to guess. State the acceptance rule as well. Under the ISO 1302 and ISO 4288 conventions the default is the 16 percent rule, which accepts a surface when no more than that share of readings exceeds the limit; adding max to the parameter makes every reading count.
Name the standard
ASME and the newer ISO 21920 documents set their own defaults for parameters, filters and acceptance rules. A drawing note naming the governing standard removes the guesswork.
Put the Callout on the Surface That Does the Work
Most drawings combine a general note, which sets a default for every machined surface, with individual callouts on the faces that seal, locate, slide or carry current. Tightening the general note to suit one sealing face makes every surface on the part more expensive. Calling out the functional faces separately keeps the cost where the function is and tells the inspector exactly where to measure.
Sealing faces and O-ring grooves: often a peak-sensitive parameter plus lay, because a radial scratch can leak while Ra still passes.
RF contact and gasket faces: the faces that carry current between a cover and a housing, often with their own finish instruction.
Bores, pins and sliding fits: roughness alongside the size tolerance, since a rough bore wears in and loses its fit.
Internal passages: the requirement plus how it will be checked if a stylus cannot reach.
Cosmetic faces: a visual reference or sample, not only a value, when appearance is what you accept.
Related: RF shielding covers and lids · Waveguide components
Conditions, Units and Short Surfaces
State whether the requirement applies before or after finishing. Anodizing, plating and blasting all change texture: blasting replaces machining marks with a matte surface, and some anodic coatings roughen a fine machined face. If the functional surface is the finished one, the limit should apply after finishing, and the supplier needs to know so the machining pass leaves enough margin.
Check the measuring set-up as well. Standards select a default cutoff and evaluation length from the expected roughness, and measure across the lay unless told otherwise. A narrow land or a short face may be too small for the default length, so the drawing or the inspection plan should state the agreed alternative. Micrometres and microinches differ by a factor of roughly forty, so a unit mix-up is never a small error.
Related: Materials and surface finishing
One requirement per face
a general note and a local callout that use different parameters or units on the same face leave the inspector to choose. Make the local callout complete.
How Roughness Is Checked on Received Parts
A contact stylus profilometer measures a profile parameter along a trace and is the reference method in most drawing standards. Optical instruments measure an area and report areal parameters, which suits small or delicate surfaces. Comparator plates help an operator judge a surface by sight and touch, but a comparison is not a measurement and is weak evidence for a tight functional limit. For internal surfaces, replica compounds or measurement before assembly are common options. A useful roughness record states:
the face or callout measured, referenced to a balloon number on the drawing.
so the result can be compared with the requirement.
across the lay, and enough readings to apply the acceptance rule.
before or after finishing.
each with its unit stated.
Why Roughness Requirements Change Quotes
Roughness drives cost through extra finishing passes, slower feeds, different tooling, polishing or lapping, and inspection time. One quote may assume the local callouts can be met in the normal finishing pass; another may add a secondary operation. Compare whether each quote lists the surfaces with special texture requirements, the measurement method, and whether a measured roughness record is included or only a visual check.
If a supplier proposes a different parameter or a looser limit on one face, ask which function that face serves and resolve it with the drawing owner. A changed roughness callout belongs in the revision record, like any dimension, so later orders and other suppliers inherit the same requirement.
Related Pages
Continue with the closest related part family or guide.
Waveguide Drawing Requirements and Tolerances
Which waveguide features, datums, finish conditions and acceptance checks to define on the drawing before a machining quote.
Surface Finishing and Dimensional Requirements
How finishes change size, how to state before- or after-finish dimensions, threads and masking, and which records show the finished condition.
What to Look for in an Inspection Report
What FAI, CMM and dimensional reports, conformance and material certificates prove, and which fields to check before accepting parts.
Questions About Roughness Callouts
Is Ra enough for a sealing face?
Often not on its own. Ra averages the profile, so a face with a few deep radial scratches can report an acceptable value and still leak. Designers commonly add a peak-sensitive parameter such as Rz and specify the lay, for example circular marks on a face seal. Check the seal manufacturer's guidance and write the chosen parameters on that face.
Our drawing gives roughness in microinches. Is that a problem?
No, provided the unit is stated clearly and used consistently. The risk comes from conversions made by hand in emails, quotes or reports. State the unit in the general note, keep it on every callout and ask for reports in the same unit, so nobody converts values between the drawing and the inspection record.
Can comparator plates be used to accept parts?
They can support a visual check, for example on cosmetic faces or during machining, but they do not produce a measured value. Where the limit is functional, such as on a sealing face or an RF contact face, ask for an instrument measurement with the parameter, cutoff and location on the record. Agree the method before ordering so it is quoted.
How do we specify roughness on an internal passage?
Put the requirement on the surface as usual, then agree how it will be demonstrated. Options include measuring each half of a split part before assembly, measuring a replica, or qualifying the machining process on a sectioned sample. Each gives different evidence, so the drawing owner should choose the one that supports the design decision.
Discuss a Roughness Requirement
Send the drawing with the faces that carry special texture callouts marked, the finish applied afterwards and the record you expect.
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Drawings are optional for an initial discussion.
Start with the information you have; file exchange can be arranged afterward.
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- If you are unsure which parameter suits a face, describe its function and the mating part.
- A short description is enough to begin.
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