Surface Roughness Measurement

Contact Stylus Profilometry for Machined and Engineered Surfaces

Surface roughness is a key characteristic of an engineering surface and can directly influence friction, wear, sealing performance, fit, fatigue behaviour, lubrication retention and the performance of coatings or bonded interfaces. AECTL provides quantitative surface roughness measurement using contact stylus profilometry for machined, ground, polished and otherwise finished engineering components.

Measurements can be undertaken on suitable flat, cylindrical, recessed and other accessible surfaces using the STARR RTD-210 surface roughness tester. The instrument traces the surface with a diamond-tipped stylus and calculates standard profile parameters such as Ra, Rq, Rz, Rt, Rp, Rv and additional roughness parameters. Results can be assessed against engineering drawings, manufacturing specifications or client-supplied acceptance criteria.

The service is suitable for manufacturing quality control, component verification, condition assessment and failure investigation where surface finish may affect component performance.

STARR RTD-210 contact stylus surface roughness tester used for profile-based surface finish measurement. Image source: Test Equip / STARR Instruments. For the published page, an original photograph of AECTL’s own instrument in use is preferable.

What Is Surface Roughness?

Engineering surfaces are not perfectly smooth. Manufacturing processes such as turning, milling, grinding, honing, lapping and polishing leave a characteristic pattern of microscopic peaks and valleys. Surface roughness describes the fine-scale component of this surface texture and is evaluated from a measured profile after the appropriate filtering and evaluation settings are applied.

A stylus profilometer measures vertical deviations as the stylus traverses the surface. The measured profile is processed to separate the roughness component from longer-wavelength form or waviness, allowing recognised roughness parameters to be calculated.

Surface Roughness Parameters We Can Assess

The RTD-210 supports a broad set of profile parameters. The parameter selected for assessment should match the engineering drawing, specification or functional requirement rather than relying on Ra alone.

Parameter	What it represents	Typical use / interpretation
Ra	Arithmetic mean roughness of the roughness profile.	Widely used general indicator of surface finish and manufacturing consistency.
Rq	Root mean square roughness of the profile.	More sensitive than Ra to larger profile deviations.
Rz	Height-based roughness parameter describing peak-to-valley characteristics over defined profile sections.	Useful where local peak and valley behaviour is important; interpretation must follow the nominated standard.
Rt	Total height of the roughness profile over the evaluation length.	Highlights the overall peak-to-valley extreme within the measured trace.
Rp	Maximum profile peak height.	Useful where protruding peaks may affect contact, sealing or wear.
Rv	Maximum profile valley depth.	Can be relevant to lubrication retention and valley-dominated surfaces.
Rsk / Rku	Profile skewness and kurtosis.	Describe the distribution and shape of profile heights where more detailed texture characterisation is required.
Rmr and related material-ratio parameters	Material proportion / bearing characteristics of the profile at specified levels.	Useful for functional surfaces such as bearing, sliding and sealing interfaces.

Additional parameters available from the instrument include R3z, Ry, RSm, RMax, RPc and Rk-family parameters, depending on the selected measurement standard and instrument configuration.

Why Ra Alone May Not Be Enough

Ra is the most commonly specified roughness parameter, but it averages the profile and can therefore mask isolated peaks or deep valleys. Two surfaces can have similar Ra values while having different functional behaviour. Where sealing, sliding, lubrication, contact fatigue or wear is important, parameters such as Rz, Rt, Rp, Rv or material-ratio parameters may provide more useful information.

For this reason, AECTL reviews the specified parameter and the intended function of the surface before interpreting the result.

Applicable Standards and Technical Specifications

Surface roughness measurement should be linked to the drawing or specification that defines the required parameter, tolerance and measurement conditions. The current ISO 21920 series provides the modern framework for profile surface texture specification and interpretation.

ISO 21920-1:2021 / BS EN ISO 21920-1:2022 – specifies how profile surface texture requirements are indicated on technical product documentation using graphical symbols.
ISO 21920-2:2021 – defines terms, definitions and surface texture parameters for profile methods.
ISO 21920-3:2021 – specifies the complete specification operator for profile surface texture, including measurement/evaluation settings and acceptance rules.
The RTD-210 manufacturer also lists ISO 4287, ANSI B46.1, DIN 4768 and JIS B601 among the instrument’s supported standards.

ISO 4287 has been withdrawn and replaced by ISO 21920-2. Accordingly, the applicable standard should be confirmed for each project rather than assuming that an older drawing callout and a current ISO 21920 requirement are directly interchangeable.

Where a client supplies a drawing using an older surface texture notation or legacy standard, AECTL can review the nominated requirement and report the measurement basis used.

Measurement Direction, Sampling Length and Filtering Matter

Reliable surface roughness assessment depends on more than placing the stylus on the component and recording a number. The profile direction, sampling/evaluation length, filter settings, stylus geometry and the surface lay can materially influence the reported result.

For surfaces with a predominant lay, the measurement direction is normally selected to provide a representative profile, commonly across the machining marks unless the drawing or specification requires otherwise. The evaluation length and filtering are selected in accordance with the applicable requirement and the expected roughness range.

Localised scratches, dents, cracks and isolated surface damage should not automatically be treated as roughness. Where these features are relevant to the engineering question, they should be documented and assessed separately from the profile roughness result.

Our Surface Roughness Measurement Equipment

AECTL uses the STARR RTD-210 advanced surface roughness tester. Its separate stylus drive unit provides greater positioning flexibility than a single-body handheld gauge and assists with measurement of surfaces that are difficult to access.

Diamond-tipped contact stylus profilometer.
Separate motorised stylus drive unit for flexible positioning.
5-inch colour touchscreen display.
Measurement of roughness, waviness and contour profiles.
22 roughness parameters available from the instrument.
Vertical measuring range of ±160 µm (320 µm total).
5 mm transverse measuring range, with up to 20 mm travel.
Selectable sampling lengths of 0.25 mm, 0.8 mm and 2.5 mm.
Evaluation length selectable from 1 to 5 sampling lengths.
Profile filters including RC, PC-RC, Gaussian and ISO 13565.
Waveform display for visual review of the measured profile.
Internal result storage with USB/Bluetooth data transfer capability.
Adjustable stand for controlled stylus positioning.
Optional stylus arrangements are available for certain bores, grooves and restricted-access geometries.

The standard stylus is specified by the supplier as a 90° diamond tip with a 5 µm tip radius and approximately 4 mN stylus force. The suitability of the standard or optional stylus depends on component geometry and the measurement requirement.

Components and Surfaces We Can Measure

Subject to access, geometry and the required traverse length, typical applications include:

Machined shafts, pins and bearing journals.
Sealing faces and gasket contact surfaces.
Bearing seats and fitted surfaces.
Turned, milled, ground, honed, lapped and polished components.
Flanges and mating faces.
Rolls, sleeves and cylindrical components.
Selected internal bores and recessed features where stylus access is available.
Gears, stepped components and O-ring groove regions where geometry permits.
Metallic components before and after machining, grinding, polishing or repair.
Qualification coupons and manufacturing process trials.

Typical Applications

Manufacturing and Quality Control

Verify whether a machined or finished component satisfies the surface roughness requirement shown on an engineering drawing or manufacturing specification.

Sealing and Mating Surfaces

Assess surface finish where excessive roughness, machining marks or inappropriate texture may contribute to leakage, gasket damage or poor sealing performance.

Bearing and Sliding Surfaces

Evaluate surface texture on bearing journals, guide surfaces and other contact interfaces where roughness can influence friction, wear, lubrication behaviour and service life.

Repair and Refurbishment Verification

Check surface condition after machining, grinding, polishing, metal spraying or other repair operations against the specified final finish.

Failure Investigation

Use quantitative surface roughness data as part of an investigation where surface finish may have contributed to wear, seal failure, fretting, contact damage or premature component deterioration.

Process Comparison

Compare the surface finishes achieved by different machining, grinding, polishing, or preparation processes, and document the effects of process changes.

Laboratory and On-Site Measurement

Measurements can be performed in AECTL’s Sydney laboratory and, where access, stability and environmental conditions are suitable, on site. Laboratory measurement is generally preferred for small components because it provides better control of component support, stylus positioning and cleanliness.

For field work, the surface must be accessible and sufficiently stable to allow the stylus to traverse without external vibration or movement. Very small features, steep curvature, deep recesses or restricted-access locations may require a specialised stylus and should be reviewed before attendance.

Test Reporting

Depending on the agreed scope, AECTL reporting can include:

Component and measurement location identification.
Specified roughness requirement or client acceptance criterion.
Measured parameter(s), including Ra, Rz, Rt or other nominated values.
Sampling and evaluation settings where relevant.
Measurement direction relative to the surface lay where applicable.
Individual readings and summary results.
Photographic documentation of the measurement locations.
Comparison with drawing or specification requirements where provided.
Engineering comments where interpretation is required.

Surface Roughness as Part of a Broader Materials Assessment

Surface roughness measurement can be combined with complementary materials and dimensional examinations where the engineering question extends beyond surface finish alone.

Visual and microscopic examination.
Dimensional measurement and component inspection.
Hardness testing.
Metallography and microstructural examination.
Coating thickness and coating condition assessment.
Wear and damage evaluation.
SEM/EDS examination of wear debris, deposits or surface contaminants.
Failure analysis and root cause investigation.

Why AECTL?

AECTL approaches surface roughness as an engineering measurement rather than simply a gauge reading. The specified parameter, surface lay, measurement direction, component function and acceptance criterion are considered when planning and interpreting the work. This is particularly important where the result is being used to assess a functional surface, verify a repair or support a failure investigation.

Request Surface Roughness Measurement

For an accurate quotation, please provide the component type, material, drawing or specified roughness requirement, parameter to be measured, number of measurement locations and whether testing is required at the laboratory or on site. Photographs or drawings are particularly useful for bores, grooves, curved surfaces and restricted-access features.

Advanced Engineering Consultancy & Testing Laboratory (AECTL)
14 Garling Road, Kings Park NSW 2148
Phone: 02 8201 3560
Email: enquiries@aectl.com.au