Surface roughness measurement and control

Surface roughness describes the set of irregularities present on the surface of a component. These irregularities may result from processes such as turning, milling, grinding, lapping, electrical discharge machining or sandblasting, but they can also be relevant on painted, coated, plastic or other material surfaces.

Measuring the roughness of a surface makes it possible to objectively assess the quality of the finish and determine whether the part is consistent with its intended function, the technical drawing or the inspection procedure. Roughness can affect fit, sliding, friction, wear, sealing, coating adhesion and the aesthetic quality of the component.

Not all instruments for roughness control are equivalent: the choice depends on the type of surface, the production process, the required parameter and the purpose of the verification. A roughness tester measures profile parameters using a stylus; a sandblasting profile gauge checks the peak-to-valley depth created by the abrasive; a roughness comparator provides a visual and tactile comparison; a certifiable roughness test block is instead used to check or calibrate the roughness tester.

How is surface roughness measured?

Surface roughness is generally measured by analysing the surface profile over a defined evaluation length. In stylus roughness testers, the tip travels across the part and detects the irregularities present, allowing the instrument to calculate the required parameters.

This type of inspection is frequently used in the mechanical industry to assess turned, milled, ground or polished surfaces, but it is not limited to metal components. The roughness tester measures the surface profile and can therefore be used on different materials, provided that the geometry and characteristics of the part allow correct stylus movement.

When the surface has been produced by sandblasting, the inspection may instead serve a different purpose: verifying the profile created by the abrasive before applying a paint or protective coating. In this case, dedicated blast-profile instruments or Grit and Shot visual and tactile comparators are used.

Surface roughness testers

Roughness testers are instruments used to measure surface finish through comparable numerical parameters. They are suitable for inspections in production, laboratories, metrology rooms, incoming inspection and the analysis of potential non-conformities.

The range includes different solutions according to the level of analysis and practicality required:

– Portable digital roughness testers are suitable for fast and repeatable checks on accessible surfaces, with different models available according to the parameters to be analysed and the required measuring range.

– A roughness tester with an external probe is suitable when the stylus needs to be positioned in a less accessible area or when it is useful to keep the display separate from the measuring point, for example on a stand, inside pipes, cavities or particular geometries.

– A compact roughness tester is designed for rapid measurements on accessible external surfaces, including vertical, inclined or external cylindrical surfaces. The V-groove allows support on pipes and round parts, with measurement performed longitudinally along the axis of the component.

– A graphic roughness tester is the most complete solution for in-depth inspections: it allows profile visualisation, measurement of numerous advanced parameters, analysis through optional software and the use of dedicated styli for small holes, grooves or curved surfaces.

The choice of roughness tester must therefore take the actual application into account: rapid checks on external surfaces, measurements in difficult-to-reach areas, graphic profile analysis or more structured metrological verification.

Roughness parameters: Ra, Rz, Rq and Rt

The Ra parameter is one of the most widely used in roughness measurement because it represents the arithmetic mean roughness of the profile. It is a summary value that describes how much the surface deviates from a mean reference line and is often the first parameter considered for a general assessment of surface finish.

Rz makes it possible to observe the average height of the profile irregularities, while Rq provides a quadratic assessment of roughness and is more sensitive to the presence of pronounced peaks or valleys. Rt instead considers the total height of the profile over the evaluated length.

For more in-depth checks, additional parameters may be required and are available on more advanced graphic roughness testers. Parameters relating to peaks, valleys, spacing, bearing area or the functional behaviour of the surface are useful when the finish affects contact, lubrication, wear, sliding or sealing.

The choice of the correct parameter depends not only on the machining process carried out, but also on the technical specification, the function of the component and the level of control required.

Roughness in mechanical machining and other production processes

Roughness measurement in the mechanical industry is essential for checking the finish of components produced by turning, milling, grinding, boring or lapping. A non-conforming surface can affect assembly, the fit between parts, component life or the correct operation of the assembly.

However, roughness control does not concern only traditional mechanical machining. Surface finish can also be relevant on components produced by EDM, on treated surfaces, painted or coated parts and components made from materials other than metal.

For this reason, the category includes both numerical measuring instruments, such as roughness testers, and comparative references intended for specific processes, such as EDM comparison specimens or comparators for sandblasted surfaces.

Measuring the blast profile before painting

When checking sandblasted surfaces, the main requirement is not always to measure the typical parameters of a roughness tester, but to verify the peak-to-valley profile created by the abrasive. This profile is important before applying paints or protective coatings because it contributes to coating adhesion and must be consistent with the specified coating system.

An insufficient profile may not provide the required anchoring, while an excessively high profile can make it more difficult to cover the surface adhesion and must be consistent with the specified coating system.

An insufficient profile may not provide the required anchoring, while an excessively high profile can make it more difficult to cover the peaks adequately. For this reason, dedicated instruments for blast verification are useful in industrial surface-preparation and painting activities.

The digital sandblasting roughness gauge checks the depth of the peak-to-valley profile according to ASTM D4417 – Method B. The base of the instrument rests on the surface peaks, while the tip reaches the valleys, providing a numerical measurement of the resulting profile.

When the procedure instead requires a comparative check, Grit or Shot visual and tactile sandblasting comparators can be used. The Grit comparator is suitable for profiles generated by angular or sharp abrasives, while the Shot comparator is intended for surfaces produced with spherical abrasives, typical of shot blasting. According to ISO 8503-2, the comparative method is used on steel surfaces prepared to Sa 2½ or Sa 3.

Visual roughness comparators for mechanical machining and EDM

Surface roughness comparators provide a visual and tactile comparison of the finish obtained on the part. They do not provide an instrumental measurement, but allow the actual surface to be compared quickly with reference specimens.

For mechanical machining, a kit is available with references for lapping, grinding, boring, milling and turning. It is useful for identifying the approximate roughness level achieved and comparing surfaces with different Ra values.

For surfaces produced by EDM, a dedicated comparator according to VDI 3400 is also available. This solution is suitable for moulds, impressions, inserts, cavities and EDM-machined components, as the surface texture generated by the EDM process differs from that produced by conventional mechanical machining.

Comparators are useful in workshops, production and quality control when a surface-finish reference needs to be shared quickly or when a preliminary assessment is required before an instrumental measurement.

Test blocks for checking and calibrating roughness testers

Roughness tester test blocks are reference specimens intended for checking or calibrating stylus instruments. Unlike visual comparators, they are measured directly by the roughness tester to verify its response against a known value.

Test blocks compliant with ISO 5436-1 feature defined profiles and nominal values, useful for checking instrument performance within the roughness range of interest. Some versions also allow verification of stylus response, an important aspect because the condition of the tip can affect measurement results.

These specimens are suitable for metrology laboratories, quality departments and companies that manage roughness testers within documented procedures. When metrological traceability of the specimen is required, they can be supplied with a calibration certificate issued by an ACCREDIA-accredited laboratory.

How to choose an instrument for roughness measurement and control

The choice of the correct instrument starts with the type of inspection to be performed:

– To measure roughness parameters such as Ra, Rz, Rq and Rt on machined or finished surfaces, the appropriate solution is a stylus roughness tester.

– For rapid checks on accessible external surfaces, including directly in production, a portable or compact roughness tester is suitable.

– For measurements in holes, cavities, grooves, internal surfaces or difficult-to-reach points, a model with an interchangeable stylus or external probe is preferable.

– To analyse the profile, manage advanced parameters and document more in-depth checks, a graphic roughness tester is suitable.

– To verify the peak-to-valley profile of a sandblasted surface before painting, a digital profile gauge or a Grit or Shot comparator must be used, according to the specified inspection method.

– To compare the approximate finish of mechanical machining or EDM surfaces, visual comparison specimens are suitable.

– To check or calibrate a roughness tester, dedicated test blocks compliant with ISO 5436-1 are required.

Correct surface roughness measurement makes it possible to assess machining quality, control surface preparation, prevent application defects and select the most suitable instrument according to the process and the actual function of the component.