Surface Roughness Measurement

Have you ever wondered why your phone screen feels smoother than a rough piece of sandpaper?

Or why some car paints appear to have a mirror-like finish while others look dull and bumpy?

The answer lies in surface roughness, a crucial factor that affects the performance, appearance, and durability of various products.

From aerospace components to medical implants, manufacturers need to accurately measure and control surface roughness to ensure quality, reliability, and safety.

In this article, I'll explore the science and techniques of surface roughness measurement, and why it matters more than you might think.

So, buckle up and get ready to dive into the fascinating world of surface roughness.

Surface roughness measurement is the process of measuring the small-scale variations in the height of a physical surface. It is different from larger-scale variations such as form and waviness, which are typically part of the geometry of the surface.

Surface roughness can be regarded as the quality of a surface of not being smooth and is linked to human perception of the surface texture.

It is a multiscale property that has different interpretations and definitions depending on the discipline considered.

Surface roughness can be measured by manual comparison against a "surface roughness comparator" or a surface profile measurement is made with a profilometer. Optical measurement instruments such as white light interferometers or laser scanning confocal microscopes can also measure the surface roughness over an area.

Areal roughness parameters are defined in the ISO 25178 series, and the resulting values are Sa, Sq, Sz, (along with others).

Average roughness (Ra) is a common parameter used to measure surface roughness, and it measures the deviation of a surface from a mean height.

It is typically measured in microns or micro-inches.

Importance in Dimensional Measurement

Surface roughness measurement is important in dimensional measurement for several reasons:

• Interaction between parts: Surface roughness often dictates how one part interacts with another. For example, if a shaft is rotating inside a bearing, a rough surface is more likely to cause metal-to-metal contact, which can lead to wear and corrosion.
• Preventative adjustments: Capturing data on surface roughness allows for analysis of trends and helps in preventative adjustments. For example, measuring roughness average (Ra) can show when a tool starts wearing out, resulting in it producing different surface characteristics. This information can then be used to decide when a tool change is necessary.
• Friction and adhesion: Roughness plays an important role in various processes such as friction and adhesion, and is widely measured.
• Wettability: The relationship between roughness and wettability is well-defined, and adding surface roughness can enhance the wettability caused by the chemistry of the surface.
• Characterization of surface texture: Surface roughness is a measurement of surface texture, and it is defined as a vertical deviation of a real surface from its ideally smooth form. Surface roughness cannot be accurately characterized by using a single parameter, so a set of surface roughness parameters is defined.
• Quantitative and qualitative methods: Surface roughness measurement can be characterized using either quantitative or qualitative methods. Qualitative techniques include optical appearance, while quantitative methods involve measuring the small-scale variations in the height of a physical surface.

Methods of Surface Roughness Measurement

Surface roughness measurement can be characterized using different methods:

• Direct Measurement Methods: These methods assess surface finish by means of stylus type devices. Measurements are obtained using a stylus drawn along the surface to be measured, and the stylus motion perpendicular to the surface is registered. This registered profile is then used to calculate the roughness parameters.
• Optical Method: This method involves the use of light to measure surface roughness. For example, a light source is used to illuminate the surface with a digital system to view the surface and the data.
• Fluid Methods: Fluid methods use the flow of a fluid over the surface to measure roughness. For example, the flow of a liquid over a surface can be used to determine the roughness of the surface.
• Electrical Method: Electrical methods use the electrical properties of a surface to measure roughness. For example, the electrical resistance of a surface can be used to determine the roughness of the surface.
• Scanning Probe Microscopy Methods: These methods use a scanning probe microscope to measure surface roughness. The microscope scans the surface of the object to be measured and records the height of the surface at each point. This information is then used to calculate the roughness parameters.
• Geometric Analysis: Geometric analysis involves the use of mathematical models to analyze the surface roughness. For example, reference lines, envelope methods, digital filters, fractals, or other techniques can be used to obtain the surface characteristic.
• Non-Contact Type: Non-contact methods do not involve actual contact with the equipment parts on the surface of the object to be measured. For example, optical methods, fluid methods, and electrical methods are non-contact methods.
• Electron Microscopy Methods: Electron microscopy methods use an electron microscope to measure surface roughness. The microscope scans the surface of the object to be measured and records the height of the surface at each point. This information is then used to calculate the roughness parameters.

Measurement Techniques

Different techniques are used based on the physical principle to assess the nanoscale, atomic scale, and microscale features of surface roughness. Profiling, area, and microscopy techniques are the three primary method types used for taking surface roughness measurements.

Surface roughness can be measured using contact methods such as stylus-type devices or replica blocks. Here are some methods and techniques used to measure surface roughness:

• Stylus-type device: This is a direct measurement method that calculates the average roughness value by tracing the surface with a stylus-type instrument. The instrument amplifies its signal to compensate for waviness and indicate only roughness.
• Replica blocks: These are used in comparison measurements and contain a specific standard roughness pattern.
• Profilometer: This is a contact measurement system that uses a diamond stylus to measure surface roughness.
• Areal roughness parameters: These parameters are defined in the ISO 25178 series and include Sa, Sq, and Sz.
• Optical methods: Optical methods include white light interferometers and laser scanning confocal microscopes. These instruments are able to measure the surface roughness over an area.

Surface roughness can also be characterized using either quantitative or qualitative methods. Qualitative techniques include optical appearance such as the fingernail test. By employing measures of surface fractality together with measures of roughness or surface shape, certain interfacial phenomena including contact mechanics, friction, and electrical contact resistance can be better interpreted with respect to surface structure.

Non-contact methods are used to measure surface roughness. Here are some examples:

1. Spatial Light Modulator: A novel non-contact surface roughness measurement method that combines the advantages of different types of methods, using a Michelson setup.
2. Laser Profilometry: A non-contact method for measuring the roughness of surfaces. However, it can be challenging to obtain accurate measurements of surface roughness parameters using a contact-free method for surfaces with high gloss.
3. Optical Appearance: Qualitative techniques include optical appearance such as visual inspection, which can be used to determine surface roughness.
4. Depolarization Effects: A non-contact method that considers depolarization effects to measure surface roughness in the sub-micron range.
5. Interpenetration of Rough Surfaces: An indirect method using the effect of interpenetration of rough surfaces that enables very simple measuring devices to be constructed.

Please note that there are also contact-based methods for measuring surface roughness.

Why Metrology Matters in Surface Roughness Measurement

When it comes to measuring the surface roughness of an object, precision is key. That's where metrology comes in. Metrology is the science of measurement and plays a crucial role in ensuring accuracy and consistency in dimensional measurement.

By using specialized tools and techniques, metrologists can measure surface roughness down to the nanometer level, providing invaluable data for industries such as manufacturing, aerospace, and biomedical engineering.

Without metrology, surface roughness measurements would be unreliable and inconsistent, leading to potential errors in product design and quality control.

So, the next time you see a surface roughness measurement, remember that behind it lies the science of metrology, ensuring accuracy and precision in dimensional measurement.

For more information:

Discovering Metrology, Units, Instruments and More

Units and Standards

The units used to express surface roughness measurements are typically either microns (µm) or micro-inches (µ-in, µ”). One micron is roughly equivalent to 40 micro-inches. The terms "micron" and "micrometer" are equivalent and both are commonly used.

Areal roughness parameters are also defined in the ISO 25178 series, with resulting values such as Sa, Sq, and Sz.

The National Measurement System for surface finish uses physical reference standards to calibrate surface roughness measurements to the defined unit of length: the wavelength of certain light sources.

Surface roughness measurements can have a significant impact on the functionality of a product. Here are some ways that surface roughness measurements can affect product functionality:

• Bearing surfaces: Many bearing surfaces require a uniform pattern of roughness which helps to retain a film of lubrication. If the surface is too smooth or too rough, the bearing would fail.
• Quality parameters: In engineering applications, there are tight quality parameters for surfaces and parts. Therefore, it is crucial that the roughness of a surface is measured accurately so it can adhere to the required standards in quality. Roughness is often undesirable, but it is difficult to control in manufacturing. Decreasing the roughness leads to the increased manufacturing cost of components, so there must be a trade-off between this cost and its performance application.
• Human perception: Surface roughness can be regarded as the quality of a surface of not being smooth and it is hence linked to human (haptic) perception of the surface texture. From a mathematical perspective, it is related to the spatial variability structure of surfaces, and inherently it is a multiscale property. It has different interpretations and definitions depending on the disciplines considered.
• Performance: The size and configuration of features have a significant influence on the quality and functionality of processed surfaces and the performance of the final products. Consequently, please measure the roughness of surfaces to meet high performance standards for resulting end products. The level of roughness must be managed based on the desired quality and performance of the surface.

Surface roughness can be measured by manual comparison against a "surface roughness comparator" or by a surface profile measurement with a profilometer. The ISO standard for surface roughness measurements is a 60° or 90° conical stylus with a spherical tip of 2μm radius.

Proper surface feature analysis identifies the possible imperfections within the material which, if carried out to a high enough standard, can make the difference between a usable product and one that will be discarded and can affect project critical factors such as cost and material usage as well as safety for the operator of the finished product.

Surface roughness is an important aspect of dimensional measurement. Here are some industry standards and methods for measuring surface roughness:

• Surface roughness comparator: A sample of known surface roughness can be used for manual comparison.
• Profilometer: A surface profile measurement can be made with a profilometer, which can be of the contact variety (typically a diamond stylus) or optical (example: a white light interferometer or laser scanning confocal microscope).
• ISO standards: The profile roughness parameters are included in BS EN ISO 4287:2000 British standard, identical with the ISO 4287:1997 standard. The area roughness parameters are defined in the ISO 25178 series.
• A2LA accredited Surface Roughness Measurement: Dimensional Measurement, Inc. (DMI) offers A2LA accredited Surface Roughness Measurement, Calibration (2D Surface Analysis) for parts.
• Average roughness (Ra): Ra measures the deviation of a surface from a mean height. It is typically measured in either microns (µm) or micro-inches (µ-in, µ”).
• Areal roughness parameters: These parameters give more significant values than profile roughness parameters.

Please note that there is no standard available for atomic force microscopy (AFM).

Improving Manufacturing Processes

Surface roughness measurements are important in manufacturing processes because they can help improve the quality of parts and products. Here are some ways that surface roughness measurements can be used to improve manufacturing processes:

1. Quality control: Measuring surface roughness is vital to quality control of machining workpieces. Surfaces in manufacturing applications must remain within desired roughness limits to ensure the optimum quality of parts.
2. Performance prediction: Surface roughness is an excellent predictor of mechanical part performance because irregularities on the surface can produce nucleation sites for fractures or corrosion. In tribology, rough surfaces wear faster and have greater friction coefficients than smooth surfaces.
3. Adhesion promotion: Roughness may be needed in some applications to facilitate adherence to cosmetic finish coatings such as plating, powder coating, or painting.
4. Contamination prevention: High purity manufacturing requires smooth surfaces within the processing equipment to avoid contamination or build-up within it.
5. Uniform procedures: Surface roughness must be maintained at all times by engineers and manufacturers to aid in the production of uniform procedures and trustworthy goods.
6. Compliance with industry standards: Surface roughness measurements can be used to determine compliance of equipment with various industry standards.

Limitations of Surface Roughness Measurement

Surface roughness measurement techniques have some limitations:

1. Lack of standardization: The main limitation of some surface roughness measurement methods is the lack of a standardized method for evaluation. This can make it difficult to compare results obtained using different techniques.
2. Limited accuracy: The real surface geometry is so complicated that a finite number of parameters cannot provide a full description. If the number of parameters used is increased, a more accurate description can be obtained. However, this is not always feasible due to practical limitations.
3. Filtering: To obtain the surface characteristic, almost all measurements are subject to filtering. It is one of the most important steps in surface roughness measurement. However, filtering can also introduce errors and distortions in the measured data.
4. Limited range: Some measurement techniques have a limited range of measurement. For example, some methods are only suitable for measuring roughness on a small scale, while others are only suitable for measuring roughness on a large scale.
5. Dependence on wavelength: The measured roughness parameters are dependent on the limitations of the short and long wavelengths. These considerations are not just a result of the measurement technique, but also of the physical properties of the surface being measured.
6. Dependence on the technique: Different techniques are used to assess the nanoscale, atomic scale, and microscale features of surface roughness. Each technique has its own limitations and is suitable for measuring specific types of surfaces.

Despite these limitations, surface roughness measurement remains an important tool in various industries for ensuring quality and performance of products.

Concluding thoughts and considerations

As I wrap up this post on surface roughness measurement, I can't help but feel confused by the intricacies of dimensional measurement. It's fascinating to think about the various methods and technologies used to measure the roughness of a surface, and how small variations in texture can have a significant impact on the functionality of a product.

But what really stands out to me is the potential for surface roughness measurement to improve manufacturing processes. By accurately measuring the roughness of a surface, manufacturers can identify areas for improvement and make adjustments to optimize their production. This not only leads to higher quality products, but also reduces waste and saves money in the long run.

However, it's important to consider the impact of surface roughness measurements beyond just the manufacturing process. Surface roughness can also affect the performance of products in various industries, from aerospace to medical devices. By understanding the impact of surface roughness on these industries, we can continue to develop new and innovative ways to measure and improve surface texture.

In conclusion, surface roughness measurement is a fascinating and complex topic with far-reaching implications. As technology continues to advance, I'm excited to see how we can apply these measurements to improve our manufacturing processes and enhance the performance of our products. So the next time you pick up a product, take a moment to appreciate the precision and attention to detail that goes into measuring its surface roughness.

Understanding Metrology Measurement Units

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Links and references

1. springer.com
2. toolingu.com
3. mdpi.com
4. wikipedia.org
5. psu.edu
6. azom.com

Recording for myself: (Article status: plan)