In product quality terms, one of the most important factors is the smoothness – or otherwise – of the surface of the product:
- For precise fits, quality of surface roughness defines some part of the fit (hole and shaft) tolerance
- For the end user, surface finish is one of the strongest quality indicators
- For moving parts, excess surface roughness will result in ‘bedding in’ wear that releases metal particles into the bearing that will then accelerate the failure process
Understanding how surface roughness is measured and reported is therefore critical in making good quality, long lasting products.
Surface roughness is a key factor in engineering manufacture. The definition of roughness describes the deviation of surface topography from an idealized surface.
The approach used precisely quantifies the irregularity or variation in the surface of a material or component. Surface roughness has huge effects on the performance and function of engineered products.
In engineering science and quality assessment, surface roughness is typically measured and characterized using specific parameters such as Ra, Rz, or Rmax (where R is Roughness).
Organizations such as ISO (International Organization for Standardization) and ASME (American Society of Mechanical Engineers) publish detailed standards for the measurement and characterizations.
These standards define the height (of peaks), depth (of troughs), and measured cyclic repetition frequency of surface features.
Surface roughness can have various effects on the performance of a component or material:
- In systems of moving parts, high surface roughness will increase friction and wear, leading to increased energy consumption and reduced lifespan of the component
- In manufacturing/assembly, excess surface roughness can affect the quality and accuracy of machined or formed parts
- In surface finishing processes such as bonding, painting and plating, the adhesion strength of coatings or adhesives can be heavily influenced – both by excess AND insufficient surface roughness
Therefore, understanding and controlling surface roughness is a critical aspect of engineering design and manufacturing processes. Various techniques and technologies are available to modify or optimize surface roughness, such as grinding, polishing, surface treatments, and coatings.
What is Surface Finish?
Surface finish in engineering terms is a systematic and standardized way to define the roughness of a surface and quantify its characteristics.
There are basically two factors that are considered in the Ra, Rz and Rmax values; the fine resolution roughness of the surface and any larger resolution waviness or undulation in the base surface height.
In general engineering, surface roughness is often represented as a Surface grade number, N1 to N12. This groups ranges into easier to interpret surface qualities, for quick comparison purposes, where precision is not required.
Why is Surface Finish Important in Engineering Processes?
Surface finish holds a center stage role in the machining and manufacturing of parts, determining how the surface interacts with anything that comes into contact with it.
On this basis alone, it’s crucial to durability and friction properties of many products and components, during normal usage or following manufacturing processes.
Surface finish can affect several properties of a product or part. These include the passive or destructive interaction with lubrication, coulomb and static friction levels with contact parts, resistance to corrosion and adhesion of surface finishes.
Finally, the user appreciation of products and components can be positively or adversely affected by choice of surface finish.
There are times when a particular form of roughness is beneficial, for example in fingerprint resistant stainless steel surfaces in appliances. There are also times when a mirror finish is not required in engineering terms, but by pressure from the market.
How to Measure Surface Roughness
Surface roughness can be measured using various techniques and instruments. These include:
Contact Profile Measurement:
This technique uses a stylus drawn across the part in several directions to assess the surface of the material, precisely measuring the height variations.
The stylus tip size must be smaller than the smallest feature of interest, to be able to traverse from the peaks to the bottom of the troughs of very fine surface features.
Also the stylus will generally be sapphire or diamond, in order to survive being dragged across (micro and macro) rough and often hardened metal surfaces.
The vertical displacement of the stylus is measured in much the same way as a vinyl record player, by displacements of fine magnetic or piezoelectric sensors. A computer calculates roughness parameters such as Ra (average roughness), Rz (average maximum height), Rq (root mean square roughness). and Rmax from the data collected.
Multiple passes in at least two directions are required to fully characterize the surface, as the larger scale surface height oscillation frequency (rise and fall of the underlying surface) cannot usually be derived from a single pass.
Optical Profile Measurement:
This technique involves using a laser or other optical devices to measure surface height variations. This is generally achieved by means of depth of focus calculations at small groups of pixels in a microscopy field of view.
Optical profilometry, also called confocal microscopy (or interferometry) is a method for assessing and measuring surface topography from several µm² to a few mm².
This generally offers lateral resolution of 200 nm and roughness depth resolutions from a few nm to several mm, selected by range settings in the optics.
Atomic Force Microscopy:
This is a system that works in much the same way as contact profilometry, in that a fine Silicon or Silicon nitride stylus is dragged across the surface and the displacement it experiences is measured.
The main difference is that the displacement is measured using highly sensitive optical methods, as a laser is reflected off the stylus support, amplifying the displacement as seen by a precision optical sensor.
This results in 3D scans of the surface, from multiple passes of the stylus. AFM is generally limited to characterizing areas of around 150 nm square, with a height resolution of a few nanometers.
Interferometry:
This method involves using the interference pattern in reflection of a dual beam to measure the height variations of the surface. A single, white light beam is split in two and then recombined, after the extracted beam has followed a longer path, creating a controlled phase difference.
The two light beams interfere with each other and create an interference pattern, which is used to calculate the surface roughness. This allows point measurements of a few nanometers target area with potentially better than 1 nm depth resolution.
The choice of measurement technique depends on budget, the type of material being assessed, the level of precision required, and the size of the surface area to be evaluated.
Surface Roughness Chart Symbols and Abbreviations
Surface roughness is represented by a range of symbols/abbreviations in engineering drawings and specifications. Selection of these is often a result of specific needs and restrictions, or to suit a particular manufacturing technique.
The common representations, measured in micrometers (μm) are:
- Ra – the arithmetic mean of the roughness profile.
- Rz – the average height of the five largest peaks and the five deepest valleys in a sampling length.
- Rq – the RMS (root mean square) value of the roughness profile.
- Rt – the maximum peak-to-valley height of the roughness profile.
- Rp – the maximum peak height of the roughness profile.
- Rv – the maximum valley depth of the roughness profile.
- Rsk – the skewness of the roughness profile, which indicates the degree of symmetry of the measured profile.
- Rku – the kurtosis of the roughness profile, indicating the degree of characteristic peakiness or flatness of the profile.
- N – the sampling length over which the roughness is measured/calculated.
On a component technical drawing or specification document, these will usually be combined with a surface finish symbol (✓) to indicate the required surface roughness. For example, ✓ Ra = 0.8 μm indicates that the surface finish should have an Ra value of 0.8 micrometers.
Surface Roughness Chart
A surface roughness chart is a table representing the different surface roughness values and their corresponding symbols and abbreviations.
The chart typically includes some of the range of roughness values from very smooth to very rough, along with the the most relevant of the corresponding Ra, Rz, Rq, Rt, Rp, Rv, Rsk, Rku, and N values.
The chart may also include a visual representation of the surface finish, such as a photograph or a sample surface.
Here is an example of a surface roughness chart that equates Ra and surface roughness grade numbers, linked from here:
5 Factors Affecting Surface Finish
After precision, surface finish is perhaps the most important property of part machining, and several factors can affect this:
- Cutting parameters such as cutter rotation speed, cut feed rate and depth significantly impact surface finish. Heat buildup often results in a poor surface finish, where low cutting speeds often lead to abrasive buildup of work hardened cuttings and increased tool wear. Similarly, excessively high feed rates can result in a rough surface finish, while poorly selected feed rates can find the natural frequency of machine aspects and result in chatter.
- Tool condition also seriously affects surface finish. Worn or damaged cutting tools result in surface roughness, reduced dimensional accuracy and potential swarf drag and tip accretion.
- Machine and sub component rigidity heavily influences surface finish. Vibrations result in cyclic machine deflections which can affect both the localized roughness and the undulation/waviness of the surface.
- The material being machined has a large effect on surface finish, both from its own cutting nature and the forces required potentially stressing the machine tool additionally. Some softer materials are more difficult to machine and can result in a poorer surface finish. Equally, very hard materials generally require more cutting force, which heavily impacts surface finish.
- The type and application of coolants will also affect surface finish. Coolants generally help to reduce operating temperatures, lubricate for smoother cutting and remove chips to prevent their entanglement in the ongoing cutting.
- Tool path and the geometry of the cutting tool can also affect surface finish. Stop starts, changes in feed speeds and complex geometry for curvature will often result in disjoints in surface finish. Optimizing tool geometry and tool path can help to minimize surface roughness, in critical areas.
- The way the workpiece is retained during machining also affects surface finish at times. Insufficient clamping or weak fixtures can result in disjoints and vibration that impact surface finish.
How to Select Suitable Surface Roughness for CNC Machining?
Specifying surface roughness in CNC machining depends on the application and the part being machined.
Better surface finishes add cost and slow down production, so specifying these should always be proportionate to the actual need:
- Consideration of function is paramount. It’s essential to consider the functional requirements of the part. Bearing or sliding parts must be smoother, non moving/rubbing structural elements can have lower surface finish quality specified.
- Design guides and specifications commonly specify a range for roughness values in functional areas.
- The type of machining process and cutter can affect the surface roughness of the part. Milling or turning suggest different surface roughness values, but much depends on the intended function. Always select a surface roughness that is achievable using the chosen process – or select the process according to the finish requirements. Centerless grinding can achieve better surface roughness than turning, but adds cost.
- The material being machined will affect the achievable range of surface finish. Harder materials require more forceful cutting methods, often resulting in rougher surfaces. Alternate processes can expand the range of possibilities, if the materials are compatible. For example, grinding softer materials like aluminum is not feasible.
- Delivering a high grade surface finish requires more expensive cutting tools and equipment and more time. Be proportionate in selecting finishes, to avoid wasted effort in manufacture.
Conclusion
Surface finish specification is a complex area, as selection of inappropriate finishes can waste manufacturing cost or doom the product to a short operational life. Take care to specify standards that meet the widely available guidance such as found here.
Choose machining processes that can deliver the required finishes – and select appropriate grades of supplier who can deliver on your specified needs and show they’ve done so.
Be very careful not to overspecify surface finishes, however – this can waste considerable time and value in delivering low roughness that actually represents no functional value, in the real part.
FAQs – Surface Roughness Chart
There are a wide range of surface roughness charts available to be consulted, such as here and here.
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