Manufacturing processes often leave irregular textures on product surfaces. With rising demand for high-quality finishes, the importance of surface finishing is becoming increasingly paramount. Surface finishing isn't just about aesthetics or achieving a smoother appearance; it significantly impacts the functionality, durability, and overall performance of a product.
Explore our guide to learn everything about surface finishing, and get tips on achieving the desired finish and selecting suitable surface roughness for CNC machining.
Surface finish, also known as surface texture or surface topography, refers to the overall smoothness, texture, and quality of a part’s surface. It's an important factor in manufacturing and engineering, as it affects not just appearance but also the performance and functionality of a product.
The key characteristics of surface finish mainly include the following three aspects:
Surface Roughness
Surface roughness refers to the small, finely spaced irregularities on a surface that might not be visible to the naked eye but can be felt if you run your finger over the surface.
Roughness is often measured using parameters like Ra (average roughness). A lower Ra value indicates fewer and smaller irregularities, resulting in a smoother surface that decreases friction and wear. When professionals refer to surface finish, they often specifically mean surface roughness.
Waviness
Waviness differs from surface roughness as it encompasses larger, more widely spaced irregularities on the surface. These can be caused by factors such as machine vibrations, deflections, or warping during the manufacturing process. Surface waviness can significantly affect how parts fit together and their sealing capability.
Lay (Surface Pattern Direction)
Lay is the predominant direction of the surface pattern, typically resulting from the manufacturing process used and can be parallel, perpendicular, circular, crosshatched, radial, multi-directional, or isotropic (non-directional).
The lay direction affects friction, lubrication, and aesthetics. In optical components, a specific lay direction can reduce light scattering and improve clarity.
As mentioned earlier, surface finish significantly impacts the appearance, performance, durability, and overall quality of a product. That's precisely why surface finish plays an important role in manufacturing processes. Here let's break down the reasons why surface finish holds such a pivotal role.
Aesthetics: The first impression of a product is often based on its appearance and tactile feel. A high-quality surface finish enhances visual appeal and can significantly influence your perception and satisfaction, especially with consumer goods.
Friction and Wear: Particularly in mechanical applications, a smoother surface finish reduces friction and wear between moving parts, thereby minimizing heat production and enhancing the efficiency and lifespan of components.
Sealing and Fitting: A proper surface finish ensures better sealing and fitting of parts, preventing leaks and ensuring precise assemblies.
Fatigue Strength: A smoother surface improves fatigue strength by reducing stress concentrations and the likelihood of crack initiation.
Corrosion Resistance: A better surface finish enhances corrosion resistance by minimizing crevices where corrosive agents can accumulate.
Adhesion of Coatings: The surface texture can impact how well coatings or paint adhere to the product.
Improved Conductivity and Heat Dissipation: In electronic and thermal applications, a high-quality surface finish enhances conductivity and aids in heat dissipation.
Control of Light Reflection and Scattering: In optical applications, the surface finish affects how light is reflected and scattered.
Given the critical impact of surface finish on manufacturing, measuring surface roughness is essential in production processes. This enables us to precisely understand the actual surface characteristics of products, ensuring they meet design and functional requirements.
Measuring surface roughness involves using various measurement techniques and data analysis to assess the relative smoothness of a product’s surface profile. The most commonly used numeric parameter to quantify this roughness is Ra.
Several methods are available to measure surface roughness. The major types of measurement techniques include are as follows:
Contact methods involve physically touching the surface with a tool, such as a stylus probe instrument. This device moves vertically in relation to the surface lay direction to trace the surface profile. The movement of the probe generates a detailed surface contour map, providing precise data on surface roughness.
These methods are primarily used in manufacturing settings where direct contact with the surface will not cause damage. However, they may not be suitable for delicate or soft surfaces that could be distorted by the probing action.
Optical Profilometer/White Light Interferometer: This technique involves projecting a light beam onto a surface and measuring the pattern of reflected light to accurately determine surface height variations, thereby creating a detailed 3D surface profile.It is suitable for delicate or soft surfaces in precision engineering, semiconductor, and optical industries. However, it requires surfaces with good reflective properties, and the equipment can be expensive.
Laser Scanning Confocal Microscopy: This method uses a focused laser beam to scan the surface, generating high-resolution 3D images of the topography. It is ideal for analyzing complex 3D surfaces in biomedical research, materials science, and precision engineering. However, it is expensive and complex to operate.
3D Laser Scanning: This technique uses a laser to capture the topography of a surface and create a 3D model. It is typically used for larger surfaces and can quickly generate a comprehensive surface profile. It is suitable for large or complex surfaces in automotive, aerospace, and architectural applications. Although it can handle large areas efficiently, it has a lower resolution compared to other methods and is not suitable for high-precision measurements or very small surface features.
Comparison methods involve comparing the surface in question with a standard set of samples that have known roughness.
These methods are quick and cost-effective, suitable for routine checks in production environments. However, they are more subjective and less suitable for applications requiring high precision.
In-process methods integrate surface roughness measurement directly into the manufacturing process. Tools like in-line profilometers or integrated sensors in CNC machines are used. These tools provide real-time data on the surface finish, allowing for immediate adjustments.
This approach is particularly useful for real-time monitoring and quality control in continuous production lines and automated manufacturing systems. However, it may be limited in situations where integrating measurement systems into the process is not feasible due to space, cost, or complexity constraint.
For all measurement methods mentioned above, please note the measurement unit when making a record. Micro-inches are used for roughness measurement in the United States, typically written as µin, while micrometers are used internationally (SI), written as µm or um. Here is a brief conversion:
If not understanding the symbols and parameters in the Surface Roughness Table as above, we will be at a loss in the complex field of manufacturing. These indicators are like markers on a map, guiding us to ensure that the quality, functionality, and suitability of surfaces meet expectations.
Ra: Average Roughness
Ra is defined as the average variation of the roughness profile from the mean line. In mathematical terms, it is the arithmetic average of the absolute values of the surface height deviations measured from the mean line over the evaluation length.
Ra is the most commonly used parameter for surface roughness because it provides a simple,general indication of the surface texture,giving a balanced view of overall roughness without being overly influenced by extreme peaks or valleys.
where :
L is the measurement length.
y(x) is the vertical distance from a given point on the surface profile to the mean line.
Because of this averaging, the Ra value is lower than the actual height of the roughness variations.
Rz: Average Maximum Height
To calculate Rz, the the evaluation length is divided into five equal lengths. Rz is the average of the maximum peak-to-valley heights within each of these five equal sampling lengths.
Rz provides a more detailed representation of surface roughness compared to Ra and is more sensitive to the peaks and valleys of the surface profile. It is often used in industries where the extremes of surface texture are critical, such as in sealing surfaces, where the highest peaks and deepest valleys can affect the performance of seals and gaskets.
In practice, for convenience, an approximate formula "7.2 x Ra = Rz" is sometimes used. However, this is a rough estimate and not always accurate.
Rp: Maximum Profile Peak Height
Rp is the height of the highest single peak in the surface profile measured from the mean line within the evaluation length.
Rv: Maximum Profile Valley Depth
Rv is the depth of the deepest single valley in the surface profile measured from the mean line within the evaluation length.
Rt : Total Roughness
Rt is the total vertical distance between the highest peak and the lowest valley within the entire evaluation length.
It is useful for overall quality control and ensuring that the surface does not have extreme deviations.
Rmax: Maximum Roughness Depth
Rmax is the largest peak-to-valley height within the evaluation length.It looks at the largest peak-to-valley difference within individual segments, and then the maximum of those segments is chosen.
Rmax focuses on the most significant localized roughness, useful for applications where specific areas of the surface need to be controlled more tightly, such as in critical sealing or contact surfaces.
RMS: Root Mean Square Roughness
RMS, also known as Rq, is the root mean square average of the surface height deviations from the mean line over the evaluation length. It gives more weight to larger deviations than Ra and is particularly useful for applications sensitive to larger surface variations,such as precision engineering and optical applications.
where:
Rq is the RMS roughness value.
L is the measurement length.
y(x) is the vertical distance from a point on the surface profile to the mean line.
The roughness symbols can be as check marks, with the point of the mark resting on the surface to be specified. Please refer to the table below for additional instructions.
In practice, from raw materials to the selection of specific processing techniques, and even the machining conditions like tool condition and machining parameters, all can greatly affect the quality of the part's surface. Under the condition that the processing material is determined, in order to obtain an ideal surface finish, we can consider the following aspects:
It is worth mentioning that since additional processing and a smoother surface will incur additional costs, it is crucial that the engineer or designer does not impose unnecessarily stringent roughness requirements. Whenever possible, the roughness specifications should be set within the limitations of the primary manufacturing process.
As indicated by previously mentioned Surface Roughness Comparison Chart, CNC machining can generate a very wide range of surface roughness. So, what kind of surface roughness is most suitable for your project? Let's find out.
Approximate Surface Roughness Conversion Chart | ||||
Roughness Grade Numbers | American System - Ra (µin) | American System - RMS (µin) | Metric System - Ra (µm) | Metric System - RMS (µm) |
N12 | 2000 | 2200 | 50 | 55 |
N11 | 1000 | 1100 | 25 | 27.5 |
N10 | 500 | 550 | 12.5 | 13.75 |
N9 | 250 | 275 | 8.3 | 9.13 |
N8 | 125 | 137.5 | 3.2 | 3.52 |
N7 | 63 | 69.3 | 1.6 | 1.76 |
N6 | 32 | 35.2 | 0.8 | 0.88 |
N5 | 16 | 17.6 | 0.4 | 0.44 |
N4 | 8 | 8.8 | 0.2 | 0.22 |
N3 | 4 | 4.4 | 0.1 | 0.11 |
N2 | 2 | 2.2 | 0.05 | 0.055 |
N1 | 1 | 1.1 | 0.025 | 0.035 |
In the chart above, the roughness grade numbers (N12, N11, N10, etc.) are often used in ISO 1302 to indicate different levels of surface roughness. Here are some typical roughness grades for CNC machining:
Ra 3.2 µm (N8)
A Ra 3.2 µm surface finish exhibits a moderately smooth surface,and is commonly used as a standard for commercial machinery. This surface finish, though leaving visible but not excessive cutting marks, is acceptable for most consumer parts and provides a sufficiently smooth surface for many applications.
Ra 1.6 µm (N7)
A Ra 1.6 µm surface finish represents a relatively smooth surface with minimal cutting marks that are barely noticeable. This finish is suitable for slow-moving and mildly load-bearing surfaces and is ideal for pump parts and hydraulic components.
Ra 0.8 µm (N6)
A Ra 0.8 µm surface finish signifies an extremely smooth and precise surface. It is the standard for many precision engineering applications, such as aerospace and automotive components.
Ra 0.4 µm (N5)
A Ra 0.4 µm surface finish provides an almost mirror-like finish. This level of smoothness requires significant effort to produce and should be requested only when it is a top priority. It is used in optical components, scientific instruments, and other high-precision applications.
Surface finish is an integral aspect of manufacturing, directly influenced by the processes used. It significantly impacts the functionality, aesthetics, and durability of the final product. However, it is important to note that a lower surface roughness is not always better,practical use and budget must be considered.
As a one-stop processing manufacturer, Chiggo not only applies a range of manufacturing processes and surface finishing services to achieve strict surface finish standards but also offers cost-effective solutions tailored to your specific project needs.
Key Take-Aways:
Manufacturing processes often leave irregular textures on product surfaces. With rising demand for high-quality finishes, the importance of surface finishing is becoming increasingly paramount. Surface finishing isn't just about aesthetics or achieving a smoother appearance; it significantly impacts the functionality, durability, and overall performance of a product.
Anodizing, also known as anodization, is an electrochemical process used to create a decorative and corrosion-resistant oxide layer on metal surfaces. While several nonferrous metals, including magnesium and titanium, can be anodized, aluminum is particularly well-suited for this process. In fact, aluminum anodizing is widely used today because it significantly enhances both the material's durability and appearance.
Electroless nickel plating originated in the mid-20th century. In 1944, Dr. Abner Brenner and Grace E. Riddell, while researching traditional electroplating, accidentally discovered a method to deposit nickel onto metal surfaces without the use of electric current. This breakthrough led to the development of electroless nickel plating. Since then, the technology has continuously evolved, and its applications have expanded—from electronics and aerospace to oil and gas, automotive, and defense industries.