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Surface roughness in CNC machining
Here we mainly focus on CNC processing. Surface roughness will affect the interaction between manufactured parts and the environment. Typical surface treatment of CNC machining, “finished processing”, smooth to the touch (Ra3.2), but there will be some machining lines visible from the actual machining (similar to the drill bit which may remain once machining completed and the tool is retracted) spiral below), you can also refer to the figure below.
The pieces are textured
This level of roughness is suitable for most parts; however, in some cases a smoother surface is required. Smooth surfaces are ideal when designing sliding parts, reducing friction between parts and improving wear resistance.
To achieve a smooth surface, additional slower processing steps or post-processing finishing steps such as polishing may be used. As roughness decreases, manufacturing costs increase. So on some parts there may be a trade-off between surface roughness and cost, so to speak. Softness requirements also determine the added value of your product.
Polish coins
For some parts, greater surface roughness is also desirable. For example, bicycle seat posts must have a high coefficient of friction to prevent them from slipping during use. A rougher surface cannot be obtained by working. Secondary treatment such as sandblasting is necessary. There is no fixed method to obtain a specific surface roughness, because the machining process and secondary finishing operations affect the surface roughness.
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Some terms on surface roughness
Ra – The numerical average of all peaks and peaks in the test length. Also known as central line averaging (CLA).
Rz – The average of the highest and lowest highs in a row. The vertical distance between the highest peak and the lowest valley, the distance between the second highest peak and the second lowest valley, etc. Generally, the five largest deviations are calculated and then averaged.
Rp – the calculated distance between the highest peak of the profile and the moving average in the evaluation length.
Rv – the calculated distance between the lowest point of the contour and the moving average in the evaluation length.
Rmax – Maximum continuous deviation in the evaluation length, i.e. the distance between the highest peak and the lowest peak.
RMS – Calculated over the rating length, this is the rms average of the profile height changes with the moving average.
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Let’s talk about surface roughness-Ra
Definition of Ra: Surface roughness Ra is measured by measuring the “average roughness”, usually expressed as “Ra”. Ra is the average calculated between the surface peak and the base.
The lower the Ra value, the smaller the change between the peaks and valleys of the surface, making the surface smoother.
PS: The Ra value of laptop touchpads in our lives is very low.
Products with higher Ra values are very textured and have a rougher surface and therefore may not be suitable for their intended use. A comparison of these Ra values illustrates the importance of determining the required surface roughness of a product before the manufacturing process begins. Without such determination, the quality of the finished surface of the product may differ significantly from that originally intended.
The example below shows the difference between the Ra value (the numerical average of all peaks and troughs along the test length) and the Rz value (the average of the highest consecutive peak and valley the lowest).
Surface roughness (Ra)
Proper surface roughness is determined based on the needs of the part, assembly or your project requirements. For example, different types of surface treatments can be applied after the part is manufactured. These types of surface treatments can improve the wear resistance and aesthetic or visual effects of the part. However, these surface treatments may not be as precise as the machined surface of the tool and may affect size, conductivity, or compatibility with certain alloys. Surface roughness averages we can achieve with CNC machining include:
1. What is the unit of Ra?
Ra, the average roughness, is a surface roughness parameter, usually measured in micrometers (µm) or millimeters (mm).
2. Ra value in surface roughness
The standard surface roughness for traditional processing is generally 3.2 μm Ra. This is the most common and is typically used on rough machined surfaces of parts that may experience vibration, heavy loads, or stress. Although this machining leaves visible cut marks on the surface, machining can save time and money.
Ra 3.2 for a certain part
Surface roughness can be achieved by finishing cutting operations to obtain a lower Ra. However, this increases costs, adds additional processing steps and results in longer manufacturing cycles. The Ra value, the average roughness value, is a key parameter in measuring the roughness of surfaces. It is calculated as the arithmetic average of the absolute value of surface height deviations from the mean line value over the specified measurement length. Essentially, the Ra value represents the average of all individual peak and surface peak measurements.
The formula for Ra is:
Ra = 1/L ∫|y(x)|dx from 0 to L
In:
L is the sampling length
y(x) is the vertical deviation from the mean line at distance x on the surface. This formula allows a more complete understanding of the Ra value and its importance in evaluating surface roughness.
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Types of machining surface treatment
1. Machined Surface – This is the surface roughness obtained directly from the machining process without any post-processing. May have visible tool marks and is generally not very smooth
2. Smooth surface – Obtained through processes such as grinding or polishing, this surface has a very fine roughness. This surface treatment is ideal for parts that require a smooth surface, whether for functional or aesthetic reasons.
3. Textured Surface – Some parts may require a textured surface for added grip, aesthetics, or other functional reasons. This can be achieved through processes such as embossing or shot blasting.
4. Mirror surface: This is a highly polished surface treatment that reflects light, similar to a mirror. Obtained by thorough polishing, often used on decorative pieces.
5. Oxidation and anodizing – For metals such as aluminum, an anodizing process can be used to form a protective oxide layer on the surface. This not only provides protection but also adds color to the room.
It is important to select the appropriate machined surface treatment, based on the part’s intended use, materials and design specifications. Surface roughness is represented by the Ra value, which provides an indication of surface smoothness.
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How to choose the appropriate surface roughness
There are several factors to consider when choosing the right surface roughness for your project. Depending on the product application, required durability, whether the part needs to be sanded or painted, the importance of precise dimensions, and the project budget, the Ra value may need to be higher or lower.
For low budget projects, 3.2 µm Ra is suitable, these projects can receive other treatments at a later stage, such as painting or sanding. The 1.6 μm Ra will show fewer cut marks and is also an economical choice.
For smoother surface requirements, like 0.8 μm Ra or 0.4 μm Ra, the cost will be higher, but this is necessary for projects that require perfect size control. This high quality finish leaves no visible cut marks and is ideal for parts subject to concentrated stress.
The finer medium roughnesses are more expensive due to the additional manufacturing processes required. They should only be specified when smoothness and perfect size are essential to the project.
Casting surface roughness comparison sample
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How to achieve different levels of surface roughness
Surface roughness is determined by the craftsman and manufacturer before manufacturing. This is a critical detail that must be maintained consistently to produce a reliable product that interacts properly with its environment.
Different types of surface treatments can determine the durability of a part. If a part requires a rougher surface, irregularities may appear on the surface, leading to more rapid wear, breakage and corrosion. Some surface roughness may also be necessary to facilitate adhesion of coatings and paints, or to improve conductivity.
The Ra value is commonly used to measure different levels of surface roughness. The surface roughness table shows different types of surface treatments with surface roughness Ra values ranging from 12.5 μm Ra (very rough) to 0.4 μm Ra (very smooth).
Retaining the product with its original machined finish ensures the tightest dimensional tolerances, achieving ±0.005mm or better. CAM can be relied upon to implement precise data paths and tool paths, thereby rendering original designs. Generally, there is no additional cost for standard surface treatment. However, there will be visible traces of tool work and the surface of the part may appear dull. For products such as prototypes, jigs and fixtures, machined products can be the most cost-effective solution, especially if there is no type of additional surface treatment.
Share 7 directions of surface lines
sandblasting
Sandblasting of surfaces is carried out using compressed air guns. Small glass beads are sprayed onto the surface, leaving a matte or semi-matte sheen and slight surface texture. This uniform surface treatment hides tool marks produced on machined parts and is primarily used to achieve a final glossy effect.
Sandblasting is not suitable for projects that require precise dimensions because the process is less controlled. Although key features, such as holes, can be masked and obscured during processing to avoid excessive changes, other parts of the part will be affected by the size and roughness of the surface.
The only controllable aspect of this type of surface treatment is the size of the glass beads.
anodizing
Anodizing is a process that adds a thin but highly protective oxide layer to metal parts. This is achieved through an electrochemical reaction when the part is immersed in an acid solution and exposed to voltage. The coating will grow evenly in all directions, meaning this type of surface treatment allows for better size control than sandblasting.
The resulting coating has high hardness and electrically insulating properties. However, this process only works for aluminum and titanium alloys.
Anodized Type II
Type II anodizing is known as the standard anodizing process. The coatings it produces can be clear or colored, with thicknesses up to 25 microns. This type of surface treatment is ideal for parts that need to be smooth, wear-resistant and visually appealing.
Anodized Type III
Type III anodizing generally costs more than Type II. The additional cost is because the process requires tighter control. Higher current densities are required and a constant solution temperature of zero degrees Celsius must be maintained to produce electrochemical reactions in coatings up to 125 microns thick.
Type III anodizing is also known as “hard coat” anodizing. Parts with this coating will have a harder outer layer that provides superior corrosion resistance and is ideal for high-level engineering applications.
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How to measure the roughness of a surface
Surface roughness can be measured by both manual and digital methods, although the most commonly used surface roughness tester is a surface roughness measuring device. This is one of the most accurate methods for measuring the surface roughness of an area. Surface roughness testers can use a variety of profiling techniques, from contact to non-contact methods.
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Contact profilometer
Contact profilometers work by measuring the displacement of a steel probe as it moves across the surface of a manufactured component. Typically, the probe can measure up to 25mm as it moves across the product surface. This displacement is then converted into a numerical value displayed on the profilometer screen. Once displayed, the measurements are then analyzed by product designers and/or manufacturers to better understand product properties.
Should you use a contact profilometer?
Contact profilometers have a certain degree of accuracy in determining surface roughness, but they also have some limitations. Firstly, the contact of the probe with the surface during the measurement process can damage the product surface, causing roughness and changes that did not exist before. Additionally, contact profilometers are also slower than non-contact technologies, so their use during mass production can slow down the assembly process.
Non-contact profilometers can be applied via several technologies, including laser triangulation, confocal microscopy, and digital holography. However, the most common application of non-contact profilometers is optical profilometry, which uses light rather than a physical probe, such as a stylus.
In this technology, light is projected onto the surface of the product. The camera detects a three-dimensional image of the surface using light reflections from well-placed reference mirrors. As a result, a three-dimensional profile of the surface is obtained and deviations from the ideal surface profile are detected.
Should you use a contactless profilometer?
Non-contact profilometers are extremely reliable and can measure surface changes down to the micron level. Non-contact surface measurement tools are also a more cost-effective option than contact methods and allow surface roughness to be calculated more quickly. Non-contact surface measurement tools can measure larger areas because they are not limited by the size of the probe.
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Handheld Surface Roughness Tester
Although still digital, portable roughness meters can perform surface measurements without being connected to an electrical outlet. It has a backlit screen to display its measurement results and can display segment calculation results and amplitude distribution curves, as well as its original surface roughness calculation. Similar to a contact profilometer, this device also uses a probe to make measurements.
A popular and simple method for measuring surface roughness is to use a digital surface roughness tester.
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Surface roughness comparison
A comparison piece for turning roughness
The Surface Roughness Comparator is used to manually evaluate the surface roughness/finish of manufactured products. Depending on the manufacturing process used and the finish required, an industry standard finish level displayed on the comparator can be selected so that the surface of the product can be compared to it.
Although the surface roughness comparator represents a cost-effective and accessible method of evaluating surface roughness, it also has significant weaknesses. Since the deviation of the product surface is calculated by touch or by judgment of aesthetic appearance, the level of precision obtained by this method is lower than that achieved using a profilometer.
Processing Industry Surface Roughness Comparison Table
The Manufacturing Surface Roughness Comparison Chart is an important guide for engineers, allowing them to compare common surface roughness values for different manufacturing processes. Being able to understand diagrams like this and convert between different units of measurement is a very useful skill for engineers.
Daguang focuses on providing solutions such as precision CNC machining services (3-axis, 4-axis, 5-axis machining), CNC milling, 3D printing and rapid prototyping services.
