You facing challenges measuring the surface finish and performance of your CNC parts? If this blog post is for you , it means CNC surface finish measurement can be the answer. The process of machining materials with cutting tools for custom CNC parts often leaves marks, burrs, and tiny chips on the machined surface. Finishing removes these unwanted marks and maintains the dimensional accuracy of the workpiece. In this blog, you’ll learn about the definition of surface finish, the key metric Ra, CNC visual guidelines, types of surface finishes for CNC machined parts , and techniques. Stay tuned!
What is surface finish?
“Surface finish describes the texture, polish, smoothness, or roughness of an object’s surface after it has been machined or processed.”surface finish is the most important factor in CNC machining . It’s the heart of the part, affecting its performance, fit, and even its lifespan. Therefore, a good surface finish can enhance product functionality, reduce friction, and enhance its professional appearance. In machining terms, surface finish refers to the texture of the machined part’s surface. The degree of surface smoothness or roughness depends on the CNC machine, machining parameters, and material type. However, this roughness can be quantified using a specific value called Ra (Roughness Average). Therefore, understanding the Ra value is crucial to achieving the best surface finish for your CNC projects.
What does the Ra value mean ?
Ra is the average value or height of scratches or cuts on a surface texture. The Ra value is a micro-measurement (micrometers or microinches) of the surface’s feel; lower Ra values indicate smoother surfaces. The feel of a surface corresponds to its Ra value; lower Ra values indicate smoother surfaces, while higher Ra values indicate rougher surfaces. CNC machined parts have various surface finish levels to meet specific needs. Some surfaces require a mirror finish, while others require only a basic finish. Therefore, understanding the correct surface finish parameters can save time, improve quality, and help avoid larger defects. Ra is most commonly measured in micrometers (μm) or microinches (μin). Ra is widely accepted, making it a preferred value among engineers and machinists. Therefore, it is considered the primary metric for CNC surface finish and is found in most charts and surface finish guides. However, many industries have different standards for different components. For example, precision aerospace components require a higher Ra finish than general mechanical components. Therefore, selecting the appropriate Ra value is crucial.
Let’s say you’re designing a mold for a bottle cap. If the surface is rough (the Ra value is too high), it can cause the plastic to stick or blemishes to appear. On the other hand, if the surface is too smooth (the Ra value is too low), it may increase costs without adding real value to the product.
Comparison of surface finish of CNC machined parts
A surface finish chart, also known as a surface roughness chart , contains information such as standard grades for various Ra values, finish descriptions, common processes, and other characteristics. This chart helps engineers and operators determine the desired finish level for their machining projects.
| Acute angle (μm) | Ra (microinches) | N-level | Complete Description | Visual appearance | Common processes | use |
| 3.2 | 125 | N8 | Standard processing | Visible tool marks | Rough milling/turning | General Parts |
| 1.6 | 63 | N7 | Fine processing | Slightly smoother | Standard milling | Structural components |
| 0.8 | 32 | N6 | smooth | Smooth eyes | Completion Pass | Moving parts, housing |
| 0.4 | 16 | N5 | Very smooth | Smooth surface | Precision grinding | Sealing surface, bearing |
| 0.2 | 8 | N4 | Super smooth | High gloss | Careful research | Aerospace parts, hydraulics |
| 0.1 | 4 | N3 | mirror-like | Specular reflection | polishing | Optical and medical components |
Standard surface finish for CNC machined parts
A question you might be asking yourself is: What’s the standard surface finish for CNC machining? This question is common, so don’t feel isolated when tackling your project. The answer depends on the function of the part. The most common surface finish standard for CNC machined parts is Ra 3.2 μm (125 μin) . The standard roughness for CNC machined surfaces is Ra 3.2 μm, or 125 μin. This is the default finish for basic milling or turning operations, without the need for any additional grinding or polishing. Therefore, this surface finish is sufficient for components where smoothness isn’t a priority. It’s commonly used for machined components, brackets, housings, and other internal parts. Best of all, this surface finish offers reasonable quality at a reasonable price, which explains its popularity.
Why is it used so frequently?
Well, this surface finish is popular because it’s easy to produce, inexpensive, and versatile. Most structural and functional components don’t require a smoother finish than this. You know, it’s rough enough for use in sealing environments, but not so rough that it breaks down during contact. Furthermore, it maintains sufficient strength to withstand the expected wear. Furthermore, if the part needs to move, be sealed, or require aesthetics, a smoother finish—ideally, Ra 0.8 μm or higher—is needed. Furthermore, you don’t always have to stick to the default settings. Remember that different industries have their own specifications. For example, medical or aerospace components have more stringent surface finish requirements. So, ask yourself, how will this part be used? If in doubt, it can be helpful to consult a CNC surface finish chart or consult your machinist. Your CNC machining technician will be able to recommend the surface finish that best meets your requirements.
How to Choose the Right Surface Finish for Your CNC Parts
CNC surface finish for your project can seem overwhelming at first, but it’s not that complicated. Xtproto’s team of engineers has compiled five simple questions to help you make your decision. Simply answer these questions and you’ll have a clear understanding of your component’s needs.
What is the purpose of the part: This should be your starting point. Will your part be useful, or will it remain useless? For internal or hidden components, a machined surface finish is often sufficient. However, for aesthetically pleasing or demanding parts, a smoother finish may be necessary.
How smooth does it need to be : It’s worth noting that if your part will be meshing with other components, a smoother surface is preferred. Therefore, you should choose a CNC surface finish that will make polishing or grinding smoother. Furthermore, improving the surface finish of your part will also make it more durable.
Does appearance matter: Appearance is important. However, for products that customers can see and touch, visuals are crucial. Typically, surface treatments like sandblasting, anodizing, and even powder coating give your parts a shiny finish. Furthermore, some surface treatments even come with color.
Will it be used outdoors or in harsh conditions: What about water, heat, or chemical resistance? Choose a surface treatment that adds a layer of protection. Best of all, anodizing or powder coating the surface after CNC machining can add a layer of protection against rust, wear, or damage.
What is your budget: As expected, some renovations may be cost-effective, while others may be more expensive. However, if you’re working on a budget, you can start with the essential renovations. In some cases, the standard renovations may be sufficient.
The Difference Between Surface Finish and Tolerance
Tolerance and surface finish are both very critical parameters in CNC manufacturing quality control. Any project requires a specific finish (Ra) and tolerance range or value, which is usually stated in the design. Let’s detail the three main differences between them;
Purpose and function
Surface finish: It is related to how rough or smooth the surface is. It affects appearance, friction and sealing.
Tolerance: Tolerance, on the other hand, determines how close the part dimensions are to the required design dimensions. Hence, it controls the fit and function of the part.
Measurements and units
Surface finish: Observed with a micrometer and expressed in micrometers or microinches, it shows the surface texture.
Tolerance: In contrast, it is expressed in millimeters or inches and indicates a specific range of dimensional deviation. Thus, one relates to surface grade and the other to dimensional accuracy.
Impact on performance
Surface Finish: Remember that a good surface finish minimizes wear and enhances sealing.
Tolerances: On the contrary, maintaining precise tolerances allows parts to remain aligned and minimizes the possibility of failure. Therefore, these tolerances ensure smooth and precise parts and reliable performance in all applications.
The Difference Between Surface Finish and Surface Treatment
Surface finish refers to the result of machining, such as the Ra value. Finishing, on the other hand, refers to the machining process, such as sandblasting and polishing. The relationship between these two terms is this: “The correct surface finishing technique must be selected to achieve the desired finish.” One is a measurement, the other an action, but both are essential for strong, reliable parts. Let ‘s take a look at the specific differences between them .
Meaning and Definition
Surface finish: As we all know, surface finish refers to the texture or smoothness of the surface of the part after machining. However, it refers to the roughness or polish of the surface.
Surface treatment: Surface treatment, on the other hand, is the action of applying a process to achieve a smooth surface finish.
Role in manufacturing
Surface finish: An important measurement after production is surface finish. It affects function, appearance and performance.
Surface treatment: In contrast, it involves processes such as grinding, polishing or coating to achieve the desired effect.
Summarize
Therefore, from the above discussion, it can be concluded that understanding surface finish is closely linked to better controlling product performance and appearance. Understanding the different types of CNC surface finishes will help you make the right decision when selecting them. As you know, surface treatment not only improves quality but also ensures that parts function as designed. Therefore, you should always consider material, function, cost, and a quality CNC machining service provider like Xtproto . Xtproto is a trusted provider in CNC machining. Contact Xtproto today for a quote or more information.