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A comprehensive guide to surface finishes for precision CNC machined parts from Elimold

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Surface finish is an integral part of the manufacturing process. It determines the texture and roughness of the manufactured part. A part’s surface can be rough, with obvious tool marks, or it can be a smooth satin finish. The importance of surface finish extends beyond appearance and aesthetic appeal. It determines functionality, wear resistance, and the finished product’s heat dissipation. Furthermore, it is crucial to inspect and measure surface finish before using parts (especially those with high tolerances). This article introduces Elimold’s basic standards and capabilities regarding surface finish for CNC custom parts. It explains what surface finish is, how to measure surface roughness, industry standards for surface finish, and the technologies that can be used to achieve the desired surface finish.

What is the standard surface finish for custom CNC parts from Elimold ?

When you want to learn about Elimold’s CNC custom parts projects, you might have one question in mind: What is their standard surface finish for CNC machining? This is a common question , and many clients have this concern when they first contact us , worrying that we cannot provide the surface finish required for their projects .

Here, I will introduce information on the standard surface finish for CNC custom parts from Elimold. In reality, the answer to this question depends on the function of the part you require . Elimold’s most common standard is Ra 3.2 μm (125 μin), the standard roughness for CNC machined surfaces. This is the default finish for basic milling or turning operations, requiring no additional grinding or polishing. Therefore, this surface finish is sufficient for components that do not require high smoothness. It is commonly used for mechanical components, brackets, housings, and other internal parts. Most importantly, this surface finish offers reasonable quality at a reasonable price, which is why we have set this standard .

Elimold offers four surface roughness levels.

Different applications have different requirements for surface roughness. Lower surface roughness is only specified under specific requirements. This is because a lower Ra value means more machining operations and stricter quality control, which inevitably increases machining costs and time. Therefore, post-processing operations are generally not the first choice when a specific surface roughness is required, as these operations are difficult to control precisely and may adversely affect the dimensional tolerances of the parts.

In some applications, surface roughness has a significant impact on the function, performance, and durability of parts. It directly relates to the coefficient of friction, noise level, wear, heat generation, and adhesion during use. The importance of these factors varies depending on the specific application. Therefore, while surface roughness may not be critical in some cases, it plays a vital role in many others. These applications include, but are not limited to, high-tension, high-stress, and high-vibration environments; components requiring precise fit and smooth movement; rapidly rotating parts; and medical implants. As mentioned earlier, different applications have varying stringent requirements for surface roughness. Below, we introduce the roughness grades offered by Elimold and all the information you need to know to select the appropriate Ra value for your application.

3.2 μm Ra

This is a standard surface finish for commercial machines. It is suitable for most consumer parts, providing a smooth enough finish but with noticeable cut marks. This surface roughness is the default unless otherwise specified. 3.2 μm Ra is the maximum recommended surface roughness for parts subjected to stress, load, and vibration. It can also be used for mating moving surfaces when the load is light and the movement is slow. High speed, fine feed, and light cutting are required during machining.

1.6 μm Ra

Typically, using this option will only result in slightly visible cutting marks. This Ra value is recommended for tightly fitted and stressed parts, and is sufficient for slowly moving and lightly loaded surfaces. However, it is not suitable for rapidly rotating parts or parts subject to severe vibration. This surface roughness is achieved under controlled conditions through high-speed, fine-feed, and light-cutting processes.

0.8 μm Ra

This level of surface finish is considered high-grade and requires very strict control during production, thus resulting in higher costs. It is suitable for parts with stress concentration. It can also be used in bearings when movement is infrequent and the load is light.

0.4 μm Ra

This level of surface roughness (technically termed “less rough”) represents a very high-quality surface finish. It is particularly suitable for parts subjected to high tension or stress, and is also required for rapidly rotating components such as bearings and shafts. Because the manufacturing process for this surface roughness is more complex, it is only chosen when smoothness is of paramount importance.

What is surface finish?

Surface roughness is a measurement of the average texture of a part’s surface, specifically referring to the surface roughness after CNC machining. Different parameters are used to define surface roughness, with Ra (average roughness) being the most common. Ra is derived by calculating the difference between the surface height and depth. When measured under a microscope, Ra is typically quantified in micrometers (x 10⁶ m). It’s important to clarify that surface roughness and surface finish are two different concepts. While finishing techniques such as anodizing, sandblasting, and electroplating can improve the surface finish of machined parts, surface roughness specifically refers to the surface texture directly present after machining.

Therefore, surface finish, also known as surface texture, is a physical characteristic of a part. Surface finish ranges from rough and matte to smooth and glossy. Surface finish is defined by three main characteristics: roughness, waviness, and texture. Furthermore, surface finish plays a crucial role in the manufacturing process. A good surface finish reduces friction and heat generation, thereby minimizing wear. It increases the visual appeal and economic value of a part. In addition, it determines the success of subsequent manufacturing processes such as adhesive bonding, coating, and electroplating. Surface finish also affects tolerance levels, especially the tolerance levels of mating parts during assembly.

CNC custom part surface roughness parameters

Certain parameters can be measured to quantify the surface finish of a material. They are often used together to obtain a comprehensive overview. The following are common surface roughness parameters used in manufacturing.

Learn about common surface treatment symbols and abbreviations

Surface finish symbols, also known as surface texture symbols, are used to represent various aspects of surface quality, including roughness, waviness, and layer thickness. These symbols are used in engineering drawings and machining processes to indicate the required surface finish grade for a specific component or surface. For example, a “C” with a center line indicates a specified surface roughness, while “Ra” indicates average roughness. Other abbreviations, such as “N” and “Rmax,” indicate maximum roughness, while “Rz” indicates average maximum profile height. Additionally, checkerboard patterns are used to indicate surfaces that require grinding.

Ra: Average roughness

Ra is defined as the average variation of the roughness profile relative to a mean line. In mathematical terms, it is the arithmetic mean of the absolute values of the surface height deviations measured from the mean line over the evaluation length. Ra is the most commonly used surface roughness parameter because it provides a simple, general indication of surface texture, giving a balanced view of overall roughness without being overly affected by extreme peaks or valleys.

Rz: Average maximum height

To calculate Rz, 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. Compared to Ra, Rz provides a more detailed representation of surface roughness and is more sensitive to the peaks and valleys of the surface profile. It is commonly used in industries where extreme surface texture is critical, such as sealing surfaces, where the highest peaks and deepest valleys affect the performance of seals and gaskets. In practice, for convenience, the approximate formula “7.2 × Ra = Rz” is sometimes used. However, this is a rough estimate and is not always accurate.

Rp: Maximum profile peak height

Rp is the height of the highest single peak in the surface profile, measured from the average line within the evaluation length.

Rv: Maximum valley depth in the profile

Rv is the depth of the deepest single valley in the surface profile measured from the average line within the evaluation length.

Rt: Total roughness

Rt is the total vertical distance between the highest peak and the lowest valley over the entire evaluation length. It is useful for overall quality control and ensuring that the surface is free from extreme deviations.

Rmax: Maximum roughness depth

Rmax evaluates the maximum peak-to-valley height over a length. It looks at the maximum peak-to-valley difference within each segment and then selects the maximum value among those segments. Rmax focuses on the most critical local roughness and is suitable for applications requiring more stringent control over specific areas of the surface, such as critical seals or contact surfaces.

RMS: Root Mean Square Roughness

RMS, also known as Rq, is the root mean square mean of the deviation of the surface height along a length from the mean line. Compared to Ra, it gives more weight to larger deviations and is particularly useful for applications sensitive to large surface variations, such as precision engineering and optical applications.

How to achieve different surface roughnesses?

The surface roughness of machined parts is typically not random, but rather carefully planned and controlled to achieve specific standards. This means that surface roughness values are preset. However, this does not mean that any value can be arbitrarily specified. In the manufacturing industry, there are a series of widely accepted Ra value standards, such as those specified in ISO 4287, which can be explicitly specified during CNC machining. These standard values range widely, from 25 micrometers to 0.025 micrometers, to meet the needs of different manufacturing and post-processing operations.

What is the true meaning of Ra?

Ra is the average or height of scratches or cuts on a surface texture. The Ra value is a micrometer (micrometer or microinch) unit of measurement for surface feel; the lower the Ra value, the smoother the surface. Tactile surfaces correspond to Ra values; the lower the Ra value, the smoother the surface; the higher the Ra value, the rougher the surface. The most commonly used measurement method, Ra, is based on micrometers (μm) or microinches (μin). Ra is widely accepted and therefore preferred by engineers and machinists. Imagine you are designing a bottle cap mold. If the surface is rough (Ra value too high), it can cause plastic to stick or become defective. On the other hand, if the surface is too smooth, Ra value too low, it may increase costs without adding any real value to the product.

How to measure the surface finish of CNC custom parts ?

Elimold offers several methods for checking the levelness or roughness of a surface. We will explain the most common methods that can be used to measure surface finish.

Direct measurement method

This involves using a stylus profilometer to examine a surface and measure one point at a time. The stylus has a small tip radius and low contact pressure. It moves freely and detects changes in roughness and waviness when analyzing the surface properties of a material. Stylus profilometers are reliable and accurate. However, they may not be suitable for brittle materials because prolonged contact can lead to dimensional inaccuracies.

laser system

Lasers use the principle of light reflection to create 3D images. In this process, you can use a laser beam to scan the surface of a part and record the displacement as the beam reflects. This process is very sensitive and will not damage the surface. Note that these laser systems may produce erroneous results when processing transparent or highly reflective materials.

Appearance inspection

You can also perform visual inspection using the naked eye, a magnifying glass, or an electron microscope. Using only the naked eye is unreliable and should only be used as a starting point. Electron microscopes provide a magnified view of the surface and show all irregularities. They are non-destructive and can inspect a wide variety of engineered materials. However, for large and complex-shaped parts, visual inspection can be time-consuming.

Regional technology

Instead of measuring from a single point, they cover a larger area. For this, you can use optical profilometers and interferometers. It involves using a light source that illuminates the surface to measure topography and roughness. Optical profilometers are expensive and may miss minute features or details.

Atomic force microscope (AFM)

AFM uses a cantilever with a pointed tip to scan the surface of a component. It maps the contours and measures intermolecular properties such as friction and adhesion. AFM is very accurate and provides 3D surface profiles. This technique can be slow and can damage brittle materials.

choose the right surface finish for your custom CNC parts ?

When you decide to contact Elimold for a collaborative CNC custom parts project, another question might pop into your head: How do I choose the right surface treatment for my CNC parts? Well, choosing a CNC surface treatment can seem overwhelming at first, but it’s actually not that complicated. Our team of engineers has summarized the following five simple questions to help you make a decision. Simply answer these questions, and you’ll have a clear understanding of your part’s requirements. Let’s get started:

What is the purpose of this component?

This should be your starting point. Does your part have a practical use, or will it remain non-functional? For internal or concealed parts, a machined surface finish is often sufficient. However, for aesthetically pleasing or demanding parts, a smoother surface finish may be required.

How smooth does it need to be?

It’s worth noting that if your part will mesh with other components, it’s best to choose a smoother surface. Therefore, you should choose CNC surface finishing to make polishing or grinding smoother. Furthermore, improving the surface finish of the part makes it more durable.

Does appearance matter?

the appearance of most CNC parts is important. However, for products that customers can see and touch, visual appeal is paramount. Surface treatments such as sandblasting, anodizing, and even powder coating typically give your parts a glossy finish. Furthermore, some surface treatments even offer color options.

Will it be used outdoors or in harsh conditions?

What about waterproofing, heat protection, or chemical resistance? Choose a surface treatment that adds an extra layer of protection. Most importantly, anodizing or powder coating the surface of CNC-machined parts adds a layer of protection against rust, wear, or damage.

What is your budget?

Finally, as expected, some surface treatment options may offer better value for money, while others may be more expensive. However, if you are working on a project that requires a limited budget, you can start with the surface treatments that are essential for CNC parts . In some cases, standard surface treatments are sufficient.

Surface finish standards in manufacturing

There are two main surface finish standards in manufacturing: ISO 1302 and ASME Y14.36. Implementing these standards helps meet customer requirements and achieve quality assurance.

ISO (International Organization for Standardization) 1302

This is a standard that includes specifications and guidelines for surface roughness assessment to ensure accuracy and consistency across the industry. It describes procedures for measuring surface roughness, indicates graphical symbols for surface texture, and defines surface roughness parameters. The standard also includes methods for calculating surface parameters and how to calibrate measuring instruments. Adherence to ISO 1302 improves product quality and reduces recall rates.

ASME (American Society for Testing and Materials) Y14.36

This standard specifies methods for controlling the surface texture of solid objects. It covers many aspects of engineering drawings, such as architectural, mechanical, and structural drawings. The standard provides guidelines for scale and format, letter and line conventions, and symbols for representing surface roughness. Implementing ASME Y14.36 can reduce errors and improve efficiency.

Elimodl is your supplier of custom, high-quality surface-finished CNC parts.

When you entrust a project to the Elimold team for manufacturing, we tend to prioritize factors such as part composition, strength, dimensions, and tolerances during the manufacturing process. However, surface roughness is also a critical factor to consider. Excessively rough surfaces can lead to increased friction and premature part failure. Achieving precise surface roughness can be challenging and costly. This is where Elimold’s professional CNC custom parts surface treatment team can help. At Elimold, we offer one-stop CNC machining solutions to meet your needs, whether you require custom precision parts or high-quality surface treatments. Our expertise covers a wide range of materials, such as aluminum, titanium, stainless steel, brass, and copper, as well as plastic, rubber, silicone, and ceramic parts. Want to discuss your project requirements and determine the best machining process and surface finish for your product? Contact us now to request a project quote, begin your project, or contact our experts. We are ready to help you achieve the best results for your project.

Summarize

After reading this article, you will not only understand Elimold’s surface finish capabilities and relevant standards in the field of custom CNC parts, but also conclude from the above discussion that understanding surface finish is closely related to better control over product performance and appearance. Understanding the different types of CNC surface finishes will help you make the right decision when choosing them.

You know that surface treatment not only improves quality but also ensures parts function as designed. Therefore, you should always consider materials, functionality, cost, and the best CNC machining services. Custom high-standard CNC parts surface treatment service providers like Elimold can provide you with a large quantity of high-quality CNC parts. Contact Elimold now for a quote or more information.

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