Why do surface roughness limitations affect your choice of high-precision CNC machining services?
In surface metrology, surface roughness is a component of surface finish (surface texture). It is quantified by the deviation of the actual surface normal direction from the ideal shape. A larger deviation indicates a rough surface; a smaller deviation indicates a smooth surface. Roughness is generally considered to be the high-frequency, short-wavelength component of the measured surface. However, in the practice of custom CNC machining projects , it is often necessary to check whether the surface roughness of the part meets the design requirements to ensure that the surface is suitable for a specific purpose. Surface roughness is an indicator of the surface texture of a CNC machined part. It is the average of microscopic deviations and irregularities on the surface of a machined part. Therefore , the engineering team at Elimold has compiled information on why surface roughness limitations can affect your choice of high-precision CNC machining services.
First, let’s briefly understand what surface roughness is.
Surface roughness refers to the minute bumps and depressions present on a material’s surface. Engineers use this concept to determine whether a surface is smooth or rough. Surface roughness affects the assembly and movement of parts, as well as the lifespan of CNC machined parts . When your design has very specific requirements for the surface roughness of a part, manufacturers will check the surface roughness to ensure good product quality. Roughness plays a crucial role in determining how a real object interacts with its environment. While high roughness values are generally undesirable, controlling roughness during manufacturing can be both difficult and expensive. Designers and process engineers often need to strike a trade-off between the manufacturing cost of a part and its application performance.
Commonly used indicators of surface roughness
Therefore, in the engineering field, engineers use different methods to describe surface roughness. The most common surface roughness indices are as follows:
| Ra (mean roughness) | Ra is the arithmetic mean of the surface height deviations measured at a specified distance, and this deviation is the same as the deviation of the mean line. The mean line is the “average midpoint” of the peaks and valleys in the profile. The average height of all peaks above this line is Ra. |
| Rz (average maximum height) | Rz is the average maximum height of surface irregularities. It is the average vertical distance between the highest peak and the lowest valley over a given length. |
| Rp (maximum profile peak height) | Rp is the highest peak of the average line over the specified sampling length . |
| Rv (maximum profile valley depth) | Rv is the depth of the deepest valley from the average line over a specified sampling length. |
| direction | Direction is the primary direction of surface texture. It is an indicator of the direction in which most surface irregularities occur. |
Reasons for the influence of part surface roughness on the selection of precision CNC machining services
Surface roughness limits directly determine the functional performance, service life, manufacturing process selection, and overall machining cost of parts, making the selection of high-precision CNC machining services crucial.
| Friction and Wear Control | For moving parts such as bearings, guide rails, and joints, excessive surface roughness will increase friction, generate a lot of heat, and accelerate the wear of parts; while appropriate micro grooves can help retain lubricating oil. |
| Sealing performance | In hydraulic and pneumatic systems, surface roughness (such as Ra value) directly affects the fit of the sealing surface. Excessive roughness can easily lead to fluid or gas leakage. |
| Fatigue life and stress concentration | Rough surfaces contain microscopic tool marks and protrusions, which easily form stress concentration points. In scenarios involving cyclic loading, high tension, or high vibration, smooth surfaces can significantly extend the fatigue life of parts. |
| Industry-specific standards | For medical implants, optical components, or high-pressure fluid valves, there are extremely stringent red lines for surface roughness parameters (e.g., from standard CNC (Ra 3.2) to finishing (Ra 0.8) or even grinding grade (Ra 0.2), which cannot be met by ordinary processes or 3D printing). |
| Tool feed and parameter limitations | Achieving lower surface roughness requires adjusting the feed rate, spindle speed, and selecting special cutting tools. Insufficient feed rate may cause tool tip squeezing and slippage, while low-speed cutting is prone to building edge formation, which deteriorates the surface finish. |
| Multiple passes and additional optimization | Standard CNC milling/turning can typically only achieve a default texture of around Ra 3.2; for higher levels, machining service providers must introduce secondary or even multiple special processes such as grinding, polishing, and electrical discharge machining (EDM). |
| Time and economic costs | Pursuing an overly refined surface (over-design) will significantly increase machine tool downtime, manual polishing costs, and scrap rate. |
| Surge in costs due to structural constraints | For example, to meet surface and structural requirements, forcibly machining a 90-degree acute internal angle can lead to expensive specialized EDM processes, causing the total cost to skyrocket. For details, please refer to the cost analysis in the 3D Printing and CNC Machining Service Guide. |
The impact of surface roughness on part performance and why it affects your decision-making.
Surface roughness is a crucial consideration in many machining projects. Besides affecting functionality and aesthetics, it also impacts machining costs, workload, and time. Some of the effects of surface roughness on CNC-machined parts and why they might influence your decision are as follows:
Effect on coordination properties
The roughness of machined surfaces means that only a small portion of the contacting surfaces actually contact each other. For example, after precision machining, only 10% to 15% of the surfaces are in contact, thus affecting the fit of the parts. In interference fits, a large surface roughness reduces the actual interference amount because the peaks are sheared and flattened during assembly, lowering the connection strength of the mating parts. In clearance fits, an excessively large surface roughness value generates significant shear stress at the surface peaks, which is sheared off at the start of operation, resulting in substantial initial wear and increasing the actual clearance.
Impact on the wear resistance of parts
The wear resistance of a part is closely related to its surface roughness. For a part to meet the required precision grade, it is not only necessary for its dimensional accuracy and geometric tolerances to meet the requirements, but more importantly, its surface roughness must also meet the precision grade requirements. The smaller the surface roughness, the larger the contact area, the better the wear resistance, and the longer the service life, making it more economical and durable.
Influence on the fatigue strength of parts
Under alternating loads, surface roughness, scratches, and microcracks on parts can easily lead to stress concentration, causing and exacerbating fatigue cracks, ultimately resulting in fatigue failure. Therefore, reducing surface roughness is significantly effective in improving the fatigue strength of parts subjected to alternating loads.
Effect on surface corrosion resistance
A higher surface roughness value allows corrosive substances to more easily penetrate into the micro-cracks or adhere to the surface of the part, resulting in poorer corrosion resistance and greater susceptibility to corrosion. Surfaces that have undergone fine grinding, lapping, and polishing are less prone to corrosion.
touch
Friction is essential for maintaining contact between surfaces, and it increases with surface roughness. For components requiring non-moving contact, such as assembly parts and transported components, sufficient surface roughness is needed to maintain grip and minimize slippage.
sports
Friction is essential for static contact, but it is highly detrimental to moving, vibrating, or load-bearing mating parts (such as joints, bearings, and assemblies) because it impedes movement, increases energy consumption, generates heat, and accelerates wear on these parts. To minimize friction and improve wear resistance, surface roughness needs to be reduced. On the other hand, roughness creates grooves, which enhance lubricant retention, essential in moving systems.
Coating adhesion
Surface roughness affects the surface’s ability to support coatings. Microgrooves on relatively rough surfaces can trap coating substances, thereby improving the coating’s absorption and retention.
Aesthetics
Surface roughness directly affects the aesthetics of a part. Gloss, mirror finish, grainy texture, or matte finish depends on the interaction between surface roughness and light. For example, lower surface roughness results in better light reflection and a smoother surface finish because there are fewer irregularities in the scattered light.
cost
Achieving a specific surface roughness affects part cost. For example, achieving a lower surface roughness requires slower machine speeds, more frequent machining operations, and other considerations. This careful tool movement minimizes surface irregularities. However, it also increases machining time, workload, and cost.
More
In addition, surface roughness also affects conductivity, sealing and leak-proof properties, hygiene, and optical performance. Although surface texture is microscopic, it plays a crucial role in various applications. Surface roughness itself is neither good nor bad; the ideal surface roughness depends on the specific application of the part.
Based on Elimold’s experience with actual projects, we will analyze how surface roughness affects CNC machining services.
Surface roughness controls friction, sealing, and appearance in functional prototypes. Industrial-grade 3D printing results in a surface roughness Ra of 3.2-6.3 μm due to the particle size used (15-45 μm), while precision CNC machining services can directly reduce the roughness to Ra 0.8 μm, and further reduce it to Ra 0.2 μm through grinding. This difference will determine whether your prototype meets medical, optical, or hydraulic requirements , issues that professional 3D printing cannot address. Here’s how machining can meet surface roughness requirements:
Feed rate and tool geometry for submicron finishing
| Parameter optimization | The feed per revolution is reduced to 0.03 mm, using an ultra-fine carbide insert (R0.1 mm tip radius). |
| Customer benefits | A surface roughness of Ra≤0.8μm can be achieved with a single cut, saving post-processing grinding and reducing production time by 25%. |
Cutting strategies for specific materials
| method | PMMA (Ra=0.05μm) was machined using diamond turning; 304 stainless steel was machined using CBN grinding. |
| result | Achieving ISO 1302 N5 surface finish is possible without additional polishing. Commercial 3D printing cannot remove the pores beneath the surface. |
One-time setup eliminates re-clamping error
| process | Roughing and finishing can be completed on a five-axis machine tool without the need to readjust the workpiece position. |
| advantage | The dimensional tolerance is ±0.005mm, and the surface quality meets the requirements. Desktop 3D printing requires the removal of the support structure and a vapor smoothing process, which takes an additional 2-3 days. |
Mirror-level performance to meet extreme demands
| technology | Use 400#, 800# and 1200# grinding wheels for progressive grinding, then polish with diamond paste. |
| result | Mirror polishing achieves a surface roughness Ra of 0.2µm, suitable for hydraulic cylinders and optical reflectors. Even with chemical polishing, the surface roughness Ra of economical 3D printed products cannot reach below 1.6µm. |
Choosing this surface finishing method bypasses the inherent minimum surface quality limitations of custom 3D printing. This technology allows for guaranteed surface finishes with a surface roughness Ra between 0.2 and 0.8 μm, eliminating the need for secondary finishing. Even the most sophisticated 3D printing services cannot match the surface control offered by machining.
More typical industry application cases
| Aerospace | An international engine manufacturer performed five-axis CNC precision turning and ultra-precision grinding on Ti-6Al-4V turbine blades, optimizing Ra from 1.2 µm to 0.08 µm. After high-cycle fatigue testing and 500 °C oxidation resistance testing, fatigue life was improved by approximately 25%, avoiding early failure caused by surface micro-dimpling. |
| Medical device field | A collaborative project at a top-tier tertiary hospital focused on titanium alloy hip joint prosthesis bases. The bases were first polished and then subjected to nano-blasting, reducing the Ra (Ra particle size) from 0.3 µm to 0.05 µm and creating uniform micro-pits approximately 5 µm in diameter on the surface. Clinical data showed that patients experienced an average 15% reduction in postoperative recovery time and a significant decrease in the rate of repeat surgeries. |
| New energy electric vehicle manufacturing | A leading automaker has adopted a CNC precision grinding and ultrasonic cleaning process for the welding surface of aluminum alloy battery boxes, controlling Ra to within 0.8 µm. This increases the lifespan of the sealant in the IP68 dustproof and waterproof test from 500 h to over 800 h, significantly improving the overall vehicle safety and durability. |
| Consumer electronics sector | After rough machining by CNC, the mid-frame of a certain flagship model has an Ra of about 1.5 µm. Subsequent two-stage fine grinding and anodizing processes bring the Ra to 0.4 µm and form a micropillar structure with a diameter of about 0.8 µm on the surface, which greatly improves scratch resistance and grip. The scratch rate decreased by 30% in drop tests. |
Selecting Elimold Company for a precision CNC machining parts project
Elimold is your reliable partner for providing high-quality, durable, and precision CNC machined parts . We offer over 40 surface finishes. Our dedicated and highly skilled team of machinists meticulously manufactures components for various industries to meet the exact requirements of our clients. With over 20 years of experience in this industry, we serve over 100 countries. Our services include rapid prototyping, custom manufacturing, and mass production of parts. We are ready to share our knowledge and meet all your machining needs. Contact us today to turn your ideas into reality in the most cost-effective way.
in conclusion
Surface roughness is crucial in the manufacturing process because it determines the function of parts and improves customer satisfaction. However, achieving precise surface roughness can be challenging and costly. In such cases, you can seek help from a professional machining team to address this challenge. Here, as a one-stop machining manufacturer, Elimold will handle your project with cost-effective solutions to meet stringent surface finish standards.