What Is CNC Machining Surface Roughness? A Complete Guide

CNC machining surface roughness is a measure of the micro-level deviations and irregularities on a material’s surface after machining. This parameter affects not only the aesthetics of a component but also its functionality and durability in various applications. This article comprehensively explores surface roughness, the different standard roughness levels, how to achieve them, and their impact on part functionality, aesthetics, and cost. The article also covers choosing the right CNC machining surface roughness for your part.

What is Surface Roughness?

Why is Surface Roughness Important? The Impact of Surface roughness on CNC machined parts

Understanding CNC Machining Surface Roughness Levels

Selecting the Right CNC Machining Surface Roughness

How is CNC Machining Surface Roughness Achieved? Factors that Influence Surface Roughness

Surface Roughness Vs Surface Finish

Conclusion

Why Choose Geomiq?

What is Surface Roughness?

Surface roughness is a measure of the texture of a part's surface after machining. It is an average of the microscopic deviations and irregularities on a machined parts surface. These deviations, inherent in CNC machining, result from the cutting tool's material removal action. CNC machining tools remove material from the surface of the workpiece by progressively scraping it off in a series of numerous cuts. These repeated cuts, the movements of the tool and workpiece, and several other factors result in microscopic surface irregularities. While CNC machining surface roughness is a microscopic feature, it significantly impacts the functionalities of parts in certain applications. CNC machining surface roughness in this context refers to the surface texture of a part as-machined or after primary finishing processes, such as blasting and polishing.

CNC machining surface irregularities

Surface roughness is a physical attribute that is quantified using several metrics. These metrics define the size and characteristics of surface irregularities and are physically measured and calculated. The most common surface roughness metrics, as defined by ISO 21920-2:2021, are as follows:

• Ra (Average roughness): Ra is an arithmetic average of the surface height deviations measured from the mean line over a specified distance. The mean line is the “average middle” of the peaks and valleys in the profile. The average height of all peaks above this line is the Ra.
• Rz (Average maximum height): Rz is the average maximum height of the surface irregularities. It is the mean of the vertical distances between the highest peaks and the lowest valleys over a given length.
• Rp (Maximum profile peak height): Rp is the highest peak from the mean line over a specified sampling length
• Rv (Maximum profile valley depth): Rv is the depth of the deepest valley from the mean line over a specified sampling length.
• Lay: Lay is the prevalent direction of the surface texture. It is a measure of the direction in which most of the surface irregularities lie.

The image below illustrates these metrics over a sample length of a part’s surface.

Surface roughness metrics

Ra is the most ubiquitous metric for defining surface roughness in CNC machining and manufacturing in general and is measured in µm (micrometres). The lower the Ra value, the less variation in surface irregularities and, therefore, the smoother the surface. As the Ra value increases, the surface becomes more coarse and textured.

Why is Surface Roughness Important? The Impact of Surface roughness on CNC machined parts

Surface roughness is a crucial consideration in many CNC machining projects. In addition to affecting functionality and aesthetics, it also impacts machining costs, effort, and time. Some of the effects of surface roughness on CNC machined parts are as follows:

• Contact: Friction is critical for maintaining contact between surfaces and increases with surface roughness. Parts that require non-motion contact, such as fitted assemblies and parts to be handled, need sufficient surface roughness to maintain grip and minimise slippage.
• Motion: Friction, critical for static contact, is very detrimental in moving, vibrating, or load-bearing mating parts, such as joints, bearings, and assemblies, as it resists motion, increases energy consumption, generates heat, and increases wear in these parts. Low surface roughness is required to minimise friction and improve wear resistance. On the other hand, roughness creates grooves that enhance lubricant retention, which is necessary in moving systems.
• Coating adhesion: Surface roughness impacts the ability of a surface to support coating. The microscopic grooves on a relatively rough surface lead to better coating absorption and retention by trapping the coating substance.
• Aesthetics: Surface roughness directly affects the aesthetics of a part. Glossy, mirrorlike, grainy, or matte appearances are determined by how surface roughness levels interact with light. For example, low surface roughness results in better light reflection and a glossier surface finish, as there are fewer irregularities to scatter the light.
• Cost: Achieving specific surface roughness impacts the cost of parts. For example, achieving low surface roughness requires slow machining and multiple passes of the machine tool, among other considerations. This careful movement of the tool minimises irregularities on the part's surface. However, it also increases machining time, effort, and cost.

In addition, surface roughness also affects electrical conductivity, sealing and leakage prevention, sanitation, and optical properties. While surface texture is microscopic, it plays crucial roles in different applications. Surface roughness levels are not inherently good or bad, and the ideal surface roughness depends on the specific application of the part.

Understanding CNC Machining Surface Roughness Levels

CNC machining surface roughness is typically measured in average roughness (Ra). Ra values for manufactured parts range from 0.1 µm Ra to 6.3 µm Ra (smooth to rough). Note that surface roughness levels are achievable outside of this range. For example, silicon wafers used in semiconductor manufacturing can be manufactured to a surface roughness of 0.01 µm Ra. The image shows CNC machining surface roughness levels for CNC milling and CNC turning.

CNC machining surface roughness levels compared

Surface roughness is not random. Instead, manufacturers take deliberate action to achieve specific Ra values. Customers can request a specific surface roughness level for their part. For standardisation, most CNC machining custom manufacturers offer four surface roughness levels.

• 3.2 µm Ra
• 1.6 µm Ra
• 0.8 µm Ra
• 0.4 µm Ra

These surface roughnesses have different textures, characteristics, benefits, limitations, and applications where they are best suited.

3.2 µm Ra

3.2 µm Ra is the standard commercially available CNC machining surface roughness. Manufacturers apply it as the default roughness unless the customer specifies otherwise. 3.2 µm Ra is characterised by visible machine cut lines but is suitable and sufficiently smooth for most consumer parts. This CNC machining surface roughness is the recommended maximum for parts that undergo stress, loading, or vibrations. 3.2 µm Ra is the baseline for parts, so it doesn't result in additional costs. The part in the image below has a 3.2 µm Ra surface roughness.

3.2 µm Ra CNC machining surface roughness (CNC milling - Aluminum)

3.2 µm Ra sample applications:

• Structural Machine Brackets: Brackets and mounts for industrial equipment often have a 3.2 µm Ra surface roughness, as these don’t require a highly polished surface.
• Automotive Engine covers: These parts are typically exposed to moderate stress and vibrations but do not require an ultra-smooth finish.
• General Tooling Fixtures: Fixtures or jigs used in manufacturing processes can be CNC-machined to a 3.2 µm Ra surface roughness, sufficient for functional use.
• Machine Chassis: External machinery frames, like the chassis or framework, are often machined to a 3.2 µm Ra surface roughness.

1.6 µm Ra

1.6 µm Ra CNC machining surface roughness level is recommended for tight fits and parts subject to stress. It is also suitable for slow-moving, slightly vibrating, and light load-bearing surfaces. Slightly visible faint cut marks characterise this CNC machining surface roughness. 1.6 µm Ra is achieved via high machining speeds, slow feeds, and shallow cuts. This CNC machining surface roughness level increases part production cost by 2.5 %. The part in the image below has a 1.6 µm Ra surface roughness.

1.6 µm Ra CNC machining surface roughness (CNC milling + turning - Stainless steel)

1.6 µm Ra sample applications:

• Hydraulic Piston Rods: Tight-fitting and subject to moderate loads, these rods need a smoother finish to ensure proper sealing and reduced friction.
• Slow-speed gearboxes: 1.6 µm Ra is suitable for light-load,  less dynamic mechanical systems with slow-moving components.
• Precision Fasteners: Custom bolts and fasteners requiring tight fits require a 1.6 µm Ra surface roughness level to ensure proper mating
• Electronic Housings: Electronic devices, especially for consumer electronics like laptops or industrial control systems, are often machined to a 1.6 µm Ra for aesthetic and assembly fit purposes.

0.8 µm Ra

Classified as a high-grade finish, this CNC machining surface roughness requires finishing cutting passes to achieve. 0.8 µm Ra is ideal for parts subject to stress concentration and loading. It is also suitable for vibrating parts and moving components. This CNC machining surface roughness adds 5% to the baseline production cost, as it requires close control and meticulous machining to produce. The part in the image below has a 0.8 µm Ra surface roughness.

0.8 µm Ra CNC machining surface roughness (CNC turning - Aluminium)

0.8 µm Ra sample applications 

• Precision Gears: Gears used in applications like robotics often require a 0.8 µm Ra surface roughness for efficient movement and minimal wear between contacting gear surfaces.
• Hydraulic Valve Components: Critical for systems that need fluid-tight seals and minimal friction. The smoother surface enhances performance in high-pressure systems.
• Medical Device Housings: Surgical tools and housings for medical devices made from stainless steel or titanium often require a 0.8 µm Ra surface roughness to prevent contamination and facilitate sterilisation.
• Jewellery Components: High-end metal jewellery parts often feature a 0.8 µm Ra surface roughness to achieve a polished, luxurious look.

0.4 µm Ra

0.4 µm Ra is considered a very high-grade smooth texture and is the finest CNC machining surface roughness most manufacturers offer. It has no observable cut marks and is usually achieved by meticulous, closely controlled machining followed by polishing. This additional manufacturing effort increases production costs by up to 15%. 0.4 µm Ra surface roughness is suitable for rapidly moving or vibrating mating parts and parts under high tension and stress. The part in the image below has a 0.4 µm Ra surface roughness.

0.4 µm Ra CNC machining surface roughness (CNC turning + polishing - Steel)

0.4 µm Ra sample applications

• Bearing Surfaces: High-precision bearings, such as those in aerospace or high-speed machinery, require an ultra-smooth finish to reduce friction and wear.
• Pneumatic Cylinder Rods: In pneumatic systems, cylinder rods with a 0.4 µm Ra finish ensure smooth operation, minimising air leaks and reducing seal wear.
• Optical Components: High-precision optical components, such as lens mounting plates, are CNC-machined to 0.4 µm Ra to avoid interference with the optical path.
• Precision Injection Molds: Molds used for high-quality plastic injection moulding, such as medical implants, are often CNC-machined to a 0.4 µm Ra surface roughness to ensure smooth surfaces.

Selecting the Right CNC Machining Surface Roughness

While surface roughness is inconsequential in certain applications, it plays a vital role in several others. In critical applications, surface roughness can impact parts' functionality, performance, durability, and aesthetics. It also impacts machining time and cost. CNC machining surface roughness level requirements vary by application. There are no inherently good or bad levels, only appropriate levels for the specific requirement. Therefore, specifying the right CNC machining surface roughness for your part is crucial. The following are important considerations in the selection process.

Functionality and purpose

The intended application of your part is the primary consideration when selecting the right CNC machining surface roughness. Surface roughness influences your part’s properties and interaction with other parts and the working environment. It can impact friction coefficient, noise levels, wear resistance, absorption levels, optical properties, load-bearing properties, durability, electrical conductivity, lubrication, and numerous other functionalities and properties. The table below outlines various functionalities of CNC machined parts, the impact of surface roughness on these functionalities, and the corresponding recommended surface roughness levels.

Impact on CNC machining surface roughness on part functionality

Aesthetics

CNC machining surface roughness significantly affects the aesthetics of a part and is vital to achieving desired looks. Shiny, reflective surfaces such as jewellery or decorative parts require a smooth surface roughness of 0.8 and below. The lower the surface roughness level, the shinier the part. If you prefer a textured look, specify a surface roughness of 1.6 and above.

Cost and production time

Smoother CNC machining surface roughnesses require more machining effort to achieve. Slower machine speeds, finer feeds, and shallow cuts are all required to achieve low roughness values. Ra levels of 0.4 and below may need further polishing to achieve. The precise manufacturing and additional processes require more time and effort, increasing production costs.

How is CNC Machining Surface Roughness Achieved? Factors that Influence Surface Roughness

Surface roughness is not a randomly occurring feature. Rather, manufacturers take precise steps to achieve specific Ra values. The following are some of the factors that influence CNC machining surface roughness. They include precisely adjustable parameters, required conditions, and necessary precautions. Note that these factors are interrelated and should be considered simultaneously.

Machining parameters

The following machining parameters affect a-machined CNC machining surface finish:

• Cutting speed: Cutting speed is the linear speed of the cutting tool relative to the workpiece. Measured in metres per minute, it accounts for the spindle speed (RPM) and the tool diameter.  Due to the conservation of angular momentum and the gyroscopic effect at play, the faster the cutting speed, the more stable the tool or workpiece is, leading to fewer vibrations and smoother cuts. Note that speeds that are too high could increase friction, leading to roughness.
• Feed rate: This is the rate at which the cutting tool moves into the workpiece. At lower feed rates, the cutting tool has more time per surface area to remove material, leading to a smoother surface roughness.
• Cut depth: Cutting depth is a measure of how deep into the material's surface the cutting tool moves per cutting pass. The deeper the cut, the higher the tendency for the tool to vibrate or deflect, as it is cutting a larger surface area. A smaller cut depth usually results in a finer CNC surface finish, while deeper cuts tend to produce rougher surfaces.

These parameters are adjustable and require a mix and match of different values to achieve the desired surface roughness. The image below illustrates these CNC machining parameters.

CNC machining cutting parameters

As illustrated by CNC machining cutting parameters and surface roughness research, the precise values of each parameter to produce specific CNC machining surface roughnesses vary by several factors, including the machining process and materials. Manufacturers and engineers use mathematical formulas to determine the ideal machining parameters to achieve a specific CNC machining surface finish in a material. The chart below shows the relationship between cutting speed and surface roughness for mild steel. It shows a proportional decrease in surface roughness with an increase in cutting speed.

Graph of CNC machining surface roughness against cutting speed for mild steel

Cutting tool

The condition and properties of the cutting tool impact CNC machining in the following ways:

• Tool Geometry: The cutting tool's shape, angle, and sharpness significantly affect surface roughness. Tools with large rake angles and sharp edges produce smoother surfaces. 
• Tool Wear: Worn tools create rougher surfaces due to uneven contact with the workpiece surface, uneven cutting action, and increased friction.
• Tool Material: The material of the cutting tool impacts how efficiently it can cut through the workpiece, influencing the final surface texture. Harder tool materials, such as carbide, diamond, and tool steel, can better overcome the workpiece’s resistance to cutting, leading to smoother surface roughness.

A selection of CNC machining cutting tools

Cutting tool hardness is a crucial consideration in surface roughness. Several studies on the correlation between tool hardness and surface roughness indicate that tool hardness significantly affects flank wear and surface roughness.

Machining conditions

Machining conditions indirectly affect the CNC machining Ra value of a part.

• Machine movement: CNC machines that vibrate excessively or shift during operations can lead to unwanted surface roughness levels. Furthermore, improper movement of machine parts, such as unbalanced spindle rotation, causes uneven cutting, which impacts surface smoothness.
• Workpiece alignment and positioning: Misalignment between the tool and workpiece can create surface irregularities due to unplanned movements. In addition, proper clamping and positioning of the workpiece, using jigs and fixtures, is necessary for achieving the desired CNC machining surface roughness level.
• Temperature control: Elevated temperatures due to friction during cutting leads to thermal expansion of the workpiece. This expansion often results in unintended CNC machining ra values. Proper temperature control using cutting fluids is necessary to achieve the intended CNC machining surface roughness.

These conditions are not usually obvious and should be treated as precautions.

Temperature regulation to improve CNC machining surface roughness, using cutting fluids

Machinability

The machinability of a material is a combination of various properties of a material that determine how easy it is to machine. These properties determine the material's behaviour during machining and can affect several outputs, including the surface roughness of the finished part. Some of the properties that determine machinability and their impact on CNC machining surface roughness are as follows:

• Hardness: Resistance to cutting and abrasion increases the vibration of the cutting tool and increases friction, leading to rougher surfaces.
• Thermal expansion: Expansion of the workpiece due to friction-induced temperature increases directly leads to uneven cutting.  
• Work hardening: The cutting action of the workpiece subjects it to shear stress and some plastic deformation. This deformation can lead to strain hardening in some materials, increasing the impact of hardness.

Post-processing

As-machined surface finish cannot always achieve the desired CNC machining surface roughness levels. Mechanical post-processing operations are often necessary, especially for lower Ra values ≤ 0.4. The following post-processing operations help achieve specific CNC Machining Surface RA values.

• Grinding and polishing
• Bead blasting
• Brushing

These processes are known as primary finishes and are part of a larger group of CNC machining surface finish operations.

Grinding and polishing involves using progressively less abrasive polishing compounds and tools to even out surface irregularities. Polishing paraphernalia usually comes with specified values roughness values that they achieve. Continuous polishing can eventually result in a mirror-like surface finish. However, this may come at the expense of dimensional accuracies. Various forms of polishing include tumbling, vibratory polishing, and grinding.

Vibratory polished CNC machined part

Bead blasting involves blasting a stream of abrasive particles at the surface. It leaves a matte, grainy finish. Being a manual process, bead blasting is not always ideal for achieving specific surface roughnesses as the final results depend on several factors, including blasting media grit and operator skill.

Bead blasted CNC machined part

Brushing involves using grit to polish the material in one direction. This process leaves a unidirectional satin finish. Brushing typically provides a CNC machining surface roughness of 1.2 µm Ra and is mainly used for aesthetics. However, this figure may vary with grit size.

Brushed CNC machined part

CNC Machining Surface Roughness Vs Surface Finish

CNC machining surface roughness and surface finish are often used interchangeably. However, they describe two different things and should not be confused with each other. Surface roughness specifically refers to the surface texture (level of surface irregularities) of a finished part after machining or primary processing. Surface roughness is a measurable feature with specific values. It describes one out of many aspects of a surface’s properties.

CNC machining surface finish: Blue anodised

CNC machining surface finish, on the other hand, describes the state of the finished parts’ surface, as well as the process involved in achieving that state. It refers not only to the texture or look of the surface but also to the overall physical and chemical state of the finished part's surface. Various CNC surface finishes are achieved via different electrical, chemical, and physical finishing processes that alter the parts’ surfaces, achieving different functional and cosmetic properties. Most of these processes do not account for the Ra value of the part. The table below describes common secondary CNC surface finishes.

Secondary CNC surface finishes

See our CNC surface finishes gallery for more details and pictures on various CNC machining surface finishes.

How is Surface Roughness Measured?

After manufacturing, the surface roughness is measured to ensure it is at the required level. It may also be necessary to measure surface roughness intermittently during processing. The following are popular techniques and technologies for measuring surface roughness.

1. Contact profilometers: These devices measure surface roughness by measuring physical deviations across the surface of the specimens using a highly sensitive diamond-tipped stylus. The stylus transverses across the specimen’s surface, recording microscopic irregularities and deviations before calculating and displaying the Ra and other roughness values. Contact profilometers may be desktop machines or portable handheld devices. While these devices are accurate, they may be time-consuming and can scratch the measured surface.

2. Non-contact profilometers: These devices work similarly to their contact counterparts, with the exception of a stylus. Non-contact profilometry uses various non-contact technologies such as laser triangulation, digital holography, interferometry, or confocal microscopy to scan the surface and measure surface roughness. These devices are faster than contact profilometers and can measure surfaces without the risk of damage, making them suitable for delicate, fine surfaces.

3. Atomic Force Microscopy (AFM): AFM  is ideal for highly polished surfaces or where extremely fine detail is essential. It provides nanometer-level resolution by using a sharp probe to scan the surface. This highly advanced technology measures several other parameters, such as magnetic and mechanical properties, in addition to surface roughness.

4. 3D Scanning: Advanced 3D scanning technologies allow for the creation of surface topography maps, enabling roughness to be measured across a larger area. This method can offer comprehensive insights into the surface structure but typically requires sophisticated equipment.

5. Comparison to Roughness Standards: In more practical settings, surface roughness can be estimated by visually comparing the machined part to pre-calibrated roughness comparison specimens. While this method lacks precision, it is fast and suitable for non-critical surface roughness measurements.

Each of these methods provides key metrics such as Ra (average roughness), Rz (average maximum height), and Rt (total roughness height), depending on the required level of detail and surface characteristics. Selecting the appropriate measurement technique depends on the surface’s requirements and the application in which the part will be used.

Conclusion

Surface roughness is a critical parameter in CNC machining that directly impacts a part’s functionality, cost, and appearance. Understanding different surface roughness levels and selecting the right level for your part ensures optimal performance. By carefully controlling machining parameters, tool conditions, and post-processing techniques, manufacturers can achieve the desired roughness levels tailored to your specific applications.

Why Choose Geomiq?

Looking for a custom manufacturer for your manufacturing needs? Geomiq is your ultimate partner for all your CNC machining needs. We combine deep expertise and state-of-the-art milling and turning equipment and techniques to achieve the exact specifications you require.

We understand the criticality of surface roughness for your part and go beyond industry standards by offering four additional surface roughness levels: 0.2 μm Ra, 0.1 μm Ra, 0.05 μm Ra, and 0.01 μm Ra. Thus providing the option of the ultra-smoothest roughness levels possible. We also offer an extensive lineup of CNC machining surface finishes.

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