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All You Need To Know About Surface Finishing For 5-Axis CNC Machining

CNC machines are known for their precision cutting and shaping ability. However, most cutting processes leave ridges or rough edges that may cause friction or look like unfinished pieces. In some cases, these rough edges play a vital role in maintaining necessary friction between different parts, but some precision parts must sit flush with each other. This is where the additional process of CNC surface finishing comes in.

In this article, we’ll help you explore every detail you need about Surface Finishing and how different CNC machining fields use it.

But before we dive in, let’s briefly understand what CNC machining is, to understand the need for CNC Surface Finishing better.

What Is Precision CNC Machining?

CNC machining, or Computer Numerical Control machining, is a process where machines are controlled by pre-programmed software to shape and cut materials into parts. It’s a key part of subtractive manufacturing, which means the machine removes material from a solid block to create the desired shape. As the material is cut away, it often leaves behind rough surfaces that require further refinement. This is where the importance of CNC surface finishing comes in. Smoothing out those rough areas and ensuring the final product meets functional and aesthetic standards. But, smoothing out rough areas is very calculated. Let’s understand the measurement of the Roughness Average (Ra) to know how CNC surface finishing ensures that we get the best of both texture and smoothness to make superior precision parts using precision CNC machining.

Surface Finish vs. Surface Finishing in CNC Machining

Although we seem to use the terms, surface finish and surface finishing, they both have very distinct meanings in CNC machining.

Understanding Surface Finish

Surface finish for CNC parts in CNC machining refers to the texture and smoothness of a part’s surface after it has been machined. It’s all about the small deviations and changes in the surface’s texture, which are a result of the machining process. Surface finish is crucial because it affects not only the appearance of a part but also its performance, including factors like wear resistance, corrosion resistance, and friction. The most used method to measure surface roughness or surface finish is thw Roughness Average (Ra), which calculates the average deviation of the surface from its ideal smooth shape. This is measured in micrometers (μm) and helps determine how smooth or rough the surface is. Tools like surface roughness testers and profilometers are often used to measure this deviation precisely.

What is CNC Surface Finishing?

On the other hand, CNC surface finishing involves the process used to achieve that texture—whether it’s anodizing, powder coating, or electropolishing. While as-machined surface finishes reflect the texture left by the cutting tool, CNC surface finishing goes further by adding protective or aesthetic layers to improve the part’s performance or appearance.

Factors Affecting Surface Finish

Achieving the right surface finish for CNC parts in CNC machining involves several factors, including:
  • Material properties: Some materials, like metals, may be more prone to surface imperfections.
  • Machining technique: Processes like turning, milling, or drilling produce different surface textures.
  • Cutting parameters: Adjusting the cutting speed, feed rate, and depth of cut can improve the final surface.
  • Tool selection: The type of cutting tool used can significantly influence the texture of the machined surface.
By adjusting these factors, machinists can manipulate the surface finish for CNC parts to meet the specific needs of the product, whether it’s for functional parts or those with aesthetic requirements.

Next: As-Machined Surface Finishes

Now that we know that even the texture of the machined parts plays a vital role in maintaining the precision of the parts, let’s discuss in detail about As-Machined Surface Finishes.

As-Machined Surface Finishes For CNC Parts

Automotive

Sometimes, parts are left as-machined, meaning they don’t go through additional finishing processes after CNC machining. These parts retain the natural texture created by the cutting tool, often leaving behind visible tool marks or a slightly rougher surface. An as-machined surface finish is acceptable for many applications, particularly where the visual appearance isn’t a priority but functionality is.


Here’s a table to allow you to understand the significance of different Ra values.
Ra Value Characteristics Application
3.2 μm Ra
Smooth, typical machine finish with visible cut marks. Standard for most consumer parts.
Suitable for parts exposed to loads, vibrations, and stress. Often used when no special surface finish is required.
1.6 μm Ra
Very faint cut marks, ideal for tight fittings. Produced using high speeds, fine feeds, and light cuts.
Best for slowly moving or mildly load- bearing surfaces. Not recommended for parts subjected to high-speed motion or vibrations.
0.8 μm Ra
High-quality finish, expensive to produce, with strict control over machining conditions.
Ideal for components subject to concentrated stress, intermittent motion, or light loads (e.g., certain types of bearings).
0.4 μm Ra
Ultra-smooth finish, requiring significant effort and precision.
Best for highly strained parts, fast- rotating components (e.g., bearings, shafts), and surfaces where smoothness is critical.

How Do You Get A Good Surface Finish With CNC Machines?

Achieving High-Quality Surface Finish in CNC MachiningAchieving a high-quality surface finish for CNC parts in CNC machining starts with the right CNC machine and the proper adjustments to cutting speed, feed rate, and tool selection.Several CNC machining processes contribute to achieving an as-machined finish, including milling, turning, and grinding. These methods not only shape the part but also directly influence the final surface texture.

Milling and Turning

Both milling and turning are fundamental mechanical finishing methods that remove material in a controlled manner, producing the part’s final shape and surface. Milling is often used for complex geometries, while turning is ideal for cylindrical parts.

The finish achieved in these processes depends on factors like cutting speed, tool sharpness, and feed rate. Parts that undergo these methods typically have Ra values in the range of 3.2 to 6.3 µm, making them acceptable for many industrial applications, such as automotive components or commercial aerospace parts, where appearance is secondary to function.

For optimal performance, Phillips Machine Tools India offers cutting-edge solutions like the HAAS UMC-750 for versatile 5-axis CNC machining for milling, and the HAAS ST-20Y for high-precision CNC machining for turning operations.

Grinding

For parts requiring enhanced precision and smoother surfaces, grinding is essential. This process involves using abrasive wheels to achieve extremely fine surface finishes, sometimes as smooth as 0.1 µm Ra. Grinding is widely used in commercial aerospace, tool manufacturing, and medical devices, where the highest quality surface finish is essential.

Phillips Machine Tools India offers advanced grinding solutions such as the HAAS Multigrind® CAand KENT KGS-84AHD surface grinder, both of which are known for delivering high-precision results. These machines are ideal for achieving superior as-machined surface finishes without the need for further post-processing.

Check out our 5-axis CNC Machines that can help you achieve the right surface finish for your manufacturing processes.

Apart from using the right precision CNC machines, reducing the feed rate while increasing the cutting speed usually results in a smoother finish. Additionally, using the right cutting tool material and applying coolants can prevent excessive heat build-up, which can degrade the surface quality.

But, if you still haven’t achieved your desired surface finish yet, the next section covers the different CNC surface finishing processes.

Common Types of Surface Finishing For CNC Parts

When as-machined surface finishes aren’t enough for the precision and aesthetics of the
machined parts, CNC surface finishing processes come into play.

Below are some of the most commonly used finishing methods, each offering distinct benefits
and applications.

Anodizing

Anodizing is a chemical process where metals, primarily aluminium and titanium, are coated with a protective oxide layer. This oxide layer enhances corrosion resistance and improves durability, making it ideal for commercial aerospace components exposed to extreme environmental conditions. Additionally, anodizing offers the flexibility of various colour options, allowing for aesthetic customization.

In the commercial aerospace industry, anodized parts help reduce weight while maintaining strength. Anodizing also improves wear resistance and can act as an insulating layer in electronics, further enhancing its versatility.

Alodine Coating (Chromate Conversion Coating)

Alodine coating, also known as chromate conversion coating, is used to treat metals like aluminium. This method creates a protective layer that enhances corrosion resistance and prepares surfaces for subsequent painting or powder coating. One of its advantages is that it maintains electrical conductivity, making it suitable for electronics applications.

In commercial aerospace, Alodine coating ensures that parts maintain integrity under high-stress conditions while bonding effectively with additional coatings, ensuring lasting performance.

Electropolishing

Electropolishing is a chemical process that removes a thin layer of material from a metal surface, resulting in a smooth, mirror-like finish. This method is particularly effective in reducing micro-roughness, improving corrosion resistance, and ensuring that parts meet strict cleanliness standards.

This finish is widely used in medical devices, where sterility is critical, as well as in pharmaceutical and food processing equipment, where smooth surfaces prevent bacterial growth.

Passivation

Passivation improves the corrosion resistance of stainless steel by removing surface impurities and creating a thin, protective oxide layer. This process enhances the metal’s performance in challenging environments, such as marine or chemical processing applications.

Passivated stainless steel is essential in commercial aerospace and medical applications, where durability and resistance to harsh conditions are required.

Bead Blasting

Bead blasting is a mechanical process that involves shooting tiny glass or ceramic beads at a part’s surface to create a uniform, matte texture. It is commonly used in consumer electronics for its aesthetic appeal, giving products a sleek, satin-like finish.

Apart from aesthetics, bead blasting prepares parts for further treatments, such as painting or plating, by smoothing out minor imperfections.

Powder Coating

Powder coating involves applying a polymer powder to a metal surface and curing it to form a durable, protective layer. It enhances the part’s resistance to corrosion, impact, and UV radiation. Powder coating is widely used in automotive components, as it can withstand harsh weather and mechanical stress.

This finishing method is also valued for its environmental friendliness, as it produces little to no harmful emissions compared to traditional liquid coatings.

Brushing

Brushing uses abrasive pads to create linear or circular patterns on metal surfaces, improving their visual appeal. This technique is particularly popular for stainless steel parts used in kitchen appliances and consumer products, where the aesthetic finish matters as much as functionality.

Brushed finishes are often used in architectural elements as well, where durability and aesthetics go hand in hand.

Polishing

Polishing is a mechanical process that uses abrasives to achieve a smooth, glossy surface. This finish enhances both reflectivity and aesthetic value, making it ideal for luxury products such as jewellery or high-end automotive parts.

Polishing also improves the part’s resistance to corrosion by eliminating surface roughness that could trap moisture or debris.

Painting

Painting involves applying a colour-based coating to enhance both the appearance and protection of a part. It is a versatile finishing method that works on a variety of materials, including metals, wood, and polymers.

In the automotive and furniture industries, painting allows for creative customization while protecting parts from environmental wear and tear.

Choosing the right surface finish depends on the material and the specific needs of the project. Each finish adds its unique set of advantages, whether it’s enhancing corrosion resistance, improving wear properties, or simply making the part more visually appealing.

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Pre-Surface Finish Processes For CNC Parts

Before CNC surface finishing methods like anodizing, plating, or powder coating can be applied, the CNC-machined parts need to be properly prepped. These pre-finishing steps are essential to ensure that the surface finish adheres correctly and performs as expected. Below are the key pre-surface finish processes:

Degreasing

During machining, the workpiece comes into contact with various contaminants, including oils, coolants, and lubricants that reduce heat and friction. However, these residues must be removed before surface treatment to ensure proper adhesion. Degreasing can be performed using chemical solvents or ultrasonic cleaning to eliminate even the smallest traces of contaminants.
  • Why It Matters: Without proper degreasing, coatings like anodizing or plating may not bond effectively, leading to poor durability or corrosion resistance.
  • Techniques Used:Solvent baths, ultrasonic cleaning systems, or aqueous cleaning solutions based on the material requirements.

Deburring

Burrs are tiny, sharp edges or protrusions left on the workpiece after machining. These imperfections can negatively impact the performance, fit, or safety of the part. Deburring ensures these edges are removed, resulting in a smoother part that is safer to handle and performs better when assembled into larger systems.
  • Why It Matters: Burrs can interfere with tight tolerances and compromise surface treatments, leading to uneven finishes or failed coatings.
  • Techniques Used: Manual filing, deburring brushes, and vibratory finishing machines depending on the size and complexity of the part.

Grinding

In some cases, additional grinding is needed to smooth out the surface and remove any material that remains after initial machining. Grinding provides the fine surface quality required for high-precision parts and helps prepare them for smooth surface finishes. This step is particularly useful when working with materials prone to contamination or when achieving a highly accurate surface profile is required.
  • Why It Matters: Grinding ensures the surface is even and free of irregularities, preparing it for processes like plating or anodizing, which require precise, smooth surfaces.
  • Techniques Used: Surface grinders, belt grinders, and cylindrical grinding machines.

Inspection

The final step before applying a surface finish is inspection. This step ensures that the part is free from any defects or inconsistencies that could interfere with the finishing process. Inspection can involve both visual checks and precision measurement tools to confirm that the surface meets the required specifications.
  • Why It Matters: Defects caught during inspection can be corrected before the finishing process, saving time and reducing the risk of rework.
  • Techniques Used: Coordinate measuring machines (CMM), profilometers, and visual inspections using magnification.
These pre-surface finishing processes ensure that the surface finish adheres correctly and provides the intended performance benefits, such as corrosion resistance, smoothness, or wear resistance. Whether you’re preparing parts for powder coating or electropolishing, following these steps guarantees high-quality results.

Achieving Perfection With CNC Surface Finishing

Surface finishing plays a crucial role in the world of CNC machining. The process of cutting and
grinding using the top precision CNC machines in the market ensures that you get the best as-
machined surface finish. But most manufacturing fields may require a smoother finish and that
is where these techniques shine.

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