BRIGHTSTAR
PROTOTYPE CNC CO., LTD
+86 137 5010 5351
EN
May. 27, 2026
When you design a part that needs to be CNC machined, surface finish is often the last parameter considered, but it is also one of the most prone to problems. The drawing may simply specify Ra 1.6 or Ra 0.8, but do you really understand what these numbers mean? What surface finish can CNC machining actually achieve? How can you ensure your part surfaces meet requirements without paying excessive costs?
These questions are asked by almost every CNC part buyer and engineer. This article will explain in detail the range of surface finishes achievable with CNC machining, the key factors affecting surface quality, comparisons of different processes, and how to select the appropriate finish for your parts.
Surface finish refers to the microscopic geometric characteristics of a part's surface after machining. In simple terms, it describes "how smooth" the part surface is.
No matter how precise CNC machining is, the tool always leaves tiny tool marks on the part surface. The height of these marks is the surface roughness. The smoother the surface, the smaller the roughness value. The rougher the surface, the larger the value.
Ra is the most commonly used surface finish parameter. Ra stands for arithmetic average roughness, which is the arithmetic mean of the absolute distances of points on the profile from the mean line within a sampling length. Simply put, the smaller the Ra value, the smoother the surface.
Visual and tactile experience of different surface finishes:
Below Ra 0.1 micrometers, the surface is like a mirror, clearly reflecting images, and feels as smooth as silk.
Between Ra 0.2 and 0.4 micrometers, the surface is very smooth, with obvious reflection but not completely mirror-like, similar to the surface of high-quality stainless steel tableware.
Between Ra 0.8 and 1.6 micrometers, the surface is smooth but shows slight tool marks, and you can feel fine texture when touching.
Between Ra 3.2 and 6.3 micrometers, there are obvious tool marks, with clear ridges and valleys when touched, similar to a standard machined surface.
Above Ra 12.5 micrometers, the surface is very rough with obvious machining marks, typically used for non-mating surfaces or as stock allowance for subsequent processing.
Different CNC machining processes, due to their different cutting principles and tool types, can achieve significantly different surface finishes.
CNC milling uses a rotating cutting tool to remove material. The surface finish of milling is affected by many factors including tool type, tool diameter, stepover, depth of cut, feed rate, etc.
For standard milling using conventional parameters and standard tools, a surface finish of Ra 1.6 to 3.2 micrometers can be consistently achieved. This finish is suitable for most non-mating and non-cosmetic surfaces, with the lowest cost.
For precision milling using smaller stepover, sharper tools, and optimized cutting parameters, a surface finish of Ra 0.4 to 0.8 micrometers can be achieved. This finish is suitable for mating surfaces, bearing mounting surfaces, and other features that require a certain level of smoothness.
For ultra-precision milling using high-precision machine tools, specialized tooling, and very small cutting parameters, surface finishes of Ra 0.1 to 0.2 micrometers can theoretically be achieved. However, in practical production, this finish typically requires polishing to be consistently achieved.
CNC turning involves the workpiece rotating and the tool moving linearly. The turned surface has continuous circumferential marks and can theoretically achieve a smoother surface than milling.
Standard turning can achieve Ra 0.8 to 1.6 micrometers, suitable for the external surfaces of most shaft-type parts.
Precision turning using sharp tools, optimized parameters, and adequate coolant can achieve Ra 0.2 to 0.4 micrometers, suitable for precision bearing journals, seal mating surfaces, etc.
Ultra-precision turning using single-crystal diamond tools can achieve optical-grade surfaces of Ra 0.01 to 0.05 micrometers. However, this process is limited to non-ferrous metals and requires extremely high equipment costs.
Grinding uses a grinding wheel as the cutting tool, which is composed of countless tiny abrasive grains. Grinding is essentially numerous tiny cutting edges cutting simultaneously, allowing for very high surface finishes.
Standard grinding can achieve Ra 0.2 to 0.4 micrometers. Precision grinding can achieve Ra 0.05 to 0.1 micrometers. Ultra-precision grinding can achieve Ra 0.01 to 0.02 micrometers.
Grinding is typically used for finishing hardened parts and applications requiring extremely high surface finish and dimensional accuracy.
EDM removes material through electrical discharge erosion, without involving mechanical cutting forces. The EDM surface is composed of countless tiny discharge craters.
Standard EDM can achieve Ra 1.6 to 3.2 micrometers. Fine-finish EDM, using multiple discharges and smaller discharge energy, can achieve Ra 0.2 to 0.4 micrometers.
It should be noted that EDM surfaces have a recast layer that affects fatigue performance. For extremely demanding applications, EDM surfaces typically require subsequent polishing or grinding.
Many factors affect the surface finish of CNC machining. Understanding these factors helps you make correct decisions during the design phase.
The sharpness of the tool directly affects surface quality. A dull tool generates higher cutting forces, leading to rough surfaces, burrs, and even chatter marks. Using new tools or regularly replacing tools is a basic requirement for ensuring surface finish.
Tool geometry also matters. A larger nose radius leaves a smoother surface but increases cutting forces. A smaller nose radius can machine finer features but results in poorer surface finish.
Feed rate is one of the most critical parameters affecting surface finish. The higher the feed rate, the larger the chip load per tooth, and the rougher the surface. To achieve better surface finish, feed rate needs to be reduced.
Cutting speed is equally important. Increasing cutting speed appropriately makes the material easier to shear, reduces built-up edge formation, and thus improves surface quality.
Depth of cut affects cutting forces. Shallow cuts generate smaller cutting forces, which helps achieve better surface finish.
For curved surface milling, stepover is the determining factor affecting surface finish. Stepover is the distance between adjacent toolpaths. The smaller the stepover, the lower the scallop height, and the smoother the surface. However, halving the stepover increases machining time by four times.
Therefore, a trade-off needs to be made between surface finish and machining time. For less demanding surfaces, use a larger stepover. For demanding surfaces, use a smaller stepover or use polishing as a post-process.
Different materials significantly affect achievable surface finish. Soft materials like aluminum alloys and brass are easy to machine to smooth surfaces but are prone to burrs. Difficult-to-machine materials like stainless steel and titanium alloys have a high tendency to work-harden and tend to produce rough surfaces during cutting. Plastic materials tend to melt or produce burrs during machining and require sharp tools and adequate cooling.
Machine rigidity directly affects surface quality. Machines with insufficient rigidity vibrate during cutting, causing chatter marks on the surface. Using shorter tools, more stable fixtures, and more appropriate cutting parameters can reduce vibration.
Adequate cooling and lubrication reduce friction and heat in the cutting zone, reduce tool wear, improve chip evacuation, and thus improve surface finish. High-pressure coolant is especially important for deep hole machining and difficult-to-machine materials.
The higher the surface finish requirement, the higher the machining cost. Understanding this relationship helps you specify surface finish requirements reasonably on your drawings and avoid unnecessary costs.
Increasing from Ra 3.2 micrometers to Ra 1.6 micrometers increases cost by approximately 20% to 30%. This is mainly due to the need to reduce feed rate, increasing machining time.
Increasing from Ra 1.6 micrometers to Ra 0.8 micrometers increases cost by approximately 50% to 100%. At this level, sharper tools, slower feed rates, and possibly an additional finishing pass may be required.
Increasing from Ra 0.8 micrometers to Ra 0.4 micrometers increases cost by approximately 2 to 3 times. This range typically requires grinding or precision milling, with higher demands on machine tools and tooling.
Increasing from Ra 0.4 micrometers to below Ra 0.1 micrometers increases cost by 5 to 10 times or more. This range typically requires lapping, polishing, or ultra-precision machining, which are high-cost processes.
Practical recommendations for specifying surface finish:
For non-mating surfaces, internal surfaces of parts, and surfaces covered by other parts, Ra 3.2 or even 6.3 micrometers is sufficient. There is no need to specify higher requirements.
For general mating surfaces, mounting surfaces, and sealing surfaces, Ra 1.6 micrometers is an economical choice. This finish meets most applications.
For bearing mounting surfaces and precision mating surfaces, Ra 0.8 to 0.4 micrometers is an appropriate range. The exact value depends on the bearing type and load.
For decorative surfaces requiring a mirror-like appearance, or sliding surfaces requiring extremely low friction, only then consider requirements below Ra 0.1 micrometers. However, note that this is not directly achieved by CNC machining but through post-processes such as polishing.
For non-mating surfaces on most mechanical parts, Ra 3.2 micrometers is sufficient. For general mating surfaces such as bolted joints and locating faces, Ra 1.6 micrometers is recommended. For sliding mating surfaces such as guide slots and sliders, Ra 0.8 micrometers is recommended.
The surface finish of bearing mounting surfaces directly affects bearing life. For standard rolling bearings, shaft journals and bearing housing bores are recommended to have Ra 0.4 to 0.8 micrometers. For precision bearings or high-speed bearings, Ra 0.2 to 0.4 micrometers is recommended. For interference fit bearings, surface finish requirements are higher because rough surfaces will be smoothed during pressing, reducing the interference amount.
The surface finish of sealing surfaces determines sealing effectiveness. For static seals (gaskets, O-rings), Ra 1.6 to 3.2 micrometers is usually sufficient. For dynamic seals (oil seals, mechanical seals), Ra 0.2 to 0.8 micrometers is recommended, depending on the seal type and media pressure.
The surface finish of mold cavities directly affects the surface quality of injection molded or die-cast products. For general appearance part molds, Ra 0.4 to 0.8 micrometers is recommended. For high-gloss appearance part molds, polishing to below Ra 0.05 micrometers mirror finish is required.
Optical parts require extremely high surface finishes, typically below Ra 0.01 micrometers, which is beyond the capability of conventional CNC machining. Optical parts are typically made by lapping and polishing processes, not direct CNC machining.
Proper marking avoids ambiguity and ensures your supplier understands your requirements.
Basic marking method:
Use the surface roughness symbol on the drawing and specify the Ra value next to or above the symbol. For example, marking Ra 1.6 means the surface finish must not exceed 1.6 micrometers.
Specify a default requirement for all surfaces:
In the drawing title block or technical requirements, you can specify "Unspecified surface roughness Ra 3.2," meaning all surfaces without individual marking default to Ra 3.2 micrometers. This avoids having to mark every surface on the drawing.
Specify higher requirements for specific surfaces:
For surfaces that require higher finish, mark them individually on the drawing. For example, mark Ra 0.8 next to a bearing mounting hole.
Avoid over-specifying:
Only specify high finish requirements on surfaces where they are truly functionally necessary. Do not habitually mark Ra 0.8 on the entire drawing, as this will significantly increase cost.
If direct CNC machining cannot achieve the finish you need, post-processing can be used.
Using sandpaper or polishing compound to manually sand can remove CNC tool marks and achieve very smooth surfaces. Manual polishing can achieve mirror finishes below Ra 0.05 micrometers. However, manual polishing has high cost, low efficiency, and depends on operator skill.
Using polishing wheels and polishing compounds for mechanical polishing is more efficient than manual polishing and is suitable for batch parts. Mechanical polishing can achieve finishes around Ra 0.1 micrometers.
Place parts in a container with abrasive media and lubricant, and use rotation or vibration to make the media rub against the part surfaces, removing burrs and improving finish. Tumbling is suitable for high-volume small parts, has lower cost, but cannot achieve high finishes, typically only Ra 0.4 to 0.8 micrometers.
Immerse the part in an electrolyte solution and use an electric current to preferentially dissolve microscopic surface protrusions, resulting in a smooth, bright surface. Electropolishing is suitable for stainless steel parts and can achieve Ra 0.1 to 0.2 micrometers finish while improving corrosion resistance.
If CNC milling or turning cannot meet the finish requirement, consider designing the part to require grinding. Grinding can achieve Ra 0.05 to 0.2 micrometers finish while maintaining extremely high dimensional accuracy.
Q: Can CNC machining directly achieve a mirror finish?
For most metal materials, direct CNC machining cannot achieve a true mirror finish. The tool always leaves tiny marks. If a mirror finish is needed, polishing is required after CNC machining. The only exception is using single-crystal diamond tools to machine non-ferrous metals, which can directly achieve optical-grade surfaces, but this process is very expensive and uncommon.
Q: How much difference does Ra 0.8 vs Ra 1.6 make for part performance?
For most applications, the difference is not significant. Ra 0.8 is smoother than Ra 1.6, but the differences in strength, fit, wear, etc., are usually negligible. Only in applications sensitive to surface finish, such as bearing mounting and seal mating, does it have a noticeable impact.
Q: Are surface finish and surface flatness the same thing?
No. Surface finish describes microscopic roughness, typically expressed as Ra. Surface flatness describes macroscopic flatness, expressed as a flatness tolerance. A part can have a very smooth surface (very low Ra) but be bent overall (poor flatness). The reverse is also true.
Q: Is the surface finish of plastic parts the same as metal?
CNC machining of plastic parts can achieve surface finishes similar to metal, but there are some differences. Some plastics tend to produce burrs or melt during machining, requiring sharp tools and adequate cooling. Plastics like PEEK and Acetal can achieve very good surface finishes, while Nylon and Polypropylene are more difficult.
Q: How can I ensure my supplier achieves my surface finish requirements?
Specify the Ra value clearly on the drawing. State that surface finish is a critical requirement when requesting a quote. Require the supplier to provide a First Article Inspection report that includes surface roughness measurement data. If possible, provide a surface finish sample as a reference
The range of surface finishes achievable with CNC machining is very wide, from standard machined surfaces at Ra 3.2 micrometers, to precision machined surfaces at Ra 0.4 micrometers, to mirror finishes below Ra 0.1 micrometers that require post-processing.
Choosing the right finish requirement requires balancing function and cost. Requirements that are too high will cause unnecessary cost increases, while requirements that are too low may affect the part's performance.
When specifying surface finish, follow the principle of "good enough." For non-mating surfaces, Ra 3.2 micrometers is sufficient. For general mating surfaces, Ra 1.6 micrometers is an economical choice. For precision mating surfaces, only then consider Ra 0.8 micrometers or lower.
Brightstar has extensive CNC machining experience and can achieve surface finishes from Ra 3.2 micrometers to Ra 0.4 micrometers according to your needs. For higher requirements, we also offer post-processing services such as polishing and grinding.
If you have questions about surface finish or need help determining the right surface requirements for your parts, please feel free to contact us.
Ready to Specify Surface Finish for Your CNC Parts?
Whether you need standard machined surfaces or high-finish precision surfaces, Brightstar can provide professional CNC machining services.
Email Amy: amy@brightstarprototype.com
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Send us your CAD files and drawings for surface finish recommendations and a quote.
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