How to Specify Surface Roughness in CNC Machining

In the world of precision CNC machining, surface roughness is a critical determinant of part quality, performance, and cost. Specifying it correctly on your technical drawings is essential for ensuring your components function as intended while avoiding unnecessary manufacturing expenses. As a leading provider of comprehensive CNC machining solutions, we guide our clients through this vital specification process.


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Understanding Surface Roughness

Surface roughness, often denoted by the symbol Ra (Arithmetic Average), measures the microscopic peaks and valleys on a machined surface. It is quantified in micrometers (µm) or microinches (µin). A lower Ra value indicates a smoother surface, while a higher value signifies a rougher finish.

Key Parameters to Specify

While Ra is the most common parameter, a complete specification involves more:

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Ra (Average Roughness): The primary indicator for general quality control.
Rz (Maximum Height of the Profile): Measures the maximum peaktovalley height within a sampling length, useful for assessing surface uniformity.
Machining Allowance: Clearly indicate the amount of material to be left for finishing operations.



The most effective way to specify roughness is by using the standardized checkmark symbol (√) on your drawings, accompanied by the Ra value, and sometimes the manufacturing process (e.g., "milling" or "turning").

Choosing the Right Roughness for Your Application

Selecting an appropriate Ra value is a balance between function and cost.

Rough Surfaces (Ra 3.2 µm / 125 µin and higher): Suitable for noncritical parts, hidden surfaces, or those requiring painting or coating. This is the most economical option.
Standard Machined Surfaces (Ra 1.6 3.2 µm / 63 125 µin): A common choice for most functional parts, offering a good balance between performance and cost. Ideal for parts subject to low loads and static fits.
Smooth Finishes (Ra 0.4 1.6 µm / 16 63 µin): Necessary for parts in dynamic contact, such as bearing surfaces, sliding components, and hydraulic seals. These require additional finishing processes like grinding or honing.
Very Smooth/Mirror Finishes (Ra < 0.4 µm / 16 µin): Reserved for highprecision applications like optical lenses, precision gauges, and aerospace components, where minimal friction and superior sealing are paramount. These are the most timeconsuming and expensive to produce.

Partner with a Knowledgeable Supplier

Unnecessarily tight tolerances can exponentially increase machining time and cost without providing any functional benefit. Our engineering team excels at Design for Manufacturability (DFM) analysis. We proactively review your specifications to recommend the most costeffective surface finish that perfectly meets your application's needs, preventing overengineering and ensuring optimal part performance.

By mastering the specification of surface roughness, you gain greater control over your product's quality and manufacturing budget. Trust us to be your expert partner in delivering precisionmachined parts that are built right and priced competitively.