What Is Surface Finish in CNC Machining?
Surface finish in CNC machining refers to the condition of a part’s surface after machining and any additional finishing processes. It describes how smooth or rough the surface is, as well as any protective or decorative coatings applied afterward.
Unlike part dimensions or tolerances, surface finish is often misunderstood or overspecified. Many buyers request extremely fine finishes without fully understanding their impact on machining time, inspection effort, and cost.
In practical terms, surface finish influences:
- Friction and wear behavior
- Fatigue life and crack initiation
- Corrosion resistance
- Sealing and mating performance
- Visual appearance
Understanding surface finish allows engineers and buyers to specify only what is truly required—no more and no less.
Understanding Ra: Surface Roughness Explained
Ra stands for arithmetical mean roughness. It is the most commonly used parameter to describe surface roughness in CNC machining.
Ra represents the average deviation of the surface profile from the mean line over a specified measurement length. In simpler terms, it quantifies how smooth or rough a surface feels and looks at a microscopic level.
Ra is usually expressed in micrometers (µm) or microinches (µin).
Common Ra Values in CNC Machining
The following Ra ranges are typical for CNC-machined parts:
- Ra 6.3–12.5 µm: Rough machining, fast material removal
- Ra 3.2 µm: Standard CNC machining finish
- Ra 1.6 µm: Fine machining finish
- Ra 0.8 µm: High-quality finish, slower machining
- Ra 0.4 µm or better: Precision finishing, polishing or grinding required
Most functional CNC parts perform perfectly well at Ra 3.2 µm or Ra 1.6 µm. Specifying lower Ra values significantly increases machining time and cost.
How Ra Affects Cost and Lead Time
Lower Ra values require:
- Slower feed rates
- Additional finishing passes
- Sharper tools and more tool changes
- Increased inspection effort
As a result, reducing Ra from 3.2 µm to 0.8 µm can double machining time without improving part function in many applications.
This is why experienced suppliers often ask buyers to justify very fine Ra requirements.
As-Machined Surface Finish
An as-machined finish refers to the surface condition directly after CNC machining, without additional surface treatment.
Characteristics of as-machined surfaces include:
- Visible tool marks
- Consistent texture following toolpaths
- Typical Ra of 3.2–6.3 µm
As-machined finishes are suitable for:
- Internal components
- Non-cosmetic parts
- Functional features where appearance is not critical
This finish offers the lowest cost and shortest lead time.
What Is Anodizing in CNC Machining?
Anodizing is an electrochemical surface treatment primarily used for aluminum parts. It increases corrosion resistance, surface hardness, and aesthetic appeal.
During anodizing, the aluminum surface is converted into a controlled oxide layer. This layer is integrated with the base material rather than applied on top, making it durable and wear-resistant.
Common Types of Anodizing
- Type II (Sulfuric Acid Anodizing): Decorative and corrosion-resistant
- Type III (Hard Anodizing): Thicker, harder, more wear-resistant
Type II anodizing is commonly used for consumer and industrial parts, while Type III is used for high-wear or harsh environments.
How Anodizing Affects Surface Finish
Anodizing does not remove machining marks. Instead, it follows the existing surface texture.
This means:
- Rough surfaces remain visually rough after anodizing
- Fine machining produces better cosmetic results
- Pre-anodizing polishing may be required for appearance-critical parts
Anodizing also slightly increases part dimensions due to oxide layer growth, which must be considered for tight-tolerance features.
What Is Plating in CNC Machining?
Plating is a surface finishing process where a thin metal layer is deposited onto a machined part using electrochemical or chemical methods.
Common plating types in CNC machining include:
- Nickel plating
- Chrome plating
- Zinc plating
- Electroless nickel plating
Plating is used to improve corrosion resistance, wear resistance, electrical conductivity, or appearance.
Plating vs Anodizing: Key Differences
While both processes improve surface performance, they differ fundamentally:
- Anodizing converts the surface of aluminum
- Plating adds a new metal layer on top of the surface
- Plating can be applied to many metals, not just aluminum
Plating thickness must be tightly controlled, especially for precision parts, as it directly affects final dimensions.
When Should You Specify Anodizing or Plating?
Surface treatments should be specified based on functional requirements, not default preferences.
Specify anodizing when:
- The part is aluminum
- Corrosion resistance is required
- Wear resistance or electrical insulation is needed
- Color or appearance matters
Specify plating when:
- The base material is steel or copper alloys
- Improved wear or corrosion resistance is required
- Electrical conductivity is important
- Tight dimensional control after coating is needed
In many cases, surface treatment is unnecessary and adds cost without improving performance.
Common Mistakes in Surface Finish Specification
Several recurring mistakes cause unnecessary cost and confusion:
- Specifying ultra-low Ra values everywhere
- Ignoring how anodizing reflects machining marks
- Not accounting for coating thickness in tolerances
- Applying cosmetic finishes to non-visible surfaces
Clear communication between buyers and suppliers prevents these issues.
How to Specify Surface Finish Correctly on Drawings
To avoid ambiguity, drawings should clearly indicate:
- Ra values for critical surfaces
- Which surfaces require finishing
- Type and standard of anodizing or plating
- Any masking or exclusion areas
Using notes such as “Ra 3.2 µm unless otherwise specified” helps simplify communication.
Surface Finish and Supplier Quotation
Surface finish requirements strongly influence CNC machining quotes.
Fine finishes and post-processing:
- Increase machining time
- Add external processing steps
- Increase inspection and handling
Suppliers price defensively when surface finish requirements are unclear. Clear, justified specifications result in more accurate and competitive quotes.
Conclusion
CNC machining surface finish is more than a cosmetic detail—it directly affects performance, durability, and cost.
By understanding Ra values, knowing when anodizing or plating is necessary, and avoiding over-specification, buyers can achieve the right balance between quality and efficiency.
In CNC machining, the best surface finish is not the finest one—it is the one that meets functional requirements with minimal waste and risk.
