By 6CProto Engineering Team · Updated August 15, 2026
CNC surface finish is measured in roughness values such as Ra, and finished with options ranging from as-machined to polished, anodized, plated, or coated. Ra 0.4–1.6 µm is common for machined surfaces, while cosmetic and sealing applications may specify tighter values or additional post-processing. The practical skill is matching the finish to the function: sealing faces need controlled roughness, wear surfaces need the right texture, and cosmetic parts need a finish specification the supplier can verify.
Ra, Rz, and What the Numbers Mean
Ra, or average roughness, is the arithmetic average of the surface profile deviations over a sample length. It is the most common specification, but it does not capture every surface character: two surfaces with the same Ra can look and behave differently.
Rz measures the average of the five highest peaks and five deepest valleys, and it is useful when extreme surface features matter, such as in sealing or coating adhesion. A drawing may specify both Ra and Rz when the application needs it.
Surface finish is not a single number but a profile with direction, waviness, and lay. The machining direction, tool marks, and waviness affect friction, sealing, and appearance, so the specification should describe what matters for the function.
Typical Surface Finishes for Machined Parts
| Finish | Typical Ra | Typical use |
|---|---|---|
| Rough machining | Ra 3.2–6.3 µm | Non-critical faces, hidden surfaces |
| Standard machining | Ra 1.6–3.2 µm | General parts, no functional surface |
| Fine machining | Ra 0.8–1.6 µm | Bearing seats, mating faces |
| Precision finishing | Ra 0.4–0.8 µm | Sealing surfaces, optics-adjacent parts |
| Polished or lapped | Ra 0.1–0.4 µm | Seals, shafts, cosmetic-critical surfaces |
These ranges are typical, not guaranteed. Achievable roughness depends on material, tooling, geometry, and the finishing pass, so confirm the value that matters on your part during DFM review.
Directional finishes deserve a note. Some assemblies specify a directional lay, such as a polished shaft running against a seal, and the drawing should state whether the surface finish is isotropic or directional. The machining and polishing process is planned around that requirement.
Post-Processing Options
As-machined finish is the baseline and is often sufficient for hidden or non-critical surfaces. When the surface must look better, seal better, or resist corrosion, post-processing adds value.
Bead blasting produces a uniform matte texture and hides tool marks, which is popular for cosmetic parts and as a pre-treatment for coating. Anodizing adds a protective oxide layer to aluminum and enables colored finishes, while passivation improves stainless corrosion resistance without changing appearance.
Polishing reduces roughness for sealing or optical applications, and plating or coating, such as nickel plating or powder coating, adds wear resistance, corrosion protection, or color. Each option has cost and lead-time implications, so specify the finish on the drawing rather than assuming a default.
Common Finishes by Material
Aluminum accepts anodizing in clear or colored finishes, bead blasting for texture, and powder coating for durability. Stainless steel is typically passivated, electropolished, or mechanically polished, and its finish is integral to the material rather than a coating.
Steel parts can be zinc or nickel plated for corrosion protection, or powder coated for color and durability, while plastics are usually used as-machined or lightly textured. Matching the finish family to the material prevents adhesion and corrosion problems later.
How to Specify Finish on a Drawing
Write the finish requirement next to the surface or in a general note: the Ra value, the measurement length, and the finishing process if it matters. ISO 1302 describes how surface texture is indicated on technical drawings, and using its symbols keeps the specification unambiguous.
Distinguish between cosmetic and functional surfaces. A cosmetic face may need a texture spec, while a sealing face needs a controlled roughness with a defined measurement method, and the two should not be treated the same.
Confirm how the finish will be measured. A roughness tester measures Ra in a defined direction, and the result depends on the sampling length and filter settings, so agree on the method with the supplier before ordering.
Choosing the Finish for Your Part
Work through three questions. First, what is the surface for: sealing, wear, corrosion, or appearance? Second, what environment will it face: moisture, chemicals, temperature, or contact? Third, what does the assembly need: a controlled roughness value, a coating, or a color?
A sealing face needs a roughness value and a measurement method; a cosmetic housing needs a texture and color specification; an outdoor part needs a corrosion-resistant treatment with verified adhesion. Writing the answer on the drawing prevents the supplier from guessing.
Finish vs. Function: Sealing, Friction, Fatigue
Surface finish affects more than appearance. Sealing surfaces need controlled roughness so gaskets and O-rings compress evenly, and too-smooth or too-rough surfaces can both leak. Wear surfaces need the right texture for lubrication, and roughness peaks can raise friction and wear.
Fatigue is a subtler effect. Sharp tool marks act as stress concentrators on highly loaded parts, so critical aerospace and automotive components often specify finer finishes or shot peening to improve fatigue life.
Corrosion also responds to finish. A smoother surface gives corrosion less area to attack and holds coatings more evenly, which is why finishing and corrosion protection are specified together on exposed parts.
Appearance specifications also affect cost. A uniform matte finish across a large part is harder to achieve than on a small part, and color matching between batches requires controlled processes and sometimes a customer-approved sample. Confirm the acceptance criteria with the supplier before production, because appearance disputes are common when the standard is only “looks good”.
Common Misconceptions
- Lower Ra is always better. Finer finish costs more and is not always functional; a sealing or wear surface may need a specific range, not the lowest number.
- Ra tells the whole story. Waviness, lay, and direction matter for sealing and friction, so specify what the application needs.
- As-machined is fine for everything. Hidden surfaces, yes; sealing faces, wear surfaces, and cosmetic parts usually need a specification.
- Finishing is only cosmetic. Anodizing, passivation, and controlled roughness change corrosion, wear, sealing, and fatigue behavior.
6CProto Expert Views
6CProto engineering perspective: Specify the finish by function, not by habit. Sealing faces need a controlled Ra with an agreed measurement method, wear surfaces need the right texture, and cosmetic parts need a process that produces consistent appearance. Put the value and the process on the drawing, and confirm the measurement direction and sampling length with the supplier during DFM.
Conclusion
CNC surface finish is a functional specification, not a decoration. Match the Ra value and post-processing to the part’s job: fine finishes for sealing and precision faces, textured or coated finishes for cosmetics and corrosion, and controlled roughness for wear and fatigue.
Write the finish on the drawing with the measurement method, and confirm achievable values during DFM review. A finish that is specified and measured consistently is what keeps the part functional in service.
FAQs
What is a good surface finish for CNC machined parts?
Ra 0.8–1.6 µm covers most general and mating surfaces, while Ra 0.4 µm or better is used for sealing and precision applications. Confirm the value for your specific part and material, and compare the cost of each step down, because finer finish is not free.
What does Ra mean?
Ra is the average roughness of the surface profile over a sampling length. It is the most common finish specification, but it does not capture waviness or surface character alone.
Should I specify Ra or Rz?
Usually Ra is enough for general parts. Rz is useful when extreme peaks and valleys matter, such as sealing or coating adhesion, and some drawings specify both, especially when the surface will be measured for qualification.
Does anodizing change the surface finish?
Anodizing adds an oxide layer and can slightly change surface texture, so confirm the finish before and after treatment. It also enables color and improves corrosion resistance on aluminum, and the coating thickness should be considered where tolerances are tight.
How is surface roughness measured?
With a profilometer that traces the surface and reports Ra or Rz over a defined sampling length. The measurement direction and settings affect the result, so agree on the method with the supplier, and request the measurement conditions with the report so the number can be reproduced.
Sources
- 6CProto Surface Finishing Services
- 6CProto CNC Machining Services
- ISO 1302:2002 – Indication of surface texture in technical product documentation
- ISO 2768-1:1989 – General tolerances