Surface Finishes for CNC Machining Parts

There are a number of surface finishing techniques to give CNC machined parts the desired professional finish. It can change the appearance, surface roughness, hardness and chemical resistance of the machining parts. Below is a quick summary of the most common  surface finishes for precision machined components from CNC machining shops.

1. As-Machined

An As Machined CNC machining part surface finish does not require any post-processing, but the tool marks will be shown on their surface, thus it is not perfectly smooth. The standard surface roughness of an as-machined parts is 3.2 um. However, additional smoothing and polishing can reduce the machined texture to surface roughness of 0.4 um. This finish is very cost-effective and most suitable for prototypes and jigs and fixtures.

As machined surface finish
As machined finish of an aluminum part

2. Bead Blasting

   Bead blasting is the operation of shooting the blast media against a surface to affect the surface finish by using compressed air typically. It results in adding a uniform matte or satin surface finish on a CNC machined part.

This process relies on using small beads to remove incremental amounts of material from the CNC machined part’s surface. As the resulting surface roughness is not guaranteed, so critical surfaces or features (like holes) need to be masked to avoid any dimensional change.

Bead blasting is a post-process to get smooth finish, but will affect the dimensional accuracy of the parts slightly, Therefore, it is used mainly for visual purposes or aesthetic applications without tight tolerance requirement.

Bead blasting finish
Aluminum part with bead blasting finish

3. Anodizing

   Anodizing is a process which is used to increase the thickness of the natural oxide layer on the surface of metal parts, it does not need to use heavy metals nor does it produce toxic waste, so it is an environmentally friendly process, also it meets the environmental and safety directives of the RoHS.

On anodizing process, an electrochemical reaction consumes the material on all exposed surface of the part and converts it into hard metal oxide, typically for aluminum or titanium. And the anodic coating grows in both directions equally and it is electrically non-conductive, so a mask need to be applied to the surfaces with critical dimensions (like threaded holes) or the surfaces that must remain electrically conductive to prevent them from anodizing.

The main advantages of anodizing CNC machined parts are with high aesthetically pleasing surface and more durable. More information about aluminum anodizing, please click here.

Black anodizing CNC aluminum part
CNC machining part with black anodizing

4. Powder Coating

   This is a process where powdered paint is sprayed onto a part which is then baked in an oven. It adds a thin layer of strong, wear and corrosion resistant protective polymer on the surface of the machined part. Powder coating is compatible with all machining metal materials, some manufacturers also combine the powder coating process with a bead blasting to create parts with smooth finish and excellent corrosion resistance.

This finish is ideal for end-use products, due to some of advantages list below:

  • Strong, wear and corrosion resistant
  • Higher impact resistance
  • Compatible with all metal materials
  • Multiple colors available

But the powder coating will change the dimension of the part, so it doesn’t fit for tight tolerance part, also it is difficult to apply to the internal surfaces.

Machined part with powder coating finish
Aluminum machined part with powder coating finish

5. Passivation

   The passivation process is a method of improving the corrosion resistance of stainless steel machined parts by removing ferrous contaminants like free iron from their surface, restoring them to their original corrosion specifications.

Passivation is a post-processing best practice for newly-machined stainless steel parts and components. Benefits include:

  • Chemical film barrier against rust
  • Extended life of the product
  • Removal of contamination from product surface
  • Reduced need for maintenance.

To passivate stainless steel machining parts, they must be submerged in a chemical solution of citric acid or nitric acid for a certain period of time and at a certain temperature. Passivation is a chemical treatment and not an electrolytic process, it does not depend on electrochemical reactions, unlike electropolishing or anodizing. And passivation is not a method to remove oxide scale from machined parts after heat treating or welding. Also, passivating stainless steel does not change the color or surface appearance of the metal. This process is not necessary for items that will be painted or powder coated.

You can passivate stainless steel to improve its corrosion resistance, but not all grades are suited for this process, and some parts may require additional cleaning operations beforehand.

Stainless steel part with passivation finish

General Rule of Thumb for Surface Finish on CNC Machining Parts:

  • As machined finish is best for the machining parts with tight tolerances, where aesthetics is not the major concern.
  • Bead blasting is best for machined parts with matter uniform finishes, when maintain dimensional tolerances is not the key issue.
  • Anodizing Type II is good for the titanium and aluminum parts which need aesthetically pleasing.
  • Anodizing Type III is better for engineering applications, where very high wear resistance and surface hardness are required.
  • Powder coating is good for the machined components which need high-impact strength or it can not be anodized.
  • The passivation process is mostly for improving the corrosion resistance of stainless steel CNC machined parts.