Surface Finishes for CNC Machining

Jack Lie CNC machining expert

Specialize in CNC Milling, CNC Turning, 3D Printing, Urethane Casting, and Sheet Metal Fabrication Services.

CNC machining is an cost-effective way of tight tolerance parts production with fine geometries details form various metal or plastic materials. In CNC machining process, minor tool marks will be left on visible surface, this need post-processing and finishing methods to improve surface roughness, visual properties and wear resistance of final CNC machined parts. Runsom provides different metal finishes to satisfy your application requirement, our available finishes including:

  1. Machined: this has minor visible tool marks, surface roughness is 3.2μm as standard, and can be increased to 1.6,0.8 and 0.4μ
  2. Bead blasting: this can provide matte finishes with light textures, normally for visual purposes.
  3. Clear or Color Anodizing: it adds corrosion resistant, ceramic layer on surface with different colors, only available for Aluminum and Titanium.
  4. Hard-coat Anodizing: it adds a wear and corrosion resistant layer of ceramic on parts surface with various color, this process provide greater protection than clear or color anodizing, only available for Aluminum and Titanium.
  5. Powder coating: this method provide higher impact resistance comparing to anodizing, which is applied to any metal with large range of colors.
Finishes methodToleranceProtectionCostApplication materialsFinishes appearance
Machines finishHighLowLowAll materialsPoor
Bead blastingMediumLowLowAll materialsMedium
Color AnodizingHighMediumMediumAl &Ti alloysGood
Hard-coat AnodizingHighHighHighAl &Ti alloysGood
Powder coatingMediumMediumMediumAll metalsGood

Machined Finish

Machined finish will left path marks of cutting tools, its quality is measured by average surface roughness(Ra), which is the measure of average deviation between machined profile and ideal surface.

Our standard machined surface roughness is 3.2μm, additional finishing cutting method can be used to reduce surface roughness, we can control this roughness down to level of 1.6μm,0.8μm or 0.4μm. There is no doubt that lower surface roughness need extra machining steps and tighter quality control, this will increase parts production cost.

Machined parts can be post-processed by smoothing or polishing to reduce surface roughness, in order to improve the surface quality and aesthetics. Smoothing and polishing canemoving material affecting part dimensional tolerance effectively.

Advantages:Tightest dimensional tolerance, No additional cost for standard finish.
Disadvantages: Visible tool marks on surface.

Bead Blasting

Bead blasting can remove tools marks and offer uniform matte or satin surface finishes on machining parts. In this process, machined parts are bombarded by small glass beads with pressurized air gun, in order to remove material and smooth parts surface. Critical surface or features should be masked in process, and avoid dimensional change affection.

Bead blasting is normally a manual process, its result always depend on skills of operators. This method is widely applied for visual purpose. As sandpaper has different sizes and grads, this process main parameters are air pressure and glass beads size.

Advantages: Uniform matte or satin finish with low cost.
Disadvantage: affection on critical dimension and surface roughness.


Anodizing is process of adding thin ceramic layers on metal parts surface, which can protect parts form corrosion and wear. These layers are electrical non-conductive with high hardness in different colors. This method is only compatible with Aluminum and Titanium.

In anodizing process, metal parts will be submerged into diluted sulfuric acid solutions, then apply electric voltage between components and cathodes. An electronic-chemical reaction can consume exposed surface materials and convert into hard aluminum or titanium oxide. This left a mask on surface with critical dimensions or electrical conduction.

Clear or Color Anodizing (Anodizing Type II): This type is also called “standard” or “decorative ” anodizing, it can dye various colors before sealing on metal parts, and present anesthetic pleasing surface finishes. Our main color are red, blue, black and gold, this coating thickness can reach up to 25μm. The typical coating thickness depends on color types, 8-12μm for black color and 4-8μm for clear. This technology provides good corrosion and limited wear resistance with smooth and anesthetic surface.

Hard-coat Anodizing (Anodizing Type III): Hard-coat anodizing provides typical coating of 50μm thickness, as specific requirement, we can improve up to 125μm. It create thick ceramic coating with high density, which has excellent corrosion and wear resistance and widely used for functional applications. The coat of this process is higher than color anodizing, in reason of its stricter process control with higher current density and constant solution temperature near zero.

Advantages: High wear resistance for top-end engineering applications.
Durable and aesthetically coating.
Easily applied to internal cavities and small parts.
Excellent dimensional control.

Disadvantage: Only limited in Aluminum and Titanium alloys.
Relatively brittle than powder coating.

Powder Coating

Powder coating provides thin protective polymer layers on parts surface, which is strong and wear-resistant. This method is compatible with all metal materials, and can combine with bead blasting to create smooth and uniform surfaces with excellent corrosion resistance. In powder coating process, optional phosphating or chromating coat will be primed on parts surface to increase corrosion resistance, then use an electrostatic spray guns to coat dry powder and cure parts at high temperature at 200℃. Thicker coating can be added by multiple layers, the typical thickness range from 18μm to 72μm. In addition, powder coating still provides different colors options.

Advantages: Strong, wear and corrosion coating for functional applications.
Higher impact resistance than anodizing
Compatible with all metal materials
Multiple color options

Disadvantage: Difficultly apply to internal surface
Less dimensional control than anodizing
Unavailable for small components