Due to the decreasing characteristics of CNC, small tool marks will appear on the surface of the workpiece after machining. The standard value of the machined surface roughness (Ra) is 3.2 μm. As a result, many parts require additional machining to achieve desired surface properties or performance, such as reducing surface roughness to 0.8, 0.4 μm or less, or for aesthetics, corrosion resistance, wear resistance, or other purposes.
In this article, we’ll take you through the most common metal CNC finishes to help you choose the best finish for your application.

sandblasting

Sandblasting will add a uniform matte or satin surface finish to machined parts, eliminating tool marks. In the process of bead blasting, compressed air is used to form a high-speed jet beam, and the material is sprayed to the surface of the workpiece to be processed at a high speed, which can remove part of the material and make the surface smooth. Critical surfaces or features (such as holes) can be covered to avoid dimensional changes, which will incur some additional cost.
It can remove the small burrs on the surface of the workpiece, improve the smoothness of the workpiece, make the workpiece show a uniform metallic color, and make the appearance of the workpiece more beautiful.
Bead blasting is mainly used for visual purposes. This is a manual process, so results are somewhat dependent on the skill of the operator. The size of the air pressure and the size of the glass beads are the main process parameters. Glass beads come in different sizes (coarse to very fine), just like sandpaper comes in different sizes and grades.

Anodizing (Type II and Type III)

Anodizing is an electrolytic oxidation process. During electrolysis, the part is immersed in a dilute sulfuric acid solution and a voltage is applied between the part and the cathode. Electrochemical reactions consume material on all exposed surfaces of the part and convert it into duralumin or titanium oxide. Masks can be used on surfaces with critical dimensions, such as threaded holes, or surfaces that must remain conductive to prevent anodization. It can improve the surface hardness and wear resistance of aluminum parts and prolong the service life.
Anodizing is the most widely used surface treatment for aluminum and titanium machined parts. Anodized parts can be dyed in different colors – black, red or gold are the most common
Coatings of different thicknesses and densities can be formed by varying the current of the solution, anodizing time, concentration and temperature:
Type II anodizing Type II anodizing, also known as “standard” or “decorative” anodizing, produces coatings as thick as 25 microns. Typical coating thickness depends on color. Thickness can be between 8-12 microns for black-dyed parts and 4-8 microns for clear (undyed) parts.
Type II anodizing is primarily used to produce smooth, aesthetically pleasing parts and provides good corrosion resistance with limited wear resistance.
Anodizing Type III (Hard Coat) Type III anodizing, also known as “hard coat” anodizing, produces coatings up to 125 μm thick. Typical Type III anodic coatings are 50 μm thick unless otherwise specified.
Type III anodizing produces a high density thick ceramic coating with excellent corrosion and wear resistance for functional applications. Compared to Type II anodizing, it requires tighter process controls (higher current density and constant solution temperature close to 0oC) and is therefore more expensive.

powder coating

Powder coating is the process of spraying powder onto the surface of a part under the action of static electricity. The process of powder coating is similar to spraying, but the “paint” is a dry powder rather than a liquid. The powder is evenly adsorbed on the surface of the aluminum workpiece, and then baked into the part through the oven. This produces a strong, abrasion and corrosion resistant coating that is more durable than standard spray methods. Various colors can be used to achieve the desired aesthetic of the part. Powder coatings are superior to wet process coatings in terms of mechanical strength, adhesion, corrosion resistance and aging resistance. It can reach 100% high utilization rate and is very environmentally friendly.
Powder coating is a strong, wear-resistant coating that is compatible with all metallic materials and can be used in conjunction with sandblasting to produce parts with smooth, uniform surfaces and excellent corrosion resistance.
Multiple layers can be applied to produce thicker coatings, with typical thicknesses ranging from about 18 μm to a maximum of 72 μm. There are many colors to choose from.

Hope the above helps you.