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Six major CNC post-processing processes

2026-04-17

After CNC machining is completed, parts usually require post-processing to improve their appearance, performance and durability. There are many types of post-processing methods and below we will focus on six commonly used processes.

Mechanical and physical processing

Polishing: The master rubs the surface of the part with a flexible tool. Some also apply polishing wax. Fluid polishing is also a common method in workshops. The highest possible mirror reflection effect. This process is often used in optical molds.

Sandblasting and shot blasting: High-pressure air is its power source. Fine sand is struck at the parts at high speed. The metal surface instantly becomes matte. Shot peening is mainly to eliminate the internal stress of the material. A lot of shot peening is done on aviation parts.

Wire Drawing: The abrasive belt runs in a straight line on the metal surface. The surface will leave a regular brushed texture. This approach is purely to enhance the appearance and texture. Many high-end digital product casings are treated this way.

Roller Burnishing: The hard rollers on the tool holder directly squeeze the inner hole. Plastic deformation occurs on the metal surface. This significantly increases surface hardness. The smoothness will also increase significantly. The inner hole of the hydraulic cylinder must be processed by rolling.

Lapping: Two flat surfaces rub against each other with abrasive in between. This method specifically deals with micron-level dimensional tolerances. High-precision gauge blocks are entirely made by grinding.

Chemical treatment

Soak parts in specific potions. A chemical reaction occurs on the surface to form a protective layer.

Blackening: Throw the steel parts into high-temperature liquid and boil them. A layer of ferric oxide will form on the surface. This black film has average rust resistance. It is usually necessary to apply anti-rust oil when packaging and warehousing in the workshop. The work clamps we make ourselves often turn black. It barely changes part size.

Phosphating: The chemical reaction produces a phosphate film. Its own anti-rust effect is limited. Factories mainly use it to prime spray paint. With this base film, the paint adhesion is extremely strong.

Passivation: Stainless steel CNC parts are most inseparable from it. Strong oxidants will promote the formation of a dense protective film. Metal that continues to rust may be completely cut off. Medical device parts must strictly implement passivation specifications.

Electrochemical treatment

This must be done with power on. Ions in liquids can adhere to or corrode surfaces.

Anodizing: The absolute core process of aluminum alloys. Divided into ordinary oxidation and hard oxidation. Hard oxygen can achieve a thickness of 50 microns. When we write a program with UG, we must calculate the margin in advance. The inner hole becomes smaller after oxidation, and the outer circle becomes larger. This must be remembered.

Micro-arc oxidation: It is the ultimate evolution of oxidation. High-voltage energization produces instantaneous high-temperature sintering. A layer of ceramic is formed directly on the surface. Wear resistance and anti-corrosion properties are thoroughly improved.

Electropolishing: Parts are energized and soaked in electrolyte. Tiny bumps on the surface will be dissolved preferentially. The entire part becomes extremely shiny. The inner wall of medical valves is extremely in need of this treatment.

Surface coating

In addition to the original material of the part, an additional layer of new material is covered.

Electroplating: Zinc plating, nickel plating and chromium plating are the most common. Hard chrome plating has an extremely hard surface. HRC hardness can easily exceed 60. When electroplating high-strength steel parts, special attention should be paid to hydrogen embrittlement.

Electroless plating (Electroless): The entire process doesn't require electricity at all. The metal is precipitated purely by chemical reduction reaction. The film thickness of electroless nickel plating is extremely uniform. Complex special-shaped parts are very suitable for this.

Vapor Phase Deposition (PVD/CVD): The machine vaporizes the metal in a vacuum furnace. The gas then adheres evenly to the part. The wear-resistant coating on the carbide blades used in our lathes is all made of this process.

Spraying and Powder Coating: After spraying or powder coating, it needs to be cured in the oven. The sheet metal shell of the machine tool is basically spray-coated. Thermal spraying melts the material and then sprays it on. Large driveshafts are worn and often thermal sprayed to resize them.

Surface heat treatment

The surface metallographic structure is changed by heating and cooling. Designed to improve local hardness.

High-frequency quenching (Induction): The copper tube coil heats up extremely quickly. Water cooling is also extremely fast. Let only the surface layer harden. The parts remain soft inside and have good toughness. Gear teeth are often treated this way.

Carburizing: Putting mild steel into a carbon-rich furnace. The surface is filled with carbon atoms. The surface becomes extremely hard after subsequent quenching. This is a great way to make high-strength parts at low cost.

Nitriding: Nitrogen atoms drill into the metal surface at high temperatures. This creates a highly hard nitride layer. Its processing deformation is much smaller than that of carburizing. Precision machine tool spindles like to use nitriding process.

Surface graphics and text processing

Add logos, numbers or visuals to parts.

Laser marking (Laser): This is also commonly referred to as laser engraving. High-energy laser directly burns surface materials. It is most convenient to print a LOGO or factory number. The writing is retained permanently and will never fade.

Silk Screen: Ink is printed onto the part through a mesh. Processing costs are extremely low. Producing flat parts in high volumes is very cost-effective.

Water transfer (Transfer): Float the patterned film on the water. The parts are pressed down and the pattern is attached. The complex curved surface of a motorcycle helmet relies entirely on it for decoration.

Choosing the appropriate post-processing method not only enhances product quality but also significantly improves its market competitiveness.