Surface Treatment of CNC Machining Parts

In many cases, CNC machining leaves tool marks on the surface, imperfections that are clearly visible on milling or turning components. While parts generally don't require a perfectly smooth surface, when these components are used in larger systems, a spotless surface is essential.

What types of surface treatments are used in modern machining?

To improve the surface quality of parts machined by CNC machines, a variety of additional finishing processes can be performed. Most are mechanical processes, while some are electrochemical. The goal of each process is simple: to improve the quality and smoothness of the part surface to reduce friction within the larger system.

These processes can be categorized as follows:

Machining—usually performed using CNC machines (such as grinders)—may also leave some tool marks, but as part of the machining process, it provides high precision and optimal part tolerances.

Basic finishing, such as sandblasting, alters the mechanical structure of the surface and significantly smoothes it. While the dimensional change is minimal, it provides a better surface quality.

Secondary finishing, such as anodizing or powder coating, can alter the dimensions of the part. For electrochemical processes (such as anodizing, as mentioned above), the effect is minimal; for painting, it is significant. Furthermore, these processes can improve a part's resistance to environmental factors and produce a perfectly smooth surface. Another advantage of this type of finishing is the ability to change the surface color.

Each method imparts different properties to the resulting surface and has its own advantages and disadvantages.

Types of Surface Finishes After CNC Machining

The above classification can be further refined by describing specific surface improvement processes, particularly after machining (both CNC and conventional).

Among them, noteworthy are:

CNC Grinding

While a component of machining, this method of removing excess material produces a smooth surface, typically without tool marks. This effect can be achieved using higher grit heights (over P240) and wet grinding. This method is an excellent method for flat or regularly sized parts and can help improve the surface without altering the final dimensions.

Extreme Precision Machining

Precision CNC machining can achieve a perfectly smooth surface during part manufacturing. This requires very high-quality equipment and cutting tools—sometimes tool marks may remain, but they are barely noticeable. This surface treatment is an additional step to machining and has minimal impact on tolerances.

Anodizing (aluminum or titanium)

This is a very popular and highly regarded surface treatment. Anodizing is an electrochemical process that improves the surface by creating a thin layer of aluminum oxide. In addition to improving surface quality, anodizing also enhances wear and corrosion resistance, mechanical properties, and removes tool marks. This surface treatment is more expensive, especially hard anodizing, but it provides a hard surface. A major drawback of this process is that it is limited to aluminum and titanium alloys. However, it has minimal impact on the final dimensions of the part (which can be incorporated into part manufacturing specifications due to its high repeatability and precise coating thickness).

Powder coating

Another post-treatment method that achieves a smooth and durable surface. The process itself is relatively simple: After applying a special primer that improves corrosion resistance, the metal turned part is coated with a fine-grained powder that adheres to the surface through electrostatic charge. The powder coating is then melted in a special oven heated to approximately 200°C.

Sandblasting/beadblasting

This is a commonly used part surface treatment method. It effectively removes surface imperfections (such as tool marks and discoloration). This surface treatment uses specialized fine particles in a high-pressure airstream.

The Impact of Surface Treatment Process on Unit Price

The quality and type of surface treatment selected still directly impact the final price of the project and individual parts.

This is due to the following reasons:

1. The additional processing time required to achieve the desired surface quality (e.g., lower processing speeds);

2. The additional cost of the specific process and type—for example, hard anodizing is more expensive than "soft" anodizing but provides a very hard surface;

3. The additional time required to prepare for the selected process.

When selecting the appropriate surface treatment type for CNC machined parts, it is important to keep in mind the system in which the part will be used. This approach can help select the optimal process.

Another aspect of surface improvement is the acceptable tolerances—if the part will be used in a complex system, the tolerances should be as low as possible.