Analysis of Common Surface Treatment Processes in CNC Machining

Analysis of Common Surface Treatment Processes in CNC Machining

The surface roughness of CNC machined parts refers to the average irregularity of their surface texture after machining. It is commonly quantified using "Ra" (Arithmetic Average Roughness), which measures the microscopic precision of the material's surface. Surface roughness not only directly affects the appearance of the part, but also significantly influences its physical properties and performance in application.

To achieve the ideal surface quality, technicians select appropriate tools and optimize machining parameters such as feed rate, cutting speed, and cutting depth to effectively control surface roughness, ensuring that the part meets functional, reliability, and lifespan requirements.

Common Surface Roughness Grades and Their Applications in CNC Machining

In CNC machining, the surface roughness of parts is not formed randomly, but is specifically controlled based on different application requirements. Different use cases have varying demands for surface roughness to ensure assembly precision, functionality, and service life. Below are several common surface roughness grades and their applicable ranges:

• Ra 3.2 μm
  This is the most common commercial-level machined surface, suitable for most consumer parts. Visible tool marks are present to the naked eye and is typically used as the default roughness standard for CNC machining. This grade is suitable for parts subject to moderate vibration, load, and stress, and is often used for mating surfaces that experience lighter loads and slower motion.

• Ra 1.6 μm
  This is a standard used in the mechanical industry for general parts that do not require high surface smoothness. Light tool marks are still visible, but the surface is finer than Ra 3.2 μm. It is commonly used for general mechanical components or structural parts with low performance requirements, especially for low-speed, light-load moving parts. It is not suitable for high-speed rotation or high-vibration environments.

• Ra 0.8 μm
  This is a higher grade of roughness that requires strict machining control. Although the cost is relatively high, it is suitable for key parts in stress areas, commonly found in automotive components and consumer electronics. This grade is also suitable for bearing components that experience light loads and intermittent motion.

• Ra 0.4 μm
  This surface grade is close to a mirror finish and is primarily used for precision parts that require extremely high surface accuracy, aesthetics, and smoothness. It is suitable for high-speed rotating parts (e.g., shafts, bearings) and effectively reduces friction and wear. However, this grade typically requires more refined machining and stricter quality control, significantly increasing production costs and cycles.

Analysis of Common Surface Treatment Processes in CNC Machining

Based on specific application needs and material characteristics, product designers select different CNC surface treatment methods. Below are common surface treatment methods for both metallic and non-metallic materials:

1. Mechanical Surface Treatment Processes

• 1.1 Natural Surface (No treatment)
  Refers to the natural surface state of a workpiece after CNC machining, typically with visible tool marks or minor defects, with an average roughness of approximately Ra 3.2 μm. It is important to note that subsequent polishing or grinding may affect the part's dimensional tolerance.

• 1.2 Sandblasting
  An economical and practical surface treatment method for metal parts with low smoothness requirements. Involves using high-pressure guns to shoot tiny glass beads at the surface, removing defects and creating a uniform matte or frosted texture.

• 1.3 Brushed Finish
  A fine finishing method that creates a uniform, unidirectional texture on the surface using fine brushes or grinding media. Particularly suitable for metals like aluminum, copper, and stainless steel, it preserves the metal's natural color while providing a unique texture.

• 1.4 Abrasive Sanding
  Also known as abrasive sandblasting, this process uses high-speed sand particles to remove surface contaminants, oxide layers, or for texture processing and pre-coating preparation. It is suitable for various metals and hard materials.

• 1.5 Polishing
  Uses polishing wheels or compounds to achieve a high-gloss finish on parts, producing a mirror effect. Commonly used in medical devices, food machinery, and high-end consumer goods to enhance aesthetics, cleanliness, and corrosion resistance.

• 1.6 Knurling
  A method where patterned tools are applied to the rotating surface of the workpiece to create regular anti-slip textures. Often used to enhance grip, this method is suitable for metals like brass, steel, and aluminum in both aesthetic and functional designs.

• 1.7 Grinding
  Uses grinding wheels or other abrasives to remove micro amounts of material from the surface to achieve a higher level of smoothness and precision. It is suitable for parts that need further surface contamination removal or roughness improvement.

2. Chemical Surface Treatment Processes

• 2.1 Passivation
  A standardized chemical treatment for stainless steel and other metals, involving immersion in a specific solution to remove free iron from the surface and form a uniform protective film, improving corrosion resistance.

• 2.2 Chromate Treatment
  Suitable for metals like aluminum, zinc, cadmium, and magnesium. The workpiece is immersed in chromic acid or other chemical solutions to form a protective conversion film, enhancing adhesion, electrical insulation, and corrosion resistance.

• 2.3 Galvanizing
  Involves immersing steel or other substrates in molten zinc to form a zinc-iron alloy layer and a pure zinc layer. This cost-effective process prevents oxidation and rust and is suitable for large-scale production of parts.

• 2.4 Black Oxide Coating
  Involves immersing ferrous metals in an oxidation salt solution to chemically form a black iron oxide protective layer. Widely used for building components and consumer electronics, providing both corrosion resistance and a matte finish.

• 2.5 Vapor Polishing
  Used for plastic parts (such as PC and acrylic) to achieve high gloss and transparency through chemical vapor that melts the surface. This method is commonly applied to car lights, medical instruments, and other products that require high aesthetic appeal or light transmission.

3. Electrochemical Surface Treatment Processes

• 3.1 Anodizing
  Primarily used for aluminum parts, anodizing involves an electrolytic process to thicken the natural oxide layer, improving corrosion resistance, wear resistance, and surface hardness, while also supporting dyeing. It is widely applied in consumer electronics and industrial equipment.

• 3.2 Electroplating
  A process where metal ions are deposited on the surface of a workpiece using electrical current, forming a uniform metal coating. It enhances conductivity, corrosion resistance, and decorative appearance. Common plating materials include copper, nickel, gold, and silver.

• 3.3 Electroless Nickel Plating
  Also known as chemical nickel plating, this process involves chemical reduction to deposit a uniform nickel-phosphorus alloy layer on steel, aluminum, or other substrates. It offers excellent corrosion resistance and uniform coverage, especially for parts with complex geometries.

• 3.4 Electrolytic Polishing
  Involves anodic dissolution to remove microscopic protrusions on the surface, making it smoother and shinier while enhancing cleanliness and corrosion resistance. This method is widely used for parts that require high sanitary standards, such as medical devices and food processing equipment.

• 3.5 Powder Coating
  Involves electrostatically spraying thermoset or thermoplastic powders onto a metal surface, which is then cured under heat or UV light to form a strong protective film. This method offers excellent decorative, corrosion-resistant, and environmentally friendly properties, suitable for various metal enclosures and structural components.

4. Heat Treatment Surface Processes

• 4.1 Annealing
  Involves heating the metal to its recrystallization temperature and then cooling it slowly (usually in sand or with furnace cooling) to reduce hardness, improve toughness and ductility, and enhance subsequent cold working properties.

• 4.2 Heat Treatment
  A series of operations involving heating, holding, and cooling to alter the microstructure of a material, thereby improving its mechanical properties, such as strength, hardness, and wear resistance. It is widely used in mold and structural part manufacturing.

• 4.3 Tempering
  Involves reheating a quenched metal to a suitable temperature, holding it for a certain period, and then cooling it slowly to balance strength and toughness, preventing the material from becoming too brittle.

• 

How to Choose the Right Surface Treatment for CNC Machined Parts?

To ensure the selected surface treatment meets design requirements and application scenarios, the following key factors should be considered:

1. Material Characteristics
   Different materials respond differently to surface treatments. For example, aluminum parts are suitable for anodizing and powder coating, stainless steel often uses passivation for enhanced corrosion resistance, and carbon steel is more suited for black oxide or hot-dip galvanizing.

2. Functional Requirements
   Select processes based on part functionality. For example, anodizing or electroplating may be chosen for parts exposed to corrosive environments, carburizing or tempering for high-wear conditions, and copper, silver, or gold electroplating for parts that require improved conductivity.

3. Appearance Requirements
   Surface treatment affects the product’s visual appearance. Polishing and electroplating can achieve high-gloss finishes, while sandblasting and powder coating can create matte or satin textures. Choose the appropriate effect based on the product's positioning or customer requirements.

4. Cost Control
   Different processes have varying costs. For example, powder coating offers good cost-performance in mass production. It is important to balance cost, production cycle time, and performance requirements to select the optimal solution.

5. Lead Time Requirements
   Processes like anodizing and electro plating generally have longer cycle times, while mechanical treatments like polishing are relatively quicker. If the lead time is tight, faster processes should be prioritized; however, if there is ample time and high precision is required, more detailed processes can be chosen.

CNC Machining Surface Roughness Measurement Methods

To verify that the surface of a part meets the required quality and performance standards, various measurement techniques are used to assess roughness, texture, and machining quality from different perspectives. Common methods include:

1. Visual Inspection
   The most direct and efficient initial screening method, which involves using the naked eye or a magnifying glass to identify obvious defects, such as scratches, dents, or burrs.

2. Profilometer
   A contact-based measuring device that uses a probe to move along the surface and record the micro-profile of the part. This method accurately evaluates roughness parameters, profile features, and machining consistency. It is highly precise and suitable for parts that require stringent surface quality standards.

3. Surface Roughness Measurement Instrument
   Specifically designed to measure microscopic irregularities on the surface, this instrument calculates roughness parameters such as Ra, Rz, and others, providing objective, numerical results. It is one of the most commonly used standard methods for evaluating the surface quality of CNC machined parts.