Understanding CNC Machining Surface Roughness and Finishing Techniques

What is the Surface Roughness of CNC Machined Parts?

The surface roughness of CNC machined parts is the average texture of the part’s surface after machining operations. It is used to quantify the fine details of the material’s surface and is denoted by ‘Ra’ (Roughness average). The surface roughness of a CNC part significantly influences its physical properties and performance. However, machinists control the surface roughness of CNC machined parts through the meticulous tool selection and optimization of parameters like feed rate, cutting speed, and depth of cut.

Typical Surface Roughnesses Achieved By CNC Machining

The surface roughness of a part is not always random after the CNC machining process because various applications require CNC parts with varying surface roughness to ensure perfect fit and function. Below is typical CNC machining surface roughness:

3.2 μm Ra

This is a standard commercial machine finish compatible with most consumer parts. Although it has visible cut marks, 3.2 µm Ra is the default surface roughness machinists apply on the CNC part.

It is the ideal surface roughness for machined parts exposed to vibrations, stress, and loads. Also, it is recommended for mating moving surfaces where the load is light and motion is slow.

1.6 μm Ra

A 1.6 µm is the industry standard roughness level for general applications. It has slightly visible cut marks and is ideal for machinery components or mechanical parts where the surface finish influences performance less critically. It is a perfect surface roughness for slow-moving and light load-bearing surfaces instead of fast-rotating parts and those exposed to intense vibration.

0.8 μm Ra

A 0.8 µm Ra is a considerably high-grade surface roughness that requires extremely close control. Although it costs more, it suits parts subjected to stress concentration, especially in automotive and consumer electronics applications. Besides, it can also be used for bearings in cases involving occasional motion and light loads.

0.4 μm Ra

This surface roughness is best suited for high-precision CNC parts for applications requiring aesthetics and smoothness. It resembles almost a mirror finish at a microscopic level. Product developers choose a 0.4 µm Ra surface roughness for rapidly rotating components, including shafts and bearings. However, it often requires more machining effort and quality control, significantly affecting production costs and time.

Different CNC Machining Surface Finishing Methods

Product designers employ different CNC machining finishes according to their unique advantages and the requirements of the intended applications. Below are these commonly used surface finishing options, metals, and non-metal materials:

1. Mechanical Finishing Methods

1.1 As Machined

As-machined finish refers to the surface appearance of machined parts as soon as the manufacturing process is finished. The parts often have surface defects, such as tiny tool marks. Machined parts with an as-machined finish have a 3.2µm average surface roughness.

Please note, that post-processing techniques like smoothing and polishing can compromise the machined surface’s dimensional tolerances.

1.2 Bead Blasting

This popular and cost-effective CNC metal finishing provides a satin or matte surface finish for parts that don’t require a glossy finish. Bead blasting involves bombarding the surface of CNC parts with millions of tiny glass beads through a pressurized air gun in a closed chamber to eliminate defects and imperfections.

1.3 Brushing

It is a precision surface finishing method that creates a uniform, directional texture on the surface of CNC parts using fine bristles or abrasive media. Brushing finish is particularly suitable for highlighting the natural luster of aluminum, copper, and stainless steel parts without necessarily giving them a high polishing gloss.

1.4 Sand Blasting

Sand or abrasive blasting is a mechanical finishing that cleans, smoothens, or shapes parts’ surfaces by propelling abrasive media such as sand at high velocity against it. It is suitable for removing contaminants, adding patterns, or preparing the surface for painting or coating.

1.5 Polishing

Polishing is a mechanical CNC machining surface finishing involving the use of abrasives or polishing compounds to achieve a high-gloss reflective finish on the surface of different parts. Machinists use rotary tools like polishing wheels or pads during the polishing process. Product designers often polish medical parts, food processing components, and luxury items because they offer aesthetic, protective, and functional benefits.

1.6 Knurling

This custom surface finishing creates a patterned texture on the CNC metal part’s surface by pressing a patterned tool against the surface of the rotating workpiece. The knurling process creates a tightly controlled and uniform pattern for better appearance or grip enhancement in metal parts, including brass, steel, and aluminum.

1.7 Grinding

This machined surface finish involves using an abrasive wheel to rid the surface of machined parts of additional material. It provides a uniform and smoother finish, particularly in materials that accumulate high amounts of contaminants.

2. Chemical Finishing Methods

2.1 Passivation

It is a standard chemical finishing treatment used to improve the corrosion resistance of machined parts. The passivation process involves dipping the material in a chemical bath that removes iron from its surface, providing a smooth, shiny finish.

2.2 Chemical Conversion Coating

Also referred to as chromate coating, it is a CNC machining surface finishing that involves dipping a metal like zinc, cadmium, aluminum, or magnesium in chromic acid or other chromium solutions. This solution reacts with the metal surface to create a protective layer that enhances paint adhesion and provides electrical insulation and corrosion resistance.

2.3 Galvanizing

Galvanizing or zinc-coating is a surface treatment method that involves immersing a solid substrate such as steel in a molten zinc solution to coat it with different zinc-iron alloy and zinc metal layers. This cost-effective finishing treatment creates a protective layer on the surface of machined parts, preventing corrosion and rust.

2.4 Black Oxide Coating

This conversion coating process creates a magnetite layer on ferrous metals through a chemical reaction between the oxidizing salts in the black oxide solution and the iron on the surface of the metal. Black oxide coating finish provides a non-reflective and corrosion-resistant surface in architectural and consumer products.

2.5 Vapor Polishing

This precision surface finishing melts the surface of CNC plastic parts with chemical vapor to achieve a smooth, glossy finish. Manufacturers utilize vapor polishing techniques for thermoplastic materials such as PC and acrylic. It provides a high-gloss finish or optical clarity for applications like automotive lights and medical devices.

3. Electrical/Electrochemical Finishing Methods

3.1 Anodizing

It is an electrochemical method that improves the natural oxide layer on metals’ surfaces, most especially aluminum. Anodizing increases metal parts’ corrosion resistance, wear resistance, and surface hardness while allowing colored parts for aesthetic reasons.

3.2 Electroplating

This custom surface finishing process allows the deposition of a metal coating on a part using an electric current. Electroplating helps to effectively control the thickness and composition of the deposited layer, enhancing electrical conductivity, aesthetic appeal, and corrosion resistance properties.

3.3 Electroless Nickel Plating

It is also called nickel phosphorous plating. It involves depositing an even layer of nickel-phosphorous alloy on the top surface of metals like steel or aluminum. The solid substrate is immersed in a water solution with nickel salts and phosphorous-reducing agents. Electroless nickel plating offers a uniform coating distribution, good adhesion, and corrosion resistance properties.

3.4 Electropolishing

Electropolishing is a standard electrochemical finishing method that dissolves the outer layer of materials to eliminate surface irregularities, providing a brighter and smoother surface. It improves the cleanability and corrosion resistance of metal parts.

3.5 Powder Coating

The powder coating method entails coating a solid substrate with free-flowing dry powder. The dry powder (thermoplastic or thermoset polymer) is sprayed electrostatically and cured at high temperatures under UV light or heat. This CNC metal finish works best with metal materials.

4. Thermal Finishing Methods

4.1 Annealing

Annealing is a CNC metal finishing process that involves heating a material until it recrystallizes, then putting it in sand to cool gradually or cooled to room temperature in the oven. Although the cooling process is slow, it helps reduce the metal’s hardness, increase its elasticity, and enhance its cold working capacity.

4.2 Heat Treating

It entails a series of processes used to modify the microstructure of materials to improve their physical and mechanical properties, including ductility, strength, and hardness.

4.3 Tempering

This heat treatment process involves exposing the metal to high heat below the critical point and held before cooling to achieve a balance of toughness and hardness in metals after hardening.

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

To successfully choose a surface treatment that matches the CNC machined part of the design requirement and application, here are some factors to carefully evaluate:

Material

Usually, the materials of machined parts respond differently to specific CNC machining surface finishing. Aluminum parts, for instance, are compatible with anodizing and powder coating for enhanced aesthetics and corrosion resistance. At the same time, passivation is typically usable on stainless steel parts for corrosion resistance and improved lifespan, and steel is compatible with black oxide, or galvanizing.

Functionality

Every CNC machined part has a specific purpose or application. Hence, a compatible surface finish must be chosen based on different functions to meet design and performance requirements. You may have to choose a surface finish like plating, or anodizing for parts exposed to corrosive substances or environment.

Also, case hardening or thermal finishing methods like tempering or annealing may be suitable if the part is exposed to high-wear applications. Electroplating machined parts with conductive coatings like copper, silver, or gold might be suitable for improved conductivity in electronic components.

Aesthetics

The desired appearance of the CNC part determines the choice of a surface finish. Surface finish treatments offer an extensive range of visual effects from matte and satin to high gloss finish. CNC machining finishes like polishing and electroplating give a high gloss finish, while powder coating, bead blasting, and sandblasting offer a uniform matte or satin finish.

Cost

Precision surface finishing processes often vary in terms of costs. For example, powder-coating parts will cost more than regular paint. But it is cost-effective for larger production runs. Hence, balancing all parameters, including costs, time, and function, would be best to achieve the best results.

Lead Time

Some CNC machining finishes take longer than others to achieve. For instance, you may need to choose a quicker metal surface finish, like polishing, when working with a shorter lead time. However, you may want to use a high-quality finish like anodizing if you have more time and need parts with a more precise and polished surface finish.

Methods to Measure CNC Machining Surface Finish

Manufacturers employ different CNC machining surface finishing measuring methods to determine if the achieved degree of surface finish allows the parts to meet the specific surface finish requirements and performance criteria. Each technique provides insights into the surface irregularities, texture, and overall quality. These measuring methods include:

• Visual Inspection: Although subjective, visual inspection of machined parts helps quickly identify significant surface flaws.
• Profilometers: Profilometers involve tracing a stylus over the part’s surface to produce a detailed profile that can be evaluated to assess surface properties.
• Surface Roughness Testers: These devices quantify the micro-irregularities on the surface of machined parts, indicating a quantitative roughness value.