Surface finishes

Surface Finishes

General Overview

• A surface finish, also known as a surface texture or finish, is the nature of a surface that defines its texture, roughness, and appearance.
• These finishes can significantly affect the physical and chemical properties of materials, such as a material’s appearance, friction, wear resistance, ability to hold paint, adhesion, and more.
• The type of machining operation and the tooling used determine the surface finish of a part.
• Surface finishes are categorized into two types: mechanical finishes and chemical finishes.

Mechanical Finishes

• These are finishes obtained through physical processes that alter the surface of a material.
• Sanding: This method uses a sandpaper to smooth the surface. It’s commonly used for wooden materials.
• Polishing: This is used to create a shiny surface by removing a small layer of the material. Favoured in metals and stones.
• Grinding: Used for hard materials. It involves a grinding wheel to eliminate material, obtaining a smooth surface.
• Blasting: This abrasive process propels a stream of abrasive material against a surface. It is commonly used to roughen surfaces or to clean them.

Chemical Finishes

• These are finishes obtained through applications of chemical agents that alter the surface of a material.
• Acid finishing: Metals are treated with acid to create an attractive and durable surface.
• Anodising: This electrochemical process thickens and toughens the naturally occurring protective oxide on metal surfaces, often aluminium.
• Painting: It provides both an improved appearance and a protective layer against factors like rusting and UV damage. Painting can also provide color coding for safety or process control.
• Powder coating: It’s a finishing process where a dry powder is applied to a surface, heated until molten, then cooled to form a solid, durable coating. Ideal for metals.
• Plating: This process involves depositing a thin layer of metal onto the surface of a part. Gold or silver plating can be used for decorative purposes, while chrome or nickel plating can enhance corrosion resistance.

Measurement of Surface Finishes

• The measurement or the quality of a surface finish can be expressed as an average roughness (Ra) value.
• The Ra value is calculated by measuring the variations in the heights and depths of the surface irregularities and averaging them.
• Smooth, highly polished surfaces have low Ra values, while rough, coarse surfaces have high Ra values.

Selection of Surface Finishes

• The selection of surface finishes must consider their impact on product performance, aesthetics, wear properties, cost and manufacturability.
• Factors such as environmental resistance (corrosion, UV light, etc.), required friction levels (wear, lubrication, etc.), and regulatory constraints (food processing, medical devices, etc.) all need to be taken into account.
• Aesthetic considerations can also play a key role, particularly in product design or for consumer-facing components.