In both prototyping and mass production, surface finish is a necessary and almost final process to achieve aesthetic or protective purposes. This article provides a comprehensive overview of surface finish principles, measurements, and techniques.
Understanding Surface Finish
What is Surface Finish?
Surface finish, or surface treatment, is a process of artificially creating a surface layer on a substrate material with different mechanical, physical, and chemical properties from the substrate. The purpose is to meet specific requirements for product corrosion resistance, wear resistance, decoration, or other special functional needs. Whether in prototyping or mass production, surface treatment is a necessary and crucial step to achieve the desired surface finish and functional properties.
Factors Affecting Surface Finish
1. Material Properties
The surface finishes that can be applied to a material are influenced by its unique characteristics. Not all materials are suitable for every type of surface treatment. For example, certain plastics may not adhere well to specific paints, leading to paint peeling. When considering both material performance and aesthetic appeal, it is crucial to carefully evaluate and strike a balance in order to make an informed decision.
2. Cutting Tools
Harder tool materials can often produce a better surface finish. But they may also be more brittle and prone to breakage.
Tools with a sharper edge tend to produce a better surface finish. But they may also be more prone to wear.
A shallower cut can often result in a better surface finish. But it may also increase the machining time.
Higher cutting speeds can lead to a better surface finish, but only up to a certain point.
Proper cutting fluid selection and delivery can help to reduce heat and improve surface finish.
3. Machine Parameters
Machine parameters, including spindle speed, feed rate, and machine rigidity, determine how the machine operates during the cutting process and directly impact the surface finish. For example, the spindle speed and feed rate can affect the tool engagement with the workpiece, thereby affecting the surface quality.
Common Surface Finish Measurement Techniques
Roughness Measurement
1. Introduction to Roughness
Surface roughness is primarily affected by the machining methods used and other factors such as friction between the tool and the surface, plastic deformation of the surface metal during chip separation, and high-frequency vibrations in the processing system. Due to differences in machining methods and workpiece materials, the depth, spacing, shape, and texture of surface features left by the machining process can vary.
2. Profilometer and its Applications
A profilometer is a measuring instrument used to quantify the shape or roughness of a surface by measuring its profile.
Profilometers are widely used in manufacturing industries to assess the roughness of various surfaces. For example, in the automotive sector, profilometers are commonly used to measure the surfaces of automotive components such as engine cylinders, wheel hubs, and brakes. In the aerospace industry, profilometers inspect and measure the roughness of aircraft turbine blades and engine cylinder surfaces.
3. Ra and Rz
Ra - Arithmetic Average Roughness: It is the arithmetic average value of the absolute profile deviations within a sampling length. In practical measurements, a higher number of measurement points leads to more accurate Ra values. Smaller Ra values indicate smoother surfaces.
Rz - Maximum Height of the Profile: It represents the distance between the highest peak line and the lowest valley line of the profile. Rz is typically used to describe relatively short surfaces. In practice, however, Ra is more commonly used for comparing surface roughness than Rz.
Surface Texture Analysis
1. Surface Texture and its Significance
Surface texture refers to the characteristics and properties of a surface, including its roughness, waviness, and form. It describes the deviations and irregularities present on the surface at various length scales.
It is significant in various industries and applications due to its impact on functionality, aesthetics, performance, lubrication, adhesion, manufacturing, and quality control. Understanding and controlling surface texture is essential for achieving desired product properties, optimizing processes, and ensuring customer satisfaction.
2. Methods for Surface Texture Analysis
Surface texture analysis involves utilizing specialized evaluation methods to obtain specific data or graphical results that ensure microscopic surface features meet the intended application requirements.
There are many ways to characterize microscopic texture, such as PSD (Power Spectral Density), slope, region analysis, and various ISO25178 parameters. The specific method used to characterize texture depends on the specific application requirements.
Types of Surface Finishes
Surface finish techniques can be classified into three main categories based on the different processing methods.
Machined Surfaces
1. Turning and Milling
When no further finishing is needed and as machined, it means there is no surface finish for the component in prototyping. It usually happens in turning and milling for metal parts. You can easily see the tool paths on the part surface.
2. Grinding and Polishing
Grinding is a material removal process that utilizes abrasive particles to remove material from the workpiece surface. The abrasive particles are embedded in a grinding wheel, which rotates at high speeds and removes material through the cutting action of the abrasive grains.
Polishing, on the other hand, involves using a polishing pad or cloth with a fine abrasive compound to remove a thin layer of material from the surface of the workpiece. The process is usually performed after grinding to achieve a smoother surface finish.
Coated Surfaces
1. Introduction to Surface Coatings
Surface coating is a process of applying a thin layer of coating material onto the surface of a material to modify its surface properties. The coating material can be liquid, powder, or gas and is chosen based on the desired outcome and the characteristics of the substrate material.
Surface coatings serve various purposes, such as enhancing the appearance, improving durability, providing corrosion resistance, reducing friction, increasing hardness, or adding functionality to the surface. The choice of coating material depends on the specific requirements of the application and the desired performance characteristics.
2. Types of Coatings and their Applications
Common surface coatings include paints, varnishes, lacquers, enamels, powder coatings, electroplating, anodizing, and thermal spray coatings. Each type of coating offers unique properties and benefits, making them suitable for different applications across industries such as automotive, aerospace, electronics, construction, and more.
Surface coating processes can be applied through various techniques, including spraying, dipping, brushing, rolling, electrostatic deposition, and vapor deposition. The selection of the coating method depends on factors such as the coating material, the size and shape of the substrate, and the desired coating thickness.
Heat Treated Surfaces
1. Heat Treatment Processes
Heat treatment is a set of industrial, thermal, and metalworking processes used to alter the physical properties of materials, sometimes including chemical properties. The most common application is in metallurgy. Heat treatment is also used in many other materials, such as glass.
The heat treatment techniques include annealing, surface hardening, precipitation strengthening, tempering, carburizing, normalizing, and quenching.
2. Effects of Heat Treatment on Surface Finish
Heat treatment typically changes the hardness, toughness, ductility, and strength of materials. The primary objectives of heat treatment are as follows:
Improving mechanical properties:
Heat treatment can improve mechanical performance by modifying the hardness, toughness, ductility, and strength of materials.
Adjusting material structure and microstructure:
Heat treatment can induce changes in the structure and microstructure of materials, allowing for the adjustment of properties such as improved corrosion resistance, wear resistance, and heat resistance.
Repairing and improving material defects:
Heat treatment can address material defects by promoting grain growth and the formation of precipitates, thereby reducing porosity, inclusions, and cracks.
Enhancing processability:
Heat treatment can improve the processability of materials by reducing their hardness, resulting in increased processing efficiency and reduced processing difficulties.
Final Words
Mastering surface finish is crucial for product performance and aesthetics. This guide covers its importance, measurement techniques, and treatment methods. Consider material compatibility, desired properties, application requirements, cost-effectiveness, process compatibility, and regulatory compliance when choosing a finishing method. By embracing these principles, you can achieve superior surface quality and unlock new possibilities in your manufacturing endeavors.
If you need a more detailed introduction to surface treatments, read our blog on An Ultimate List of Surface Treatments for Rapid Prototype Machining.
