Surface finish is an integral part of manufacturing. It defines the texture and roughness level of a manufactured component. The surface of a part can either be rough with visible tool marks or fine and satin-like. The importance of surface finish extends beyond cosmetics and aesthetic appeal. It determines the functionality, wear resistance, and how well a finished product can dissipate heat.
Before using a part especially those with high tolerance requirements, it is important to inspect and measure the surface finish roughness. This article will explain what surface finish is, how to measure surface roughness, industrial standards of surface finish, and techniques you can use to achieve the desired surface finish.
What is Surface Finish
Surface finish, also known as surface texture, is the physical characteristics of a part. It can range from rough and matte to smooth and glossy. Surface finish is defined by three primary features; roughness, waviness, and lay.
- Roughness is a fine-scale surface irregularity that you can feel.
- Waviness describes medium undulations that are visible to the naked eye.
- Lay is the direction of the dominant pattern which can be circular, parallel, or perpendicular.
These parameters show how much the part’s surface deviates from a perfectly flat one.
Furthermore, surface finish plays an important role in manufacturing. A good finish minimizes wear and tear by reducing friction and heat generation. It increases the visual appeal and economic value of a component. Additionally, it determines the success of subsequent manufacturing processes such as adhesive bonding, coating, and plating. The surface finish also influences tolerance level, especially for mating parts during assembly.
Common Surface Roughness Parameters
There are certain parameters you can measure to quantify the surface finish of a material. They are usually used together to get a comprehensive overview. The following are common surface roughness parameters in manufacturing
Ra (Roughness Average)
Ra defines the mean vertical deviation of a surface from an ideal one. It is the measurement of the average hills and valleys across the length of a surface. The value can range from 12.5 to 0.4 micrometers. Note that the lower the Ra value, the smoother the surface. Ra is important for aesthetic applications.
Rz (Roughness Depth)
Rz is the measurement of the maximum height difference between the tallest peak and the deepest valley on a surface. You calculate Rz by selecting the highest value after measuring the peak-to-valley height at 5 different locations. It is a better parameter for evaluating surface roughness for critical applications when compared to Ra.
Rmax (Roughness Width)
Rmax is similar to Rz. However, it measures the maximum vertical distance between the highest peak and the lowest valley. Rmax gives an idea of the worst-case scenario surface roughness and helps to detect scratches and burrs. Note that the higher the Rmax value, the rougher the surface.
Rp (Roughness Peak)
Rp measurement represents the maximum peak height along a sampling length. It is often compared with other surface roughness parameters for quality control and ensuring proper surface finish. The Rp value affects the part’s wear resistance and ability to seal.
Rv (Roughness Valley)
Rv value often complements Rp value. Rv is the measurement of the deepest valley depth of a surface from the mean line. A lower Rv value indicates a smoother surface and vice versa. Rv value helps to detect surface imperfections and is linked to functional properties such as friction and sealing.
Standards of Surface Finish in Manufacturing
There are two major surface finish standards in manufacturing. The ISO 1302 and ASME Y14.36. Implementing these standards helps to meet customers’ needs and achieve quality assurance.
ISO (International Organization for Standardisation) 1302
This is the standard that contains specifications and guidelines for the assessment of surface roughness to ensure precision and consistency in industries. It describes the procedures for measuring surface roughness, indicates graphical symbols for surface texture, and defines surface roughness parameters. This standard also contains calculation methods for the surface parameters and how to calibrate the measuring instruments. Following ISO 1302 standards improves product quality and reduces incidence of recalls.
ASME (American Society for Testing and Material) Y14.36
This standard specifies methods to designate surface texture controls on solid objects. It covers many aspects of engineering drawings such as architectural, mechanical, and structural. This standard provides guidelines for scales and formats, lettering and line conventions, and provides symbols that indicate surface roughness. Implementing ASME Y14.36 reduces errors and increases efficiency.
Factors that Affect Surface Finish
Many factors affect the final surface roughness of a part. It may be as simple as inadequate preparation of the workpiece surface before machining. The following factors can impact the success of a component.
Material Properties
The intrinsic properties of the material will affect how it behaves in terms of surface finish. The level of surface roughness depends on the ductility and hardness. Ductile materials such as aluminum and brass have a finer finish than hardened metals. Furthermore, the grain size of the material also has an effect. Too large a grain size may lead to an undesirable surface finish.
Cutting Techniques
The use of mechanical cutting operations increases the likelihood of marks on the part’s surface. This is because the tool continually engages and disengages to result in a cutting action. The newer non-contact processes such as laser cutting give a cleaner result and a better surface finish.
Cutting Parameters and Tool Condition
The tool condition determines the final finish. An old or blunt tool will result in defects or uneven finishes. Additionally, the cutting rate and depth are also important variables. You can optimize these parameters based on the material you are working with. Using too high a rate and depth can lead to overheating and surface flaws. However, very low speed affects productivity.
Cutting Conditions
It is important to use coolants when machining to ensure sufficient heat dissipation. Without an efficient cooling system, the material can expand unevenly which can lead to inconsistent surface roughness. The rate of chip removal and management also plays a role. Chip build-up leads to scratches and marks on the surface of the workpiece.
Operator Skills and Expertise
The results that a skilled operator produces will always be different from an inexperienced one. A skilled machinist has a deep understanding of the process and can adjust important parameters as needed to get a high-quality finish.
Jigs and Fixtures
Not only will vibrations cause human and environmental harm, but it can also result in flaws and imperfections. Always use jigs and fixtures to hold the cutting tools and workpiece and prevent vibrations. This ensures accuracy and consistent quality.
How to Measure Surface Finish
There are different ways to check for the level or roughness of a surface. We will explain the most common methods you can use to measure surface finish.
Direct Measurement Methods
This involves the use of a stylus profilometer to examine a surface and measure one point at a time. The tip of the stylus has a small radius and makes contact with low pressure. It moves freely and detects variations in roughness and waviness as it analyzes the surface characteristics of the material. The Stylus profilometer is reliable and accurate. However, it may not be suitable for brittle materials as prolonged contact can lead to dimensional inaccuracy.
Laser Systems
Lasers work on the principle of light reflection to create a 3D image. Here, you scan the surface of the part using a laser beam and record the beam displacement as it reflects away. This process is sensitive and does not damage the surface. Note that these laser systems may give false results when working with materials that are transparent or highly reflective.
Visual Inspection
You can also inspect visually with the use of the naked eye, magnifying glasses, or electron microscope. The use of just the naked is not reliable and can be a starting point. Electron microscopes provide a magnified view of the surface and show all irregularities. They are non-destructive and can examine a wide range of engineering materials. However, the use of visual inspection may be time-consuming for large and complex shaped parts.
Area Techniques
Rather than taking measurements from single points, they cover a larger area. For this, you can use optical profilometers and inferometers. It involves using a light source that illuminates the surface to measure topography and roughness. Optical profilometers are expensive and may miss out on tiny features or details.
Atomic Force Microscopy (AFM)
AFM uses a cantilever with a sharp tip to scan the surface of a component. It maps out contours and measures intermolecular properties such as friction and adhesion. AFM is accurate and provides a 3D surface profile. This technique may be slow and can damage brittle materials.
How To Get The Desired Surface Finish
Many times, the machined surface finish may not be good enough. You can further improve the surface characteristics mechanically, chemically, or with the use of coatings. They are different and give unique results.
Mechanical Finishes
There are different techniques under this category with the most common being machining, grinding, and lapping.
- Machining involves the gradual removal of materials by milling or turning to give a fine and high-precision finish.
- Grinding removes impurities and smoothens a surface with a rotating abrasive wheel.
- Lapping describes the application of loose abrasive powders between two surfaces and rubbing them together.
Chemical Finishes
You can also use chemicals to enhance the surface finish of a part either by etching, pickling, or passivation.
- Chemical etching uses acids, alkalis, or salt solutions to improve surface roughness. It is a fast process and ensures a quick turnaround.
- Picking is an aggressive method that uses acids to remove impurities and contaminants leaving a satin-like appearance.
- Passivation builds up oxide layers to create a shield for aesthetic and protective functions.
Coatings
Another way of improving a surface finish is to add an extra layer of coat that forms a strong bond with the surface. The coating can come in the form of painting, plating, or powder coating.
- Painting involves applying liquid paint on the surface of a part to conceal defects and provide a durable finish.
- Plating deposits a layer of metal over the surface of an object. It can either be electroplating or electroless plating. Electroplating needs electric current while electroless plating uses chemical reactions.
- The powder coating process uses a gun to spray charged dry powder onto the surface of a part to create a strong and protective coating.
Conclusion
Surface roughness is important in manufacturing because it determines how a part functions and improves customer satisfaction. To achieve a good surface finish, consider reducing the feed rate, using a chip breaker to control chip sizes, and avoiding vibrations by securing the tool and workpiece in place. However, there may be a need to carry out further post-processing to optimize surface finish. There are different ways you can go about this. It may be as simple as surface grinding or using complex plating techniques to add an extra layer of protective metal.
Zintilon is your reliable go-to partner when it comes to delivering quality and durable products. We have a great variety of surface finishes over 40+. Our team of dedicated and skilled machinists carefully fabricate components across various industries to meet the exact requirements of our customers. We have been in the industry for more than 20 years and have served 50 countries and counting. Our services include rapid prototyping, custom manufacturing, and mass production of parts. We are ready to share knowledge and attend to all your machining needs. Contact us today and bring your ideas to life in the most cost-effective way.
FAQs
What are the different types of surface finish?
There are different types of surface finish in manufacturing. They include rough machining, semi-finishing, fine finishing, superfinishing, and specialized finishes.
What industries require precise surface finish control?
Many manufacturing industries specify the surface roughness of the final part to ensure quality. Some common industries are medical, defense, aerospace, automotive, and food processing.
How does surface finish impact the functionality of a component?
The surface finish of a part determines how well it functions and how long it can last. Surface roughness determines sealing capabilities, level of corrosion resistance, heat transfer efficiency, aesthetics, and degree of wear and friction.
Great, Together
