Surface finishing is a critical aspect of steel CNC machining. It produces high quality parts while enhancing their appearance and durability. Different manufacturing industries including medical, marine and automotive prioritize surface finishing in part manufacturing. A well planned surface finish ensure that parts perform as intended and withstand the rigors of their operating environment.
Before using any surface finishing technique for your application, you must understand how it works to avoid mistakes. This article explains the importance of surface finishing in steel CNC machining and the common methods you can employ.
Importance of Surface Finishing in Steel CNC Machining
Surface finishing is an important aspect of product development. This is because the ‘as-machined’ finish may not be sufficient for performance and aesthetics. The following are some reasons you should carry out various operations to improve surface roughness in CNC machining.
Enhance Corrosion Resistance
Most steel parts are susceptible to rust on exposure to corrosive substances. Hence, you must add an extra layer of protection to shield them from moisture, chemicals and environmental factors. These surface finishing techniques help to prevent oxidation and rust formation. The final parts are suitable for applications with unavoidable exposure to extreme weather and high humidity.
Improve Wear Resistance
The different surface finishing processes also improve the wear resistance of components. Resistance to wear is important for parts that often experience friction and impact when used continuously. You need to apply proper finishing to prevent parts from wearing out with time. Some techniques harden the surface and make it more resistant to mechanical wear.
Achieve Tighter Tolerances
Surface finishing in CNC machining also allows you to produce parts with smoother surfaces and tighter tolerances. The machining operation tends to create minor imperfections and tool marks. All these can affect part performance and lead to failure over time. The use of surface finishing techniques reduce roughness and create uniform textures.
Aesthetic Appeal
Another benefit of surface finishing is to enhance aesthetic appeal. This is especially important for components that visual appeal is as important as their functionality. The appearance of a product determines how a customer perceives it. A high quality surface finish improves the look and feel of a part. It makes it more appealing to customers.
Improves Cleanability
There is an increasing demand for steel components that are easy to clean. Industries including pharmaceuticals, medical devices and food processing requires parts that are amenable to sterilization. Rough surfaces are carriers of contaminants including bacteria and debris. They make components unclean and unhygienic. The use of surface finishing in steel CNC machining eliminates crevices that can arbor bacteria.
Common Surface Finishing Techniques for Steel CNC Machining
You can finish the surface of steel CNC machined parts in different ways. We’ve grouped them into three for better understanding. Let’s discuss these techniques in detail.
- Mechanical Finishing Methods
This is a popular way of finishing steel components. What you do here is to physically modify steel surfaces using tools, abrasives and impart forces. Grinding, polishing and blasting falls under mechanical finishing methods.
Grinding
Grinding describes the process of using a rotating abrasive wheel to make steel surfaces smoother. You can use surface, cylindrical or centerless grinding depending on the required finish. This technique removes surface irregularities and marks. It can also be a step before further finishing processes like polishing or coating.
Polishing
The result of the polishing process is a smooth and reflective finish. You carry out polishing with buffing compounds and rotating pads. This operation gradually reduces surface roughness by removing micro-scratches and imperfections. Polishing reduces surface friction and makes components last longer. It enhances the aesthetic appeal of steel parts.
Blasting
Blasting can be sand or bead blasting depending on the type of abrasive you use. It entails using high pressure air or mechanical force to propel abrasive particles onto the steel surface. In sandblasting, the media is usually silica sand. On the other hand, bead blasting uses glass beads to create a matte and uniform finish. Generally, blasting produces uniform textures with controlled surface roughness.
- Chemical Finishing Techniques
This describes techniques that alter the steel’s surface through controlled chemical reactions. It provides a smoother surface and improves corrosion resistance. Passivation and electropolishing are common chemical finishing techniques for steel CNC machining.
Passivation
Passivation is a popular process you can employ to finish stainless steel. This chemical treatment uses nitric or citric acid to remove free ion from the surface of the component. This action produces a chromium oxide layer that enhances the corrosion resistance properties of stainless steel. Additionally, passivation removes surface contaminants that can lead to rust.
Electropolishing
The process of electropolishing is somewhat similar to that of mechanical polishing. However, the tools here are electrolyte bath and electric current. This technique involves submerging the steel part in an electrolyte bath and passing electric current. It dissolves microscopic peaks on the surface leaving an ultra smooth finish. Electropolishing produces clean and contamination free components.
- Coating and Plating Techniques
Another broad category of steel CNC machining surface finishing is coating. Here, you apply protective or functional layers to enhance the steel components’ properties. Examples of coating and plating techniques are powder coating, zinc plating, and nickel plating.
Powder Coating
The powder coating process begins with preheating the part. You then apply electrostatically charged dry powder particles onto the steel surface. The coated part is then loaded into the curing oven. This allows the powder to melt and form a durable, smooth layer that hardens as it cools. Powder coating is versatile, as there are various colors and textures available. The final part is resistant to chipping and fading.
Zinc Plating
You can also refer to zinc plating as galvanization. What you do is apply a thin layer of zinc onto steel by electroplating or hot dip galvanization. The zinc layer acts as a sacrificial coating. When a zinc plated part is exposed to corrosive substances, the outer zinc layer corrodes before the underlying steel. Zinc plating also enhances visual appeal with the shiny finish it creates.
Nickel Plating
The major difference between zinc and nickel plating is the depositing metal. As the name implies, nickel plating deposits a thin layer of nickel using electroplating or electroless plating. The final parts are corrosion-resistant and highly durable. Furthermore, nickel plating improves the electrical conductivity of components. This makes this technique beneficial for electronic parts like connectors and contacts.
How to Select the Right Surface Finish for Your Application
With so many surface finishing techniques available, choosing the most appropriate for your project can be challenging. There is no straightforward approach to this as there are some factors to consider. Let’s discuss six key considerations when selecting a surface finishing technique.
Corrosion Resistance Requirements
An important factor to look out for in the corrosion resistance requirements of the part. Steel metal is prone oxidation and corrosion especially in chemical and marine environments. You should prioritize corrosion resistance for parts exposed high humidity and moisture. Best surface finishes for corrosion resistance are zinc plating, passivation and nickel plating.
Wear Resistance Needs
You must also keep in mind wear and abrasion resistance needs. This is important for components used in heavy equipment and machinery as they are usually subjected to impact and repetitive motion. Using the right techniques like nickel plating and galvanizing increases surface hardness while providing wear resistance.
Aesthetic Appearance
The aesthetic and surface appearance of the part is also crucial in some situations. In this cases, the finish must enhance color, texture and reflectivity. The technique should mask all surface imperfections and produce a consistent look. The ideal finishing techniques for visual appeal are polishing, bead blasting, and powder coating.
Functional Requirements
Functional requirements covers everything involving friction, conductivity and cleanliness. Check if the techniques can improve friction, enhance electrical conductivity and maintain cleanliness. Common industries that require functional surface finishes are medical and pharmaceutical, electronics and precision engineering.
Environmental and Chemical Exposure
Assessing the condition and environment where the components will be used is vital. Look out for exposure to extreme temperatures, chemicals and UV radiation. The use of passivation and powder coating with UV stabilizers resists chemical exposure. They prevent sun induced fading and degradation.
Cost and Production Efficiency
The cost factor should not stand alone. You should use it with other factors explained above. The cost of surface finishing should align with your budget and production volume. It should not significantly increase manufacturing expenses. Mechanical techniques such as grinding, polishing and blasting are cost effective finishing options.
Conclusion
Selecting the right surface finish is key for CNC-machined steel components. Steel is most times not used as it is. You have to further process it to improve appearance and functionality. Industries such as aerospace, automotive and marine depend on the various finishing techniques to ensure steel components meet high performance standards. When choosing the best application method, you must consider wear and corrosion resistance requirements. You should also keep cost and production efficiency in mind.
