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Why CNC Machined Parts Need Surface Finishing
CNC machining processes, including milling and turning, often leave visible cut marks that impair the surface quality of machined parts. Although CNC machining offers precise parts, these machined parts need surface finishes for various reasons.
In this section, we will discuss why surface finishing CNC machined metal parts is essential:
Improved Appearance
Surface finishes like sanding, polishing, electroplating, or painting help conceal sharp edges and machining marks left behind during CNC machining procedures. As a result, these surface finishing treatments make CNC machined parts more visually appealing.
Enhanced Corrosion Resistance
Most CNC machining materials are vulnerable to corrosion substances. Hence, product designers often protect the surfaces of machined parts with finishes like anodizing, polishing, and passivation, ensuring their extended lifespan.
Improved Cleanliness and Hygiene
Product manufacturers apply different finishes on CNC machined parts to ensure they are easy to clean and maintain. Moreover, this is essential in applications like food processing equipment and medical devices where hygiene is key.
Functional Performance
Manufacturers apply different CNC machining surface finishes to improve material properties like conductivity, reduce friction, and add other desirable properties, optimizing their functional performance.
Customization
Choosing a desired surface finish for your machined parts allows you to customize them to meet specific preferences and requirements. You can achieve different finishes to achieve a series of surface properties, textures, or colors.
Common Types of Surface Finishes for Metal CNC Machined Parts
Different surface finishes are applicable to different CNC machining materials, each with varying surface roughness values. However, choosing the surface finish that best ensures the optimal performance, functionality, durability, and visual appeal of machined parts is critical. Below are commonly used types of surface finishes for metal CNC machined parts:
Polishing
Polishing is a typical mechanical finishing process involving chemical agents or abrasives to create a high-gloss, mirror-like finish on the surface of metal parts. This finishing process enhances the physical properties of metal machined parts, increases corrosion resistance, improves cleanliness, and mitigates friction.
The polishing method is well-suited for metals like aluminum, stainless steel, and brass. Product designers and manufacturers in the food processing, medical, and luxury goods industries widely use polishing due to its functional and aesthetic properties.
Although polishing provides a smooth, reflective surface that improves the metal part’s visual appeal, the process can be time-consuming and labor-intensive. This is especially true for machined parts requiring extremely high finishes or complex geometries.
Bead Blasting
Bead blasting is a commonly used mechanical finishing technique for metal materials. It involves using a pressurized air gun to blast the surface of machined parts with small glass beads to improve its visual appearance. It is a versatile finishing treatment and applies to different materials, including metals and plastics.
Bead blasting creates a smoother surface with a consistent matte or satin finish and a light texture on CNC machined parts, removing visible tool marks, burrs, and other imperfections. The bead is usually sprayed on the machined parts in a closed chamber. However, ensure you cover critical features or surfaces, including holes, to prevent dimensional changes.
More so, bead blasting focuses on enhancing product aesthetics rather than functionality. The operator’s skill often determines the outcome of this finishing treatment since it is a manual process. Typical parameters of the method include the size of the glass beads and the amount of air pressure. Like sandpapers, glass beads vary in size and grade, ranging from very fine to coarse.
Anodizing
Anodizing develops a protective oxide layer on the machined metal parts’ surface. This protective coating shields the surface of the metal parts from corrosion and wear. The anodic coating is compatible with different colors. It is electrically non-conductive and has high hardness (type III). Anodizing is ideal for developing a corrosion-resistant coating for machined aluminum and titanium parts.
To anodize a CNC metal part, immerse it in a diluted sulfuric acid solution and apply an electric voltage between it and the cathode. An electrochemical reaction occurs on the exposed surface of the machined part, transforming it into hard titanium or aluminum oxide. However, remember to mask surfaces with critical features or dimensions like threaded holes that must retain electrical conductivity when anodizing.
The anodized machined parts can take various colors, such as gold, red, black, or blue, before sealing. Additionally, you can achieve varying thicknesses and densities on anodized metal parts by changing the anodizing time, consistency, and anodizing duration. However, there are three variants of anodizing, each with a different process, coating thicknesses, and properties. Here are these variants:
- Type I (Chromic Acid Anodize): It is the thinnest layer and doesn’t alter the machined parts’ dimensions. This type of anodizing appears grayer and doesn’t accept other colors.
- Type II (Boric Acid Anodize): It is a safer type of anodizing and allows you to add various colors to a machined part due to its better paint adhesion. It is called decorative or standard anodizing and can create coatings as thick as 25 µm.
- Type III (Hard Sulfuric Acid Anodize): This is the most commonly used anodizing type, especially for aluminum and titanium alloys. It offers the clearest finish, which allows it to be compatible with the widest range of colors. Type III anodizing finish is slightly thicker than Type II, with coating ranging from .001 to .004 inches. Combining Type III with PTFE/Teflon creates a dry lubricating surface.
Moreover, type III anodizing is not ideal for specific applications because its thicker coating can cause the workpiece to brittle, resulting in chips or cracks under severe stress. As a result, it is crucial to consider the application before choosing between type II and type IIII anodizing.
Alodine
Alodine or chem film is the brand name of the chromate conversion coating. This chemical surface finish involves dipping the machined parts in chemicals made with proprietary formulas with Chromium as the principal component. Ensure the process meets MIL-DTL-5541F standards when requesting Alodine for your metal CNC machined parts. It represents the US Military Specification of Chemical Conversion Coatings on Aluminum and Aluminum Alloys.
Alodine’s protective coating acts as a corrosion inhibitor. More importantly, you can use it with decorative finishes since it improves adherence to paints and adhesives. Unlike other finishes that suppress the thermal and electrical conductivity of aluminum machined parts, Alodine enhances the conductivity of aluminum components. This surface finish is cheaper, and the coating is susceptible to scratches and superficial damage.
Powder Coating
It is an electrostatic process that forms a thin and uniform protective layer on the surface of CNC machined parts with dry powder. The powder coating process is compatible with all metals. Besides, powder coating CNC machined parts helps enhance their strength, corrosion, and wear resistance.
Unlike anodized parts, powder-coated parts exhibit higher impact resistance properties and are compatible with an extensive range of colors. Powder coating can be performed with bead blasting to produce machined parts with uniform and smooth surfaces and excellent corrosion resistance. The powder used in this process can be either thermoset or thermoplastic polymer.
Although powder coating is similar to spray painting, it involves applying dry powder to the metal’s surface and curing it in an oven. You have to prime machined parts with an optional chromating or phosphating coating to enhance their corrosion resistance. Then, coat these parts with dry powder using an electrostatic spray gun and cure them in an oven at 200ºC. However, you can apply multiple layers to achieve the desired coating thickness ranging from 18 µm to 72 µm.
Electroplating
Electroplating is a finishing process that involves depositing a metal coating on a machined metal part, increasing its thickness. Applying this surface finish on CNC machined parts protects their surface from corrosion, shock, heat, and tarnishing and makes them resistant to wear and tear over time. This process works best with chrome, cadmium, tin, copper, nickel, and gold. Electroplating CNC machined parts helps to improve adhesion between the base material and its additional exterior coating. However, you can make your machined part magnetic or conductive depending on the metal plated on the base material.
Unlike other surface finishes in CNC machining, electroplating is not environmentally friendly because it produces hazardous waste. Hence, it can cause severe pollution if not correctly disposed of. Additionally, electroplating is time-consuming and relatively expensive because of the required equipment, metals, and chemicals, especially if the metal parts require multiple layers.
Passivation
Passivation protects surfaces of ferrous materials like steel and stainless steel from corrosion or rust, ensuring improved appearance, performance, and cleanliness. This chemical treatment involves immersing machined metal parts in an acid bath, such as Nitric or citric acid, to free iron from their surface to achieve a smooth and polished finish.
Since passivation is not a coating, it doesn’t require masking and won’t add any thickness to the machined part. The acid-based bath removes traces of iron and rust from the part’s surface, producing a layer composed of chromium or nickel on the metal part’s surface. Despite nitric acid being the traditional passivation choice, citric acid bath has become the widely embraced choice due to its shorter cycle times.
Passivated parts are rust-resistant and are ideal for outdoor applications. More so, passivation is a commonly used surface finish across various industries, from the aerospace industry, with demand for high-quality steel and the tightest dimensional tolerances, to the medical sector, where sterilization and longevity of products are paramount.
However, passivating machined parts can result in extended part production time because these machined metal parts must undergo pre-treatments such as cleaning to remove debris, greases, or other contaminants. Although submersion is the most commonly used passivation technique due to its faster cycles and consistency, acidic spray is a better alternative.
Electroless Nickel Plating
The electroless nickel plating process involves creating a protective layer of nickel alloy on CNC machined parts to increase their corrosion resistance properties. It deposits a layer of nickel-alloy, usually nickel-phosphorous, on metal parts using a nickel bath and chemical reducing agent such as sodium hypophosphite. This process uniformly adds the nickel-alloy coating on complex parts with intricate features like holes and slots. There are several types of electroless nickel plating, each with varying percentages of phosphorus. These include low-, medium-, and high-phosphorus nickel.
Parts finished with nickel plating often exhibit remarkable hardness and wear resistance. Besides, you can make them harder with heat treatment. The Electroless nickel plating process is suitable for finishing different metals, including stainless steel, aluminum, and steel. Despite the enormous benefits of this method, it has certain limitations, including subsequent reduction in plating rates, contaminant growth in nickel baths, and rising phosphorous content. In addition, electroless nickel plating is not best suited for finishing rough, uneven, or poorly machined surfaces.
Best Practices CNC Machining Surface Finishes
Each surface finish has its benefits and limitations; a finish is only ideal if it meets the requirements of your part’s requirements or intended application. Here are the best practices for achieving top-tier surface finishing:
Use Multiple Surface Finishing for Perfect Appearance
Using more than one surface finish can help improve the surface roughness of CNC machined parts and add varying properties. For instance, you can perform media blasting before other finishes to conceal minor tool marks, create specific matte finish and improve surface texture before applying another finish.
In addition, your parts’ surface can have a smooth, matte, and colorful appearance by combining media blasting with anodizing. Similarly, you can combine Alodine and Type II anodizing even though you have to mask areas that need to keep their electrical and thermal conductivity.
Choose a Surface Finish that Matches Your Intended Application
Understanding the purpose of your machined parts helps determine the ideal surface finish that matches your appearance and functional requirements. While anodizing offer excellent dimensional control, bead blasting is a perfect surface finish in cases when dimensional control is not a concern.
Type II anodizing suits your titanium and aluminum parts when you need a protective and cosmetic coating with various colors. Meanwhile, powder coating is a perfect choice for applications when you need parts with high-impact strength.
Choose Appropriate Tooling and Parameters
It is essential to use the proper cutting tool and fine-tune machining parameters, especially during the finishing cutting pass, to improve the surface roughness of machined parts. Besides, implementing proper machining techniques, including toolpath optimization, minimizing chattering/vibrations, and proper fixturing, help achieve high-quality surface finish.
Consider Budget/Costs
Ensuring your desired surface finish matches your budget constraints is critical when finishing your machined parts. A surface finish is not suitable for your product if it doesn’t fit within your budget range. Hence, consider each surface finish’s various properties and associated costs to determine the ideal one that fits your budget.
Conclusion
Metal CNC machined parts can take on any surface finishing to ensure they meet the demands of your project. We have discussed the most common surface finishes for metal CNC parts earlier in this article, each with unique pros and cons. However, it is crucial to understand how these surface finishes work and their outcomes to determine the right fit for your specific application.
AT-Machining is the right expert to contact whenever you need the help of an expert with the surface finishing of your finished metal parts. Different applications require varying roughness values, so we offer the standard surface roughness levels specified for CNC machining applications, including (0.4, 0.8, 1.6, and 3.2 µm Ra). Our state-of-the-art CNC machining factory and network of suppliers allow us to deliver quality plastic and metal machined parts with top-tier finishes tailored to your preferences. Submit your CAD files today for instant quotes.
FAQs
How Do You Prepare the Surface of CNC Machined Metal Parts for Finishing?
Before applying surface finishes to CNC machined metal parts, it is vital to prepare their surface to ensure they meet aesthetic and quality standards. Here are common ways to prepare your custom-machined parts for surface finishes:
- Cleaning: First, clean the machined parts thoroughly to rid their surface of any residue, oils, and machining fluids. Solvent-based or ultrasonic cleaning methods are commonly used to remove contaminants that can hinder the finishing process.
- Smoothing Edges: Sharp edges and burrs can have adverse effects on the finishing process. Hence, to ensure the uniformity of the applied surface finish, practice deburring techniques like tumbling, edge rounding, or manual sanding.
- Ensuring Compatibility: Confirm that the CNC machined parts’ material matches the intended finishing process. For example, some metals may require certain pre-treatments before plating or anodizing to ensure superior finish quality and optimal adhesion.
What Are the Common Surface Finish Parameters?
Surface finish or surface texture parameters are grouped into three characteristics, including waviness, roughness, and lay. The waviness comprises more significant undulations, while the roughness often comprises smaller irregularities. Lay represents the direction of the texture on the part’s surface created by different manufacturing operations like CNC turning, grinding, and milling.
What Are the Common Surface Roughness Symbols for CNC Machined Parts?
The surface finish of machined parts is quantified by specific units and symbols that define the surface’s average or peak-to-valley roughness. Understanding these metrics helps to specify, achieve, and verify the desired surface quality.
- Ra (Roughness Average/Average Surface Roughness): This metric measures the average roughness of a CNC part, represented in microinches (µin) or micrometers (µm). RA is the most widely used parameter for general surface finish, and it quantifies the deviation of a roughness trajectory from the mean line.
- Rz (Average Maximum Height): Quantifies the maximum average height of a surface trajectory. The average peak-to-valley height of the surface irregularities over a certain length is determined by considering the average outcomes of the five most significant disparities between the lowest valleys and highest peaks throughout the entire area.
RMS (Root Mean Square): It is similar to Ra but measured differently. It provides a slightly different perspective on surface roughness.
