When talking about CNC machining finishing options, you might have heard about “as-machined”, “anodizing”, coatings, or blasting among others. But before going into detail for each option, we need to clarify two terms: “surface finish” and “surface finishing”. Sometimes they are misused, which can create confusion.
- Surface finish: Surfaces have characteristics produced by the manufacturing process; these characteristics are: lay, roughness and waviness. Each of these is a “surface finish” and basically they quantify how irregular a surface is (on the micro scale). Depending on the function of your product, you might need to target specific values for these characteristics.
- Surface finishing: This term corresponds to the processes that protect and improve the appearance of surfaces. Some of these processes add material, some remove material, and some involve heat, electricity or chemicals. This article will expand on these common processes with the goal of helping you to choose the best for your product.
Surface finish should not be confused with the use of geometrical tolerances such as flatness, profile, or total runout. Different methods are used to measure surface finish, and it describes the irregularities of a surface at the micro level rather than dimensional inaccuracies. But you might be asking, if geometrical tolerances are defined on your drawing, and if the exterior surfaces of your machined component look ok, why should we bother with micro levels and layers?
Surface finish and finishing are particularly important if your product is in contact with other components. For example: the purpose of a ball-bearing is to reduce rotational friction and support radial and axial loads. When one of the races rotates, the balls also rotate because they are in contact. Therefore, the friction between these components is critical. If the surfaces of the balls or the races have poor surface finish characteristics, then the friction is increased. This results in issues like wearing and decreasing the life of the components, even if these components were fabricated within geometrical tolerances.
Another important characteristic of surface finish is protection against gradual destruction such as corrosion. With the right option you extend the life of your components by adding a protective layer that increases corrosion resistance.
Figure 01 shows a quick schematics of three surface finish characteristics:
- Lay: direction of the predominant surface pattern. Some examples: radial, vertical, horizontal, cross-hatched, circular and isotropic.
- Roughness: measure of the total spaced surface irregularities. These deviations can be plotted as a profile (see Figure 01). There are different methods to quantify roughness, with the most popular defining averages such as arithmetical mean deviation Ra, Root mean squared Rms, and others. Always check what method and parameters are being used in your project as this will affect your product specifications and result in different values.
- Waviness: Similar to roughness, these are surface irregularities but with a greater spacing.
Figure 02 shows a visual comparison of surfaces with different roughness values. Notice how light reflects on the surfaces of the ball causing different appearances for different scenarios. Roughness also plays a key role in contact mechanics since higher roughness values cause faster wear on components, higher friction, and irregularities in surfaces that might become nucleation sites for corrosion and cracks. On Figure 02: “Surface 01” might be the ideal scenario, while “Surface 04” is rarely desirable. Higher values of roughness are not always bad though; when you are interested in adhesion, roughness might be a benefit, but you have to be careful and choose your materials and surface finishing option appropriately.
- Alodine: When a material becomes “passive”, it is less affected or corroded by the environment. In other words, a chemical reaction between the base material and the introduced coating agent produces an outer layer that acts as a shield. This is the case for alodine, or chromate conversion coating, where a thin coating is used to passivate aluminum. The protective layer serves as a corrosion inhibitor. Alodine also improves adherence for paints and adhesives, which benefits a decorative finish. It is typically a cheaper process, but it is prone to scratches and other superficial damages. The bath of chemicals used is often made with proprietary formulas. Chromium is one of the main components, which can cause allergic reactions, so these chemicals should be handled in a safe manner. When requesting alodine for your machined parts, you might see the term MIL-DTL-5541F. This code refers to a standard: Military Specification of Chemical Conversion Coatings on Aluminum and Aluminum Alloys.
- Anodizing: Similar to alodine, anodizing is a passivation process that creates a protective layer on an aluminum part. In this case, an acid electrolyte bath with a passing electric current is used (hence the name: anode). Anodizing is a controlled way to oxidize a base material to improve durability and corrosion resistance. In the case of aluminum, an outer layer of aluminum oxide protects the aluminum substrate. This outer layer is fully integrated with the substrate, so it does not flake or chip like other coatings such as paint and plating. Due to the porous nature, it can also be painted and sealed.
Black Oxide: This process typically applies to ferrous materials such as steel. The goal is to create a black oxide layer called magnetite (Fe3O4) which is more stable than the natural rust red oxide (Fe2O3). The chemical bath is usually at high temperature, and contains alkaline cleaner, water, caustic soda and a sealant such as oil that improves corrosion resistance. There are variations of this process, particularly at cooler temperatures. However, this offers less abrasion resistance. Stainless steels can also be protected with black oxide.
Electroless Nickel Plating: This process is the deposit of a nickel-alloy coating by chemical reduction without using an electric current. Typical coatings are nickel phosphorus where the higher phosphorus content improves corrosion resistance, but decreases hardness. If you are using electroless nickel plating to improve corrosion resistance, do not heat treat after! That will just reduce your corrosion resistance all over again. Aluminum, steel, and stainless steel can all be electroless nickel plated.
Electropolishing: In this process, which can be applied to steel or stainless steel, material is removed, so it can be seen as the opposite or reverse of electroplating. It does use electric current, but in this case to dissolve a controlled layer of the base material, rather than adding a coating. With electropolishing, several factors must be considered, such as base material chemical composition, electrolyte chemical composition, electrolyte temperature, time of exposure, and current density. This particular process relates well to the surface roughness concept explained at the beginning of the article. Peaks are removed leaving a smoother surface, minimizing sites where corrosion can happen as a result.
Media Blasting: Many different metals can be media blasted. This is another process to remove debris and change roughness of parts. In this case, a pressurized jet is used to force an abrasive toward the surface of a part. Sand is a typical abrasive, as well as moderately abrasive materials such as glass beads and plastic beads. The effect is similar to using sandpaper, but at high speed, providing a more even finish with no problems on corners or fillets. A variation of this process includes water which lubricates the surface and traps dust. Keep in mind that using wet blasting on mild steel will result in immediate corrosion.
Passivation: As explained above, passivation is a process that creates an outer layer by chemical reaction with the base material or from spontaneous oxidation in the air. The outer layer functions as a shield or micro-coating that improves corrosion resistance. While it is common for aluminum to naturally form a thin surface layer of aluminum oxide on contact with oxygen in the atmosphere, that is not necessarily the case for all aluminum alloys. Some of them do not form this particular oxide layer well, and therefore are not protected against corrosion. On stainless steel, a corrosion mechanism known as “rouging” happens due to external contamination or destruction of the original passive layer. A passivation process before placing the part in service is helpful to increase the life of the component.
Powder Coating: In this process, powdered paint is applied electrostatically and then cured under heat or with ultraviolet light. The dry powdered paint is applied to a metal surface without the need of a solvent, allowing uniform thick coatings with increased durability. Powder coating has the disadvantage of changing dimensions, so make sure to control your tolerances and roughness values, especially on critical features. If you need electrical conductivity in your parts, this process is not a good choice, due to the conductivity properties of the coating. Steel, stainless steel, and aluminum can all be powder coated.
Tumbling: This finishing option consists of rotating parts in a barrel filled with an abrasive or non-abrasive medium. Tumbling can also be called barrel finishing). It’s a relatively cheap process, but sides and faces can be left uneven, so be sure to check your geometrical tolerances requirements. For 3D applications, this process can correct artifacts and visible defects.
Zinc Plating: There are two similar processes that unfortunately use the same name in industry, which might be confusing:
- Electro-galvanization, sometimes called zinc plating, is done by applying zinc using an electrical current. The advantages are that this process is cheaper and parts are easier to weld. The disadvantage is that the coating is less wear resistant, and therefore should not be used on parts that come in contact with others.
- Hot-dip Galvanization, also called zinc plating, consists of submerging the part in a molten zinc bath. This process creates a more resistant outer layer. In addition, galvanized steels have a unique characteristic: when the coating is damaged, exposing steel to the atmosphere, the underlying steel does not corrode first. Instead, zinc is corroded. This is due to the difference in electrical potential. If your application is meant to work in an aggressive environment, you should consider Hot-dip galvanization.
Hopefully this article helped you to understand surface finish, surface finishing, lay, roughness and waviness, as well as how to choose appropriate finishing options.
Consider evaluating your designs for surface finish and finishing specifications: will your components function in an environment likely to cause corrosion or another material destruction? We also recommend you read about ASME Y14.36M standard to understand how to specify surface finish in drawings with the appropriate set of symbols and descriptions.
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