CNC Machining has given the world’s creators - scientists, inventors, and engineers – the opportunity to achieve factory-grade results in their manufacturing process. CNC machines automate the conversion of metal, plastic, wood, ceramic, and other materials into highly useful modern components.
The results are highly precise, and each part is identical to the next. But there is one more step needed to make a newly machined part truly impeccable — surface treatment.
What is CNC Surface Treatment?
CNC machined parts come embedded with plenty of battle scars. Scratches and other minor defects will usually be present depending on the material type and the CNC cutting, drilling, milling, and CNC turning tool employed. Its surface must also undergo post-processing treatment to get the desired finish.
Look around you, and you’ll see objects composed of parts with a variety of finishes, such as smooth, glossy, or matte. Surface treatment alters the surface of the CNC machined part to enhance its aesthetics, durability, and usability.
And in engineering, the surface finish can have a significant impact on a part’s performance. It also impacts the function of the overall product, especially if its function affects another part’s function. There are several surface treatments available, and the one you choose will depend on the properties of the machined component you want to maximize. For example, a mirror-glass finish is pleasing to the eye and has a lower friction coefficient, but a textured finish is preferable for a better grip. A machined part with a rough surface is more prone to breakage and corrosion, but they are a better candidate for increased adhesion.
Here are the fundamental reasons why CNC surface treatment is a necessary post-processing step:
- Removes surface defects
- Enhances surface adhesion to coatings such as paint
- Increases resistance to corrosion and chemicals
- Improves electrical conductivity
- Boosts strength, increases wear resistance, and keeps friction effects to the minimum
What Are The Major CNC Machining Surface Treatments?
There are three major surface finishing processes you can apply to a CNC machined part - mechanical, chemical, and electrochemical surface treatments. Each of these treatments generally follows two principles:
- Adding a layer or altering the existing surface- through electroless plating, electroplating, painting, passivation, and pickling
- Removing a layer or reshaping the existing surface- through shot blasting, electropolishing, buffing, and lapping
Mechanical surface treatment
Mechanical surface treatment mechanically alters the surface properties of a workpiece, modifying it to become smoother, flatter, or thicker. The surface is manipulated to achieve a particular property, including increased adhesion, wear, and corrosion resistance improved electrical conductivity, deburring, and more.
Here are the most common mechanical surface finishing techniques:
- Buffing: In buffing, a loose abrasive is mixed with binders and formulated into a paste or liquid. This compound is then smeared onto a revolving buffing wheel or belt, which is then used to buff a piece to smoothen medium to deep nicks, lines, and scratches. The abrasive used may be low-grit, medium-grit, or high-grit. Buffing gives a smooth and mirror-like surface finish.
- Shot Blasting: This process involves forcibly bombarding abrasive materials, such as steel beads or glass beads onto CNC machined parts to remove dust, dirt, or other contaminants from its surface. It is also used to shape or smoothen a surface. This technique uses a device that’s similar to a spinning wheel to produce centrifugal force to “throw” instead of “blow” abrasive material upon the part. The type of abrasive material used will vary depending on how defective the part is and the amount of high pressure it can withstand.
- Tumbling: This process is used to soften shaft metal peaks on a CNC machined part. In tumbling, workpieces are placed with abrasive materials in a barrel, called a ‘sifter,’ rotating at a preset speed. The rotation causes the parts to collide among themselves, against the abrasive media and the compound, causing the rough edges and other defects to smoothen. The speed of the rotation, type of abrasive material, among other variables, dictates the results.
- Lapping: This technique involves using loose abrasive powders to grind a CNC machined part at a very low speed. Lapping is used to achieving a particular surface roughness, thickness, or flatness. Usually, lapped parts exhibit a dull, matte finish. A slight reflectivity may be seen when the CNC machined part is lapped with micron-size aluminum oxide.
Chemical surface treatment
Chemical surface treatment modifies metal surfaces with the use of chemicals. It’s often used to pre-treat workpieces for another surface treatment such as powder coating or electrochemical surface treatments. Here are the common types of chemical surface treatment:
- Pickling- Pickling is a metal surface treatment that removes superficial stains, scale (oxide layer), and inorganic impurities from newly CNC machined metal parts. The process is called pickling because it calls for the use of an acidic solution called ‘pickle liquor.’
How pickling works: Pickling works by submerging a metal part into a pickle solution, whose exact composition varies depending on the type of metal. For example, parts made from high-carbon steel are pickled in solutions containing phosphoric and nitric acids, while pickle liquor for low-carbon steel incorporates sulfuric or hydrochloric acid.
Once submerged, the acid solution eliminates any oxide (scale) or impurities present on the metal’s surface, which, if left alone, can impact its functionality. This process can result in a slight dull in the visual brightness of the metal. Following pickling, the metal usually undergoes an oiling process to protect against oxidation.
- Passivation- Passivation is used to increase corrosion resistance in parts made of stainless steel, aluminum, silicon, nickel, and ferrous materials. It’s usually the final treatment process where free iron is removed from the surface, thereby decreasing the treated surface’s tendency to rust. Where pickling can work on sub-levels, passivation works exclusively on the surface level.
How passivation works: The component is first cleaned to remove contaminants from the surface that may have been introduced during CNC machining. Then the part is made to undergo a passivating bath.
In this, the part sits immersed in a Citric acid or Nitric acid-based solution or paste. The bath removes iron particles from the metal’s surface and forms a thin protective inert film — a passive oxide layer — over the component.
Electrochemical surface treatment
Electrochemical surface treatment or electrolytic polishing treatment is a finishing process that uses a combination of electricity and chemicals to polish the surface of metal parts. It works on nearly any metal, including stainless steel, aluminum, brass, copper, nickel alloys, carbon steels, cobalt chrome, and copper. Electrochemical polishing dissolves a thin layer from a metal’s surface, thereby enhancing its smoothness, reflectivity, passivity, and purity. This process can reduce roughness up to 50% and make parts 30 times more corrosion resistant than passivity.
While mechanical surface treatment techniques are macro-metal removal processes, electropolishing is a micro process and can work in tandem with mechanical methods such as grinding, sanding, and buffing.
Electrochemical polishing results in microscopic smoothness, deburring, and a hyper-clean surface that’s easy to sterilize. It also removes surface defects, eliminates friction and discoloration from oxidation or heat tint.
How Electrochemical surface treatment works
Electropolishing removes a microscopically precise layer from a part’s surface. Its freedom of precision, coupled with its high ability to remove a variety of imperfections, makes it usable to polish critical components that are fragile or have intricate geometric patterns. It’s an aggressive alternative to passivation and removes the outer layer of the metal part and all its contaminants.
Here’s how the process works — the metal part that needs to be treated acts as an anode. It is connected to the positive terminal, while a negatively charged cathode, typically made from stainless steel or zirconium, is connected to the power supply’s negative terminal. The component is immersed in a container full of sulfuric or phosphorous acid connected to a power supply.
Electric current is passed from the anode to the cathode with the electrolyte solution. The current triggers ions on the metal’s surface to oxidize and dissolve into the solution. This process allows for micron-level precision in how much metal surface is dissolved.
Burrs, peaks, and high points attract the current faster than micro-depressions or valleys, enabling quick smoothening of irregular surfaces.
The amount of material removed is directly proportional to the length of time the part is exposed to the current, electrolyte temperature, electrolyte chemical composition, the density of the current, and the metal part’s composition.
Modern surface treatment
Some of the most attractive modern surface treatments used to manipulate CNC machined surfaces include:
- Vapor deposition techniques: Chemical and Physical Vapor deposition are used to coat surfaces with a thin film. For example, if you want a layer of silicon on your component, trichlorosilane will be injected into a heated vacuum where it will breakdown into silicon, chloride, and hydrochloric acid. The silicon will deposit onto the part’s exposed surface, and the other two constituents will be vented out of the chamber.
CVD and PVD produce surfaces with superior hardness, smoothness, oxidation, and corrosion resistance. They are also used to process semiconductors or nonconductive metals.
- Ion implantation: In this surface treatment process, particular ions (usually nitrogen or carbon) are made to penetrate the surface of a component, into its matrix, to make it wear, fatigue, and corrosion-resistant. This treatment can take between 2 to 10 hours. It preserves the surface finish and mechanical properties of the component, and there is no risk of delamination (since it’s not a coating) or scaling. It can be used on metals, elastomers, and polymers.
What Are The Most Common Surface Treatments For Metal?
The following techniques are often used alone or in a group to treat metal components to maximize their aesthetics and functionality.
Anodizing is an electrochemical process that is usually applied to aluminum but can also work on other nonferrous metals like magnesium and titanium. Pure aluminum usually has strong corrosion-resistant properties because of its oxide layer. But, it’s often combined with other alloys that may not have strong anti-corrosive properties. Anodizing can help form a thick aluminum oxide layer to guard against rust, which is hugely beneficial if the part will be exposed to marine environments.
Here’s how anodizing works — aluminum is immersed in an electrochemical bath. Aluminum acts as the anode, while a cathode is connected to the inside of the anodizing tank. When a current is passed, the charged bath triggers the release of oxygen ions from the electrolyte, which reacts with aluminum atoms present on the part’s surface. Anodized oxidation is essentially a very controlled method of a naturally occurring phenomenon- oxidation. This produces parts that are durable, easy to maintain, glossy, and visually striking due to their uniform color.
Polishing is used to produce a reflective surface. A micron or a sub-micron abrasive material combined with liquid is used for this process, making it a “wet” technique.
The abrasive material is loaded onto a pad that’s softer than the working piece. Since polishing removes very little material, it is usually done on surfaces that are already of high quality through another treatment like lapping. Polishing cosmetically enhances the surface and also seals high-pressure liquids and gases. Additionally, it can help create uniformity across surfaces, and sharpen the edges formed by cutting tools.
Vacuum plating involves chemical vacuum deposition or physical vacuum deposition that this article touched on above. Both processes enable you to create a thin surface on top of your machined part by heating the coating metal to its boiling point inside a vacuum chamber until it vaporizes.
The vapors then condense into a coating that sits adhered to your CNC machined part’s surface. The coating can range from a single atom to one that is several millimeters thick.
Electroplating uses electricity to coat metal, such as copper, with a layer of another metal, such as gold. It can make inexpensive metals look expensive and also make them durable and rust-resistant. Two terminals, known as electrodes (cathode and anode), are placed in an electrolyte solution and connected to a power supply. Both the electrodes and electrolytes are carefully selected elements, and when together, form a circuit. The part that needs to be plated is the cathode, and the anode is made of the metal that will be dissolved and be plated onto another.
When the power is turned on, electricity flows through the circuit, causing the electrolyte to release some metal atoms which get deposited onto the anode. All conductive metals like gold, zinc, copper, silver, lead, nickel, and platinum can be electroplated.
Also known as chemical or autocatalytic plating, electroplating involves binding a superior metal coating over an inferior metal surface solely using chemical reaction rather than electrical energy. The metal part that needs to be plated is placed in a bath containing the metal being applied. This gets catalyzed by specific materials and is deposited onto the part’s surface.
Since it doesn’t use electric current, electroless plating works on both non-conductive and conductive objects. It produces an even layer of coating even on a complex surface.
Some metals are soft and pliable, while others are hard and strong. Surface hardening is a technique that enables you to change the mechanical properties of a metal. Also known as case hardening, surface hardening is a heating treatment that reinforces the vulnerable outer surface of metal without changing its softcore.
This technique produces a thin layer of metal on the surface that’s significantly harder than the metal underneath. Metals that undergo surface hardening include low carbon steel, high carbon steel, stainless steel, cast iron, and tool steels.
Conventional surface hardening processes include nitridation, oxidation, bronzing, ion implantation, or coating the surface with wear-resistant materials like chromium.
In metal painting, paint is applied over a treated surface using various techniques, such as:
- Powder coating- dry polymer powder is applied electrostatically on the part’s surface, which is then baked using heat or UV light. This is tougher than conventional paint.
- Wet painting- the traditional method of applying liquid paint onto the surface of a metal. A spray pump or a pressurized vessel is used to apply paint for an even coating.
Need Help Choosing The Right Surface Finish? Call Us Today
Different materials need different surface finishes. RallyPrecision is the industry leader in high-quality CNC metal surface finishing treatment services. We understand that you want your parts to be as immaculate as you intended them to be. We can help you reach your goals.
Our dedicated team of engineers and operators is standing by to help you discover the best finishing services based on your unique requirements. Contact us now, and let’s get going on your next CNC machining surface treatment project.
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