Surface finishing is the final step of CNC machining. Finishing can be used to remove aesthetic flaws, improve a product’s appearance, provide additional strength and resistance, adjust electrical conductivity, and much more.
With all of the surface finishing options available, how can product managers and designers make sure they’re choosing the best one? Luckily, there are a few common finishes that offer unique advantages, and it’s just a matter of understanding the specifications of each option.
Common finishes for CNC machined parts
CNC machining produces a part with an “as-machined” or “as-milled” finish as soon as the manufacturing process is completed. The part will have small but visible tool marks and blemishes. The average surface roughness is around 3.2 μm. As-machined parts have the tightest dimensional tolerances and are extremely affordable to produce because post-processing isn’t necessary.
This finish is a good choice for those who are more concerned with dimensional integrity than aesthetics. However, parts with as-machined finishes don’t rank very highly when it comes to protection. Their roughness and lack of protective coating renders them susceptible to nicking, scuffing, and scratching.
Anodizing is an electrochemical process that thickens a CNC machine part’s natural oxide layer to make it thicker, denser, electrically non-conductive, and more durable.
This process can only be done using aluminum or titanium alloys because they conduct electricity well. During anodizing, the alloy is submerged in an acid electrolyte bath and acts as an anode. Once a cathode is placed in the anodizing tank and an electrical current passes through the acid, oxygen ions from the electrolyte and atoms from the alloy combine at the surface of the part.
Anodizing comes in two different varieties — Type II and Type III. The overall finishing process is the same, but Types II and III require that the part be submerged in a diluted sulfuric acid solution.
Anodizing Type II, also known as “decorative anodizing” (as the finished coating can be clear or colored), produces coatings up to 25 μm thick. The coating thickness range for clear parts is 4-8 μm and 8-12 μm for parts that have been dyed black. This process produces a part that is smooth, elegant, and resistant to corrosion and wear.
Anodizing Type III, also known as “hardcoat anodizing,” can produce anodic coatings up to 125 μm thick. Parts with this coating have high density and are even more wear-resistant than anodizing Type II.
All in all, anodized finishes are durable and promise good dimensional control. Anodized finishes are best used in high-performance engineering applications, particularly for internal cavities and small parts. They are among the most aesthetically pleasing finishes for CNC machined parts, but often come at a higher price tag.
Powder coating is a lot like spray painting. First, the part is primed with a phosphating or chromating coat to make it more resistant to corrosion. Then, the part is “painted” with a dry powder coating from an electrostatic spray gun and cured in an oven heated to at least 200°C. Multiple layers can be applied to increase thickness, which can reach 72 μm.
On its own, this finish creates a thin protective layer on the CNC machined part that is strong, wear-resistant, and aesthetically pleasing. This process can be combined with bead blasting to increase the part’s corrosion resistance and create greater uniformity in texture and appearance.
Unlike anodizing, a powder-coated finish is compatible with all metals, less brittle, and offers greater impact resistance. This finish is suitable for many functional applications but may be particularly well-suited for military applications.
However, powder coating generally yields less dimensional control than an anodic finish, and powder coating is not recommended for use in small components or internal surfaces. What’s more, powder coating’s higher price point might make it prohibitively expensive for larger production runs.
Bead blasting is used to add a matte or satin surface finish to a CNC machined part. During this process, a pressurized air gun shoots millions of glass beads at the part, effectively removing tool marks and imperfections, creating a consistent grainy finish. In contrast to other finishes, including anodizing and powder coating, bead blasting adds no chemical or mechanical properties to the part — it’s purely visual. Unlike powder coating, which adds material to a part, bead blasting is a reductive finish, meaning it removes material from the part. This is an important consideration if your part has strict tolerances.
Bead blasting is one of the most affordable surface finishes, but it must be executed manually. As such, those considering bead blasting as a surface finish should be prepared to incur the cost of engaging an operator who has been formally trained in this process, and recognize that the final result will largely depend on how skilled the operator is. Bead size and grade will also affect the final finish.
Gain expert advice on finishing your CNC machined parts
In short, a post-processing finish such as anodizing or powder coating will likely prove an effective option for parts that don’t need to be picture-perfect but must maintain their original dimensions. Protecting or reinforcing an aluminum or titanium part may benefit from anodizing. If the part cannot be anodized but requires strength and impact resistance, powder coating provides an effective alternative. Finally, if cost-effectiveness is a higher priority than tolerance — and the part does not require a glossy finish — bead blasting may be the preferable route.
Engineers, designers, and product managers would do well to consult a manufacturing expert to ensure they are making the right choice for their next project. The experts at Fast Radius are well-versed in all things manufacturing, from product design and prototyping to manufacturing at scale. Our team can help you choose which finish is best for your CNC machined part and make sure you go to market with a strong, elegant finished product.
Contact us today — we’d love to talk about your vision. If you’re ready to learn more about CNC machining and other manufacturing processes, take a look at the Fast Radius resource center.