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CNC Machine Shop Finishing Applications: Anodizing Machined Parts

[April 14, 2015] To the layperson, once the chips stop flying and another precision machine part comes out of a CNC mill or other high speed machining tool, they may think this is the end of the metalforming process. For almost all parts though, there is one last crucial step in the production process that has a big impact. At APM’s Colorado machine shop, nothing is delivered until final surface finishing is complete. The processes involved in CNC milling and other machining operations impart undesirable irregularities onto a part’s surface – that’s the nature of the work. It is often undetectable to the naked eye, but has the ability to affect the long-term functionality of the part and is aesthetically unpleasing. Therefore, as the last step in the CNC machining process, surface finishing applications are used to guarantee functionality, protection, a refined look, and other qualities depending on what the part will have to endure over its life. On the machine shop floor, CNC machinists employ a variety of surface finishing methods to produce the most accurate and best looking parts. These run the gamut and are as varied as the parts they are treating: Surface grinding, electroplating, powder coating, blackening, buff polishing, bead blasting and metal brushing are some commonly employed techniques to attain a desired surface. There is one particular finishing method not on this list that is unique in the milling and machining industry because it is such a versatile process lending itself to a multitude of varying applications. For those unfamiliar with anodizing, we’d like to provide a brief overview.

In a nutshell, the success and versatility of anodizing machine shop parts is due to combining the natural properties of aluminum alloy with time-tested techniques rooted in the science of chemistry. Anodizing, as stated, is mainly utilized on aluminum alloys, and imparts a finish that protects the metal by enhancing its naturally occurring surface oxide layer. It’s a conversion coating – not paint. In addition to a precision machine part that looks great, it results in an extremely hard and durable surface, lessening wear on the part and aiding in corrosion resistance. An anodized finish still remains porous however, allowing for the incorporation of color or other treatments into the end product. There are more benefits; an anodized part is relatively inexpensive to finish, it’s an environmentally safe process, and they are easy to maintain.

In simple terms, a precision manufacturing facility anodizes aluminum by immersing a part in an acid solution then applying electricity, producing an electrolyte solution. Anodizing is categorized into types based upon the specific acid used. Let’s take a quick look at the three most common applications.

Type I – Chromic Acid Anodizing

  • The use of chromic acid is the oldest method employed, but for decades has been declining due to concerns over the environmental impact of chromic acid. It use today is limited mostly to military applications.

Type II – Sulfuric Acid Anodizing (Conventional)

  • Performed with sulfuric acid at room temperature, Type II accounts for the majority of anodizing work and is based on specs used for medical, aerospace, automotive and defense applications. Thicknesses produced range from about .0001" to .0008" mm, and result in a coating that is resistant to stain, heat, corrosion, is easy to clean and provides good electrical insulation.

Type III – Hard-coat Anodizing

  • Hard-coating involves reducing the temperature of sulfuric acid to 32 oF. This allows for the anodizing process to build up into a heavier thickness, to almost .002 mm. This produces the benefit of extra durability and high wear resistance. Because of the chemical processes, hard-coats are rarely colored and reserved for parts that take a lot of abuse such as machine and automobile.

Because of the costs involved in supporting non-CNC machining operations, and some regulatory, safety and environmental concerns, many machine shops, including Advanced Precision Machining choose to outsource anodizing work.

Want to learn more, or have a question about our surface finishing options? Contact the expert CNC machinists at APM’s Colorado machine shop for all of your milling, machining and finishing needs.

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About the Author

Gerry Dillon is a co-founder, current owner and certified CNC machinist at Advanced Precision Machining (APM), a full-service machine shop located in Longmont, Colorado. Before making his home in the United States in 2000, Gerry was born and raised on the emerald isle of Ireland and took an interest in milling and machining from an early age, ranking #1 in the Irish National Apprenticeship Program. In 2005, he partnered with his friend and colleague, Kirk Tuesburg, currently APM’s machine shop manager, together launching what’s grown into a leading Colorado machine shop. Gerry brings over 30 years of machining experience to the shop floor, and is certified in all aspects of geometric dimensioning and tolerancing. 

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