9 Common Surface Finishes In Metal CNC Machining


Each CNC machined metal parts are expected to last a long time and meet specific requirements. These requirements can range from a variety of resistance to aesthetic needs. The surface of the finished part may have scratches and characteristic patterns. These scratches may be caused by the type and sharpness of the cutting tool or the material used for the product. Metal finishing is a method of changing the surface of a machined metal part. It is the final step in most metal fabrication processes.

Metal finishing helps improve the aesthetics of the final part and provides environmental protection. In addition, the process helps to reduce surface roughness that cannot be handled by the machining operations of a particular part. In this guide, we will review some of the major surface finish methods used in metal fabrication. This guide also provides applications and factors to consider when selecting a finishing process. Let’s get right to it!

What are some common metal surface finishes?
There are a variety of metal finishing processes that can be used for different purposes. The following are some of the most commonly used methods:

  1. Polishing

Polishing involves modifying the surface of a custom CNC part using polishing tools and abrasive materials. Polishing is a very efficient process and you can easily increase its efficiency.

For example, different types of polishing, such as glass, will be more efficient when you use a mirror shine or polishing wheel. These materials will improve the size and accuracy of the finished part.

  1. Electroplating
    This process is also known as metal plating. It involves the use of electricity to improve the quality of the metal surface. An electric current is passed through a liquid bath. The bath contains ions of the added metal and the treated material (substrate). The substrate acts as a cathode to attract the ions in the bath. In this way, you will achieve the desired surface finish.

Electroplating has countless benefits. It increases durability, improves corrosion resistance, reduces surface friction, and provides better aesthetics. The process is suitable for a wide variety of metals. However, copper, gold, nickel and silver are the most commonly used.

  1. Anodic Oxidation
    Anodizing is a chemical oxidation process. In this surface treatment method, a thin metal oxide is added to the surface of the metal part. This metallic oxide layer protects the metal material from corrosion and other forms of wear and tear.
    There are two main types of anodizing available for custom CNC parts. The first is Anodizing Type II, which is the standard anodizing. It produces a coating that is uniform and corrosion resistant. It can be dyed in different colors for the part, hence the name “decorative anodizing”.

The second type is anodizing type III, often called hard coat anodizing. This type of coating is usually a thick ceramic coating. They provide high density and excellent wear and corrosion resistance to metal parts.

  1. Passivation
    Passivation is a chemical treatment used on stainless steel materials and other alloys. The process enhances the corrosion resistance of the treated metal parts. The chemical properties of any passivated layer on a metal surface can be enhanced using this method. There are different methods of passivation. They include Tank immersion – helps to create a uniform finish on the fabricated surface and provides corrosion resistance benefits.
    Recirculation – involves circulating a chemical solution through a piping system.
    Spray application – This is ideal for on-site treatment of metallic materials. High velocity oxygen fuels spray semi-molten droplets of coating material to produce a coated surface with optimum quality.
    Gel Application – This is a manual treatment that involves brushing a gel onto a metal surface. It is ideal for treating welds and some other complex areas of spotting.
  2. Lapping
    The grinding process involves the use of a lapping machine to apply a precise surface finish to metal parts. The lapping machine is equipped with two plates. These plates roll in different directions at the same time. Between the plates there is an abrasive liquid that gives a very precise mirror-like appearance to the workpiece that enters between the plates. The first lap results in an exact finish on one side. However, depending on the material and application of the component, the surface finish can be double sided.
  1. Black Coating
    This method of metal finishing uses a method called chemical transformation to coat parts. It is suitable for steel, aluminum, stainless steel, brass, copper and several other materials. If you don’t like the plating process, this is the method for you. In this method, salt is contained in a black oxide solution that acts as an oxidizing agent. This solution reacts with the iron present in the metal material and forms a coating on its surface. It has the advantages of reducing friction and corrosion resistance.
  2. Powder Coating
    It involves injecting a thin, protected polymer layer onto the surface of a machined metal part. In addition, a dry powder is sprayed on the surface of the component.

The first thing to do is to prepare the part with a chromate coating or using pacification. Then, electrostatic spraying helps to coat the material with the dry powder. A variety of colors are available. This method is very robust and provides a corrosion and wear resistant surface finish.

  1. Brushing
    Designers recommend brush coating to obtain the desired metal surface pattern. Brushing creates different types of line patterns on the surface. It may be straight, swirling or chaotic. Most manufacturers use brush coating on machined metal parts after plating. Brushing offers more aesthetic advantages than protection.
  2. Parkerized Phosphate Surface Treatment:
    Parkerizing is another method that helps improve surface roughness, wear and corrosion resistance, and prevent defects. This method is best suited for ferrous metals, including steel and steel alloys, cast iron and cooked iron. It provides additional protection against corrosion for these materials. However, it is not applicable to non-ferrous metals such as aluminum, brass, copper and plastic products. Many manufacturers mark it as one of the best methods for firearms and automotive parts.
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