Surface Finishing for Machining – Guide

When a material undergoes the process of machining, no matter what method it uses, it comes out with it with a certain roughness. Bottom line is the product you get right after machining cannot be the final product. The surface finish of the product needs to be worked on to make it more suitable for use. While there may be different methods for metal machining and plastic machining, we have put together the most common post-processing methods that’ll help in improving surface finishing of both kinds.

In this article, we will be studying Surface Finishing in detail, going over the different methods of improving Surface Finishing, the technicalities related to them and how they are being used in the industry. Keep reading!

Introduction

Surface Finish is basically used to refer to the surface of the machined product. There are certain parameters used for the precise calculation of Surface Finish. There are multiple processes used in the industries that modify or work on a surface of a manufactured item to attain certain properties. A post-processing step is usually the last step in the process of a product manufacturing timeline. It is done to give a product the final touches before passing it on for the next step.

Surface Finish is a measure of the complete texture of a surface which is mainly dependent on three characteristics:

  • Lay: It refers to the predominant or more frequent surface pattern. It depends on the production method that has been used to process the surface.
  • Surface Roughness: It is actually a measure of the fine space irregularities found on a surface.
  • Waviness: While Surface Roughness focuses on the finer irregularities, Waviness refers to the broader possibilities of irregularities.

The result of post-processing methods in a product is an improved appearance, corrosion resistance, wear resistance, controlled surface friction, fix other surface flaws and much more. Surface Finishing is a very important part of the manufacturing process. It is broadly divided into two categories depending on how it is affecting the product it is applied to:

  • Removing / Reshaping Finishing
  • Adding / Altering Finishing

Surface Finishing is further done by various methods that we will cover one by one in this article. Each of the methods has its advantages and disadvantages. We will be listing the suitable materials that can undergo the processes. Let us start!

Different Types of Surface Finishing

Bead Blasting

Source: Balotinovani

In this process, the machined workpiece is bombarded with tiny glass beads with the help of a pressurized air gun. This is done for removing some part of the material and smoothening the surface. While the work-piece undergoes this process, critical parts of the material (like holes) can be abstracted (by masking) to make sure there aren’t any dimensional changes.

Bead Blasting helps in removing the tool marks that are left due to the machining process while at the same time provides a uniform matte or satin surface finish. Bead Blasting is a post-processing method that is used mainly for visual purposes. There are certain parameters that will affect the extent of how Bead Blasting works:

  • Size of the glass beads. They can be anything ranging from coarse to very fine.
  • Size of the air pressure.

It is also very important to note that since Bead Blasting is a manual post-processing method, the result will depend on the skills of the operator.

Pros:-

  • Low-Cost
  • Uniform Finish

Cons:-

  • It can affect critical dimensions and surface roughness

Used for: This post-processing method can be used for any material. There are no restrictions as such. Some examples are:-

  • It is used for pre-treatment of Nickel and Chrome before galvanization.
  • It is used in the final surface finishing of Stainless Steel.
  • It is also used in Aluminum components before they are put through the process of anodic oxidation.
  • Brass, Bronze and Copper have a major need for Bead Blasting.

As Machined Finish

Machined Part

As Machined Component From Chizel’s Hub

As Machined Finish is a very simple tweak to the usual machining procedure. While operating on the work-piece, it is made sure that machining is done rather slowly. This is done to ensure that the roughness value of the surface is less and in turn, the surface is smooth.

The quality of the surface acquired at the end is measured in terms of a parameter called Average Surface Roughness (symbolized by Ra). It actually refers to the average deviation of the machined work-piece from an ideal surface.

The parts resulting from this process can further be smoothed or polished using other post-processing methods to improve their surface quality even more. Some material may be removed affecting the dimensional tolerances.

Pros:-

  • Tight dimensional tolerances
  • No additional costs required

Cons:-

  • The marks resulting from machining may not be completely removed.

Used for: Any plastic or metal material can be processed by tweaking the machining process in this way. It is a useful tweak that makes it easier to apply the other post-processing methods if required.

Anodizing

Anodized Parts at Chizel
Anodized Parts at Chizel

Anodizing refers to a process in which a thin metal oxide film is added on the surface of metal parts. This layer protects them from corrosion as well as other wear and tear that they may undergo. Also, this coating, which is referred to as Anodic Coating, is electrically non-conductive, has high hardness and can also, be dyed into different colours which gives them a nice aesthetically pleasing look.

We are going to talk about two important types of Anodizing in this section. These types are referred to as Anodizing Type II and Anodizing Type III methods.

The actual process is that the work-piece is submerged in a diluted solution of the sulfuric acid solution and an electric voltage is applied between the cathode and that work-piece. An electrochemical reaction takes place which consumes the material from the exposed surface of the work-piece and converts it into hard aluminium or titanium oxide. It is recommended to mask the critical parts of the work-piece to make sure that they do not get anodized.

There are three parameters involved in the above process: Electric Current, Anodizing Time and Consistency & Temperature of Solution. By varying these parameters in a different way, we can get coatings of different thickness and density. The Anodizing Type II and Type III are categorized by the categories of various thickness and density.

Anodizing Type II

It is also referred to as Standard Anodizing or Decorative Anodizing. The ceramic layer coating produced under this type is of thickness up to 25 µm. In this case, the coating thickness depends on the colour of the part. If it is dealing with clear or un-dyed parts, the thickness is around 4-8 µm and if it is dealing with parts that have been dyed black, the thickness will be around 8-12 µm.

Type II Anodizing is known for producing surfaces that have aesthetically pleasing looks, good corrosion resistance as well as limited wear resistance.

Anodizing Type III

It is referred to as Hardcoat Anodizing. The typical or default anodic coating, in this case, is 50 µm thick but it can be specified to produce coatings up to 125 µm thick.

The coatings produced in this method are thick, ceramic coatings having high density, excellent corrosion and wear resistance as well as support for functional applications. Anodizing Type III requires greater process control in comparison to Type II hence the costs involved are higher.

Pros:-

  • A coating is durable as well as aesthetically pleasing.
  • Easily applied to smaller parts including internal cavities
  • Good control on dimensions
  • High resistance to wear which helps in top-end engineering activities(Type III)

Cons:-

  • Compatibility with specific metals
  • Type III Anodizing is very expensive in comparison to other alternatives
  • Relatively brittle in comparison to powder coating

Used for: Anodizing is only compatible with Aluminum and Titanium alloys.

In Anodizing, the oxide layer penetrates 50% into the work-piece but at the same time, builds up 50%. While the percentage may be closer to 67% and 33% in case of Type II Anodizing, Type II Anodizing sticks to 50%-50%. The buildup can be reduced by removing a very minute (about few tenths) of aluminium before the anodic layer is applied and formed.

Powder Coating

Enclosures
Enclosures Made In-house

In this post-processing method, a thin layer of protective polymer is infused on the surface of the machined part. The coating produced is very strong, provides a wear resistant finish and can even be combined with bead blasting resulting in surfaces that are smooth, uniform and have tremendous corrosion resistance.

Powder Coating involves spraying a dry powder on the surface of the work-piece. The parts are first readied by a phosphating or a chromate coat. This increases their corrosion resistance and is followed by a coating of dry powder using an electrostatic spray. In the next step, the work-piece is cured in at a high temperature which is usually around 200 degree Celsius.

The typical thickness of the layer formed by powder coating varies from 18 µm up to 72 µm. Apart from that, the use of a wide range of colours is allowed in powder coating.

Pros:-

  • Impact resistance greater than other alternatives
  • A wide range of colours available
  • High wear and corrosion coating which helps in supporting functional applications

Cons:-

  • Difficult to apply to internal parts
  • Not supportive of smaller components
  • Dimensional control is lesser than Anodizing

Used for: It is compatible with all metal materials. The only criterion that needs to be fulfilled by the metal is that it can survive the thermal curing process.

  • Over the recent few years, the use of Powder Coating for aluminium extrusion has experienced an exponential rise.
  • It is used in tractor vehicles as a method for corrosion protection, high-temperature performance as well as outdoor durability.

Conclusion

In this article, we have clearly established that surface finishing is needed to be kept in mind and taken the utmost care of as it plays a vital role in the final presentation of the product. We have learned about the most important methods used for improving the Surface Finish of machined products. It is a vast field that must be studied properly. Research should be performed to find the method most suitable for your product and execution shall be carried forward.

We hope you’ve taken away ample knowledge about post-processing and surface finishing methods from this article. Please do subscribe for more.

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