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A Complete Guide to Surface Finish of CNC Machined Parts

A Complete Guide to Surface Finish of CNC Machined Parts

Table of Content

Key Takeaways

  • CNC – stands for Computer Numerical Control.
  • Surface finishing is used to elevate the aesthetic looks and performance of the machine parts.
  • Helps to get rid of surface roughness like rust, corrosion, visual properties like enhanced aesthetic looks, etc. along with additional quality.
  • Surface finishing also helps in premature wear and tear of the machine parts.


With technological advancements, machines have become a part and parcel of life. But most often it becomes difficult to manage a machine and its parts in the long run. Since most machines and their parts are made with metal, there are high chances of occurring various problems like rust, corrosion, etc. Fortunately, there are many remedies in the market to prevent machine parts from such problems. One of the most popular solutions for machine parts is “surface finishing”.

Today, we will walk through the “step by step surface finish of CNC machined parts”. Let’s get started!

Surface Finish – This A.K.A “surface texture” and “surface topography”. In simple terms, it is the nature of a surface. This involves small local deviations of a surface from the flat ideal.

CNC Milling – Computer numerical control (CNC) is program code. This represents instructions for precise movements to be carried out by machines. In other words, this code defines how to automatically create, produce, or transform a virtual object into a real one. CNC Milling Machines are very useful for both commercial and industrial production.

Machine Parts – In general, machine parts are described as machine elements or hardware that refer to an elementary component of a machine.

Let’s move ahead and glance at other vital topics.

Standard Surface Finishes

In general, a surface finish is described as a measure of the texture of a surface. It is normally identified by the layer of the surface pattern, roughness and waviness.

Standard surface finish includes the features that are frequently used about the machining finishes using different production methods.

Surface Finish Units

  • Roughness Average (Ra)
  • Root Mean Square Roughness (RMS)
  • Maximum Roughness Depth (Rmax)
  • The Mean Roughness (Rz)

Techniques For Finishing A Metal Surface

  • Sanding
  • Ultrasonic Polishing
  • Rumbling And Tumbling
  • Magnetic Polishing
  • Sandblasting
  • Lapping
  • Filing

Need for Surface Finish

Surface finishes are not only for aesthetic purposes. Instead, a surface finish serves so many additional important purposes.

Corrosion resistance – Corrosion is a very serious problem for metal. In addition, rust is also a problem. It reduces the quality of such components, hence it affects the performance and functionality. In this case, surface finishing ensures adequate protection and long-lasting performance.

Enhances the aesthetics – The aesthetics play a very important role because appearance is nowadays very crucial to improve the performance of the product. With the surface finishing technique, CNC machined parts look attractive.

Easy manufacturing process – A machining surface finish makes manufacturing effortless. For example, sandpaper sticks better with paints. This process assists in relieving manufacturing stress.

Do’s & Don’ts for Surface Finish

There are several things we need to keep in mind to get perfect CNC machined parts. For instance correct dimensions and tolerances, shape, quality of the raw materials used, etc. But this needs to be worked upon after the machined parts are produced.

List of Do’s for Surface Finish

Measurement of Surface Finish – The first step is surface finish measurement and it includes profile analysis, area and microscopic inspection, focusing on the roughness peak (Ra) and its separation (D). But it is important to choose the right technology that is most suitable to achieve the desired results.

Increase Speed & Reduce Feed – This step is crucial especially when processing expensive parts. Surface finish helps to ensure following the predefined, as well as correct feed & speed. Increasing speed & reducing feed includes the correct way of handling the entire finishing process to fasten the surface feet per minute (SFM) and decrease inches per revolution (IPR) thus, extends tool life and reduces the chance of catastrophic tool failures damaging finished parts.

Use of Chip Breakers – One of the most important steps in controlling chips for a good surface finish. Since the generated chips hinder the entire machining process to a large extent, it is highly recommended to use a high-quality chip breaker for great results.

Increase The Nose Radius – Since there is a direct relationship between the blade tip radius, it results in a good surface finish. As a result, a smaller nose radius reduces the pressure on the tool. Then, the blade feeds at half of the nose radius. In this case, a larger nose radius performs heavier cuts.

Reduce Vibration With Balanced Tools – Using balanced tool technology reduces the apparent vibration during finishing.

Sharp Blade, Leading Angle And Positive Angle – The need for sharper blades, larger lead angles and positive rake angles is very important to get desired surface finishing.

Tool Holder And Work Holder – Most underrated factor while the surface finishing process is the tool holder. For example, if the knife holder is old and the groove for holding the blade is damaged, the blade will move, hence the movement of the blade will cause chattering and poor surface finish. Thus, always consider it a good tool and work holder.

List of Don’ts for Surface Finish

Do Not Use The Same Tool – During the process of surface finishing, the tools and parts may become rough with blades as well as a large nose radius. In this scenario, a finishing tool with the right lead angle and radius wiper flatness. This flattens the required part of the machine, hence resulting in a great surface finish. On the contrary, the blade will push the material instead of cutting. It will lead to poor surface quality, burr edges and shortened blade life.

Avoid Pauses – For a smooth surface finish, unnecessary pauses hinder the correct finishing process. In case this happens consistently, ensure that the tool does not stop throughout the cutting process.

Avoid Lowering The Centerline – One of the best ways to correct the cutting process is to follow a 70:30 ratio instead of a 50:50 method. Because cutting the middle will tap the insert along the edge of the material and cause burns and result in an inappropriate surface finish.

Role of Surface Finish for CNC Machined Parts

Rust Protection – Most CNC machine parts and components are made with metal when the equipment is in the open air, it is very easy to get corroded. Therefore, the whole machine needs anti-rust protection. E.g. Electroplating, coating, and other surface finish technology are used worldwide for rust protection for metal parts.

Exterior Decoration – With the evolution of CNC machine technology, the requirements of users for products and product appearance have changed. Nowadays, people often look for attractive products, the market competition has increased. The appearance depends highly upon the design. This means aesthetics lies in the application of advanced surface finishing technology.

Surface Strengthening – CNC machines are known for a complex structure because some CNC parts require stiffness but some require hardness and wear resistance on the surface. In this case, the surface heat treatment is the most popular and commonly used technique. In addition, shot blasting is also used for surface strengthening of CNC machine parts.

Part Repair – Surface finishing technology isn’t only popular for the CNC manufacturing process but also repair and damage of CNC parts along with re-manufacture. The process of using construction machinery extends the service life of CNC machine parts and improves the utilization value and reduces resource consumption.

Factors Affecting Surface Finish of CNC Machine Parts

CNC finishing is the most important process or method that is essentially used to enhance the look, feels and function of a metal. But it is necessary to choose the best finishing technique for fruitful outcomes.

The Product’s Application – In a CNC machine, every part is subjected to different environmental conditions such as vibrations, heat, moisture, UV light, etc. It is important to carefully make the right choice. Because it is often intended for outdoor use and increased exposure requires a standard surface finish to improve the hardness. The product application for surface finish should also increase the wear and tear resistance.

Durability Concerns – Durability must be a very necessary consideration such as how long you want your product to last. In this process, even the raw material plays a crucial role. This means adding value to the finished product involves durability as well as the finishing.

Raw Material and Metal Surface Finish – There are so many materials that are very hard to polish. So this may be extremely challenging to anodize some other surfaces because the paint might not stick well to certain materials. Therefore, CNC finishing is a must-to-follow step.

The Part’s Dimensions – The machining surface finish may change a part’s dimensions. For example – a thick finish such as powder coating increases the surface thickness of the metal material.

Cost – Never forget to consider the budget! The best surface finish methods often need careful preparations, complex procedures & higher-quality materials. This means always checking and comparing the prices and then making a decision.

Types of Surface Finishing for CNC Machined Parts

Milled – This CNC machining surface finish technique does not require any type of post-processing. It indicates that the part will remain in its machined state as CNC machining. Thus, it can manufacture parts with tight tolerances of up to ± 0.025 mm. In addition, the milled components cater to the best dimensional precision. Some common materials that use the milled technique are Aluminum, Brass, Magnesium, Nickel, Steel, Thermoset plastics, Titanium, and Zinc.

Bead Blasting – Bead blasting is a kind of shot blasting. This process discharges fine glass beads to clean or finish a surface at very high pressure. So this process consists of shooting small spherical media at the surface of a CNC machined part using compressed air. The bead blasting technique aims to leave a smooth, shiny, and cleaner surface. The final result of bead blasting is a smooth texture with either a matte or satin finish. In general, this technique is most commonly used for materials like metals, plastic, glass, and rubber to create an excellent surface finish.

Powder Coating – Powder coating is a dry coating method used for metal finishing. This is generally used for industrial equipment. The powder coating process involves applying a thin layer of dry polymer powder to the machined part. A surface finish method adds strength and wear-resistance to the CNC machined parts. The powder coating methodology is very popular especially, for delivering high-quality finishes for functionality, as well as outlook. The powder coating finish is not only flexible but also a solid solution. Thus, it is an ideal treatment for both indoor and outdoor applications. In addition, it is used for various materials such as metal, concrete, steel, and plastic because it is one of the most cost-effective finish options in the market.

Anodizing (Type II and III) – Anodizing is a very popular electrochemical technique to enhance the outlook of the metal surface into a decorative, durable, corrosion-resistant, anodic oxide finish. This technique involves adding a very thin ceramic layer to the surface of machined parts. Anodizing technique helps in corrosion and wear-resistant. It is one of the most environmentally friendly processes because it does not produce any hazardous waste. The anodizing process involves the usage of an aluminum object as the anode and a carbon electrode as the opposing pole to perform electrolysis submerged in electrolyte baths. When the acid solution is electrolysis processed during anodizing, the anode emits oxygen. But this doesn’t happen in the case of aluminum anodes. Then, oxygen from the aluminum anode instantly binds before gasifying with dissolved surface layer metal and forms an aluminum oxide. Aluminum and other nonferrous metals can be anodized such as magnesium and titanium.

Passivation – This is a chemical treatment for stainless steel and other alloys that improves the proficiency of the treated surfaces to oppose corrosion. In stainless steel, the passivation method uses nitric acid or citric acid to remove iron. This chemical treatment creates a protective oxide layer that does not chemically react with air to cause corrosion. So it helps stainless steel resist rust or corrosion. Passivation helps to remove surface contamination and boosts system maintenance. The common materials that are passivated include aluminum, zinc, cadmium, copper, silver, magnesium, and tin alloys.

There are various types of passivation –

  • Tank Immersion
  • Application of Spray
  • Circulation
  • Application of Gel

Top Surface Finish Technologies

  • Surface Spray Technology
  • Chemical Processing Technology
  • Surface Strengthening Technology
  • Surface Shot Blasting Technology
  • Surface Heat Treatment Technique

Process of Surface Finish

In general, there are 3 major steps to be followed for excellent surface finishing for CNC machined parts.

  • Material Selection
  • Pre-Finish Preparation Steps
  • Selection of the best Surface Finishing Option

Material Selection – First of all, it is necessary to select the right material for surface finishing. While selecting material, you need to analyze the properties required for the product based on the use.

Pre-Finish Preparation Steps – Next comes, pre-finishing preparation. In this step, choose the right type of product requirement and design. Also, access the material quality such as temperature, weight, strength, environmental conditions, etc.

Selecting the best Surface Finishing Option – Since there are many techniques for surface finishing, it is important to pick the best and right to get great outcomes.

Surface Finish Characteristics

Surface Roughness – This referred to finely spaced surface irregularities.

Surface Waviness – This is also one of the Surface irregularities with spaced irregularities.

Lay – Lay is simply referred to as the pattern directions that are produced from the different operations that take place on the surface such as turning, Planning, Grinding, Milling, etc.

Final Thought

The surface finish of CNC machined parts caters to an end number of advantages to the products. But most often people as well as big businesses face a dilemma when it comes to choosing the right technique for surface finishing. There are many reasons to get confused such as the endless number of solutions available in the market, etc. In such a scenario, refer to this article or consult with industry experts.

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