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Surface Finishing

This article aims to provide an overview of surface finishing options for CNC machined metal parts and help you select the right finishes for your applications.

Table of Contents

Overview   |   Finish vs. Finishing   |   Methods   |   FAQs

Overview

The term “surface finish” is sometimes used interchangeably to talk about the literal surface finish of a part, and the finishing method applied to a part once it’s come off the machine.

Surface finish and surface finishing methods are different things, and equally important considerations in the manufacturing process.

CNC surface finish and finishing methods are crucial when your part interacts with other components. For instance, ball bearings are designed to minimize rotational friction and support loads. As the races rotate, the balls also turn due to their contact. If the surfaces of the balls or races are not finished correctly, friction increases, leading to more wear and a shorter lifespan, even if the components meet geometric tolerances.

Surface Finish vs. Surface Finishing

While surface finish refers to the characteristics that are produced by the manufacturing process, surface finishing is a term used to encompass the various post-machining processes that can be applied to a component that protect it and improve its appearance.

Let’s dig into surface finish a bit more.

Not to be confused with GD&T terms like flatness and profile, surface finish is concerned with the surface inconsistencies that happen at the micro-level rather than the dimensional.

A machined component's surface finish (and the finishing methods applied to it, if any, after the machining operation) are crucial if the component is part of an assembly or in contact with other components.

Consider a ball bearing; it’s designed to reduce rotational friction and support radial and axial loads. But if the surface of the race or the balls has inconsistent or poor surface finish characteristics, you’ll generate more wear due to increased friction and ultimately reduce the part’s lifespan, even if the part is within tolerance.

Let’s briefly review the three characteristics of surface finish.

Roughness measures surface irregularities as deviations that can be plotted as a profile.

Waviness is similar to roughness in that it measures surface irregularities that are more spaced out.

Lay is the direction of the predominant surface pattern; examples include horizontal, vertical, radial, circular, cross-hatched, and isotropic.

Waviness spacing-1

Roughness is particularly important to pay attention to, as it greatly impacts contact mechanics. More roughness equals increased friction, which equals faster wear on your components. Higher roughness values also leave sites on your part where corrosion and cracks can develop.

The part’s end use will largely determine what type of surface finish is needed. To use roughness as an example again, if you need your part to adhere to another component, roughness can be a beneficial attribute if you’ve chosen the right material and surface finishing method.

Surface Finishing Methods

Surface finishing methods protect and enhance the look of a machined component's surfaces. These methods may add or remove material, or use heat, electricity, or chemicals to alter a part's surface finish.

Methods

Media Blasting

Media blasting uses compressed air or water to shoot abrasive materials, known as "media," at high speeds onto a surface. This process cleans and prepares the surface for further treatment or finishing.

The type of abrasive material chosen for media blasting varies based on the task and the results you want to achieve.

Purpose: to achieve a smoother surface

Compatible Materials: most metals and plastics

Black Oxide

Black oxide is a coating applied by dipping a part into a sodium hydroxide and potassium nitrate solution. This process creates a black, smooth finish that enhances corrosion and wear resistance.

Purpose: to improve corrosion and wear resistance

Compatible Materials: mild steels, carbon steels, stainless steel, copper

Powder Coating

Powder coating involves using powdered plastic mixed with chemical agents like pigments and additives to create a strong finish on metal parts.

The powder is sprayed onto a part and sticks due to an electrostatic charge. The coating tool charges the powder particles while the part is grounded, drawing the particles to it. The part is then baked, turning the powder into a solid, durable, and corrosion-resistant coating. This process allows for various pigments and finishes. Sometimes, parts are preheated before spraying.

Purpose: to increase strength and provide a good base for dyeing

Compatible Materials: aluminum, stainless steel, steels, copper, brass, zinc

Electroless Nickel Plating

Electroless nickel plating is a technique that adds a nickel layer to a part's surface. This enhances the part's corrosion and wear resistance, as well as increases its hardness.

Purpose: a cost-effective way to improve surface finish

Compatible Materials: aluminum, stainless steel, mild steels, copper, brass, titanium

Eletropolishing

Electropolishing is a process used to improve the surface finish of metal parts. It involves the removal of a thin layer of material from the surface of a metal object using an electrochemical process.

Purpose: to achieve an even, shiny surface that’s corrosion resistance and highly weldable

Compatible Materials: stainless steel, aluminum, copper, brass, nickel alloys

Passivation

Passivation treats metal surfaces with an acid solution to remove free iron and contaminants, restoring a thin, protective oxide layer. This layer shields the metal from environmental reactions, reducing corrosion risk. It is often used on stainless steel and other metals to enhance resistance to rust, stains, and deterioration.

Purpose: to improve corrosion resistance

Compatible Materials: stainless steel, aluminum, titanium

Anodizing

Anodizing creates a thin layer on metal surfaces, offering protection against corrosion and wear. It only works with aluminum and titanium.

In Type II and Type III anodizing, the part is placed in a diluted sulfuric acid solution, and an electric voltage is applied. This causes a reaction that turns the surface into hard aluminum or titanium oxide. Masks can be used on areas that need to stay conductive or have precise dimensions, like threaded holes, to prevent anodizing. The anodized parts can also be dyed in various colors before sealing.

Type II anodizing, also known as "standard" or "decorative" anodizing, creates coatings up to 25 μm thick. The thickness varies by color: 8-12 μm for black-dyed parts and 4-8 μm for clear parts. This process is ideal for achieving a smoother surface, offering good corrosion resistance and some wear resistance.

Type III anodizing, known as "hardcoat" anodizing, creates coatings up to 125 μm thick, with a standard thickness of 50 μm. This process forms dense layers that offer superior corrosion and wear resistance, ideal for functional uses. It requires more precise control than Type II anodizing, involving higher current density and maintaining the solution temperature near 0 degrees Celsius, which increases the cost.

Adjusting the electric current, anodizing time, solution consistency, and temperature allows you to create coatings with varying thicknesses and densities.

Purpose: to impart a shiny, aesthetic finish that improves corrosion resistance

Compatible Materials: aluminum, titanium

Frequently Asked Questions

What is a surface finish in CNC machining?

Surface finish encompasses the texture and quality of a machined component's surface, shaped by the precision of machining processes and the effectiveness of post-processing techniques to meet specific aesthetic and functional requirements.

Why is surface finish important in CNC machining?

The significance of surface finish lies in its impact on a component's visual appeal, operational efficiency, and overall performance. Achieving a refined surface finish can minimize friction, bolster wear resistance, and elevate the part's quality.

What are the common types of surface finishes in CNC machining?

Surface finishes vary, including rough and smooth machining, polished, brushed, anodized, and coated options. Each finish offers unique features and uses.

How is surface finish measured?

Surface finish is assessed using parameters like Ra (average roughness), Rz (maximum profile height), and Rt (total profile height). These are measured with tools such as profilometers and optical comparators.

What factors influence the surface finish of a machined part?

Surface finish is affected by the material being machined, the shape of the cutting tool, the speed and feed rate, the depth of the cut, and the machining process used.

How can surface finish be improved in CNC machining?

To enhance surface finish, adjust cutting parameters, use top-quality cutting tools, apply advanced machining techniques, and conduct post-processing like polishing, grinding, and coating.

What is the role of cutting tools in achieving a good surface finish?

Cutting tools are essential for achieving a smooth surface finish. The material, shape, and condition of the tool greatly affect the quality of the machined surface. Tools that are sharp and well-maintained deliver superior finishes.

How does material selection affect surface finish?

Choosing the right material impacts the surface finish. Softer materials often result in smoother finishes, while harder materials might need extra finishing steps.

What are some common post-processing techniques for improving surface finish?

Post-processing techniques like polishing, grinding, sanding, anodizing, and coating improve the look and function of machined parts by enhancing their surface quality.

How does surface finish impact the performance of a machined part?

The surface finish of a machined part affects its friction, wear resistance, corrosion resistance, and durability. A superior surface finish enhances the part's performance and longevity.

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