A Guide to Surface Finishing for Precision Machined Parts

Precision machining entails very exact craftsmanship, with numerous steps that must be completed accurately to achieve the desired results. With CNC machining, surface finishes are the final process in fabricating a product that will perform optimally. Machining finishes provide numerous advantages, including augmenting conductivity, improving appearance, increasing strength, providing increased resistance, and removing any aesthetic imperfections. Understanding these various surface finish processes helps manufacturers choose the best finishing technique for their application.

Factors That Can Affect Surface Finish Processes

When machining parts, the final techniques component manufacturers use generally seek to either alter or add to the material’s surface finish. In machining, surface finishing isn’t just done to improve a component’s look; performance can also be affected by the part’s surface finish. Processes for finishing parts made from metals or alloys are either chemical or mechanical, and choosing the best machining surface finish requires evaluating several vital factors of a part’s design.

Factors to consider about what surface finish processes to use include: 

• Appearance: Aesthetics is often a factor, especially with external components that are readily visible, with particular machining surface finishes done to make them more visibly pleasing.
• Budget: For most manufacturers, balancing costs with other factors like appearance or functionality is integral to investigating surface finish processes.
• Environmental impact: Regulations and customer demand for more eco-friendly surface finish processes make environmental considerations increasingly important.
• Functionality: Part performance is often the most critical aspect when choosing machining finishes for precision parts, so surface finish processes that increase resistance to chemicals, corrosion, wear, or other parameters affecting a part’s functionality are commonly prioritized.
• Geometry: In CNC machining, surface finish choice must often consider a component’s shape, especially for parts that require tighter tolerances.
• Lead times: The time it takes to apply CNC machining surface finishes must be considered, especially for parts that require a quick turnaround. When time isn’t a factor, surface finish processes that take longer but offer a higher quality finish may be more desirable. 
• Material: Some CNC machining finishes work better on certain materials than others; for example, anodizing tends to be used for parts made from aluminum.

Most methods for finishing surfaces can be applied to parts made with lathes, mills, and other equipment used in CNC machining. Surface finishes must also consider component design and requirements, like the material from which a part is made.

Mechanical Properties to Consider for CNC Machining Metals & Alloys

When fabricating a part via CNC machining, surface finish processes rely on the material from which a component is made. Metals and alloys differ significantly in what can be done to achieve an optimal surface finish. To achieve a specific surface finish, processes must always consider these material properties in machining a material. 

Properties to consider include: 

• Damping: This involves the elimination or reduction of movement, noise, or vibrations by converting energy from these into heat; the better the damping properties of a material, the fewer adverse effects result from movement, noise, or vibrations to equipment.
• Density: This directly corresponds to how much a component will weigh; for example, aerospace parts are generally made from less dense materials like aluminum and its alloys as these have lower densities.
• Ductility: This relates to a material’s ability to change shape, making softer materials easier to machine.
• Elasticity: Referred to commonly as the elastic modulus, this property relates to a material’s ability to resist deformation; the lower the elastic modulus, the more malleable a material will be, with heat treatments during fabrication explicitly used to reduce elasticity and increase strength.
• Fracture toughness: Measuring a material’s capacity to tolerate impacts, parts more prone to collisions require more excellent impact resistance.
• Hardness: Generally, more rigid materials better resist abrasion, cutting, scratching, and wear, so these also tend to be more challenging to machine.
• Strength: Harder materials are generally stronger but require specialty cutting tools, slower processing times, and higher labor costs.

When considering a part’s desired surface finish, finishing processes should consider the material’s hardness or softness. Most harder metals are thus combined with softer ones like aluminum into alloys, as well as through physically altering the material by bending, hammering, or stretching.

Types of CNC Machining Surface Finishes

After machining, surface finishes need to undergo a few processes before finishing. The part must first be cleaned of coolants, oils, or other lubricants, along with any other contaminants that result from CNC machining. Finishes can only follow this degreasing process, which commonly utilizes solvents, though ultrasound methods are sometimes employed instead. What degreasing method is utilized often depends on what surface finish processes will follow, which may require washing the part.

After decontamination of the component, deburring begins. This involves removing protruding elements and sharp edges on the part’s surface that can pose a danger or affect its performance. An abrasive wheel grinds down and removes any additional material, helping manufacturers achieve an even smoother surface finish. In machining components with tight tolerances, an inspection process follows to aid manufacturers in identifying and correcting defects and any other issues. Once complete, chemical or mechanical surface finish processes can begin. 

Chemical Finishing

Chemical surface finish processes provide an added layer to protect the part. In CNC machining, finishes that utilize chemicals include anodizing, chemical plating, electroplating, and passivation.

Anodizing

In this chemical process, after machining, surface finishes involve immersing the part in an electrolyte solution, which causes a protective oxide layer to form. There are two types of anodizing. These surface finish processes may create a decorative layer, known as type II anodizing, or make a hard coat for more high-performance applications, known as type III anodizing. Both machining finishes use electrolysis and sulfuric acid to uniformly form an oxide layer on the part’s exterior.

The difference between these two anodizing techniques lies mainly in the thickness of the surface finish. Processes for anodizing with type II generate oxide layers up to about 25 microns (about a thousandth of an inch) thick, whereas type III can achieve finishes up to 150 microns (approximately three-five hundredths of an inch) in thickness. The machining surface finish technique provides much better resistance to abrasion and corrosion. However, when stressed, type III anodizing can sometimes make the material more brittle, chipping, and cracking.

Galling

Anodizing makes components more resistant to corrosion and wear and helps prevent threaded components from galling. A form of wear resulting from two surfaces adhering to each other and friction results from a damaged crystalline structure below the metal’s surface. As aluminum tends to gall easily, aluminum components are anodized to protect their surface integrity.

Electroless Plating

This method for finishing results in plating a part to resist corrosion and wear. It forms a metal coating evenly over the component, regardless of its shape. Unlike the electroplating process, where electrical current is used for plating, this technique utilizes a chemical bath combined with a catalyst to create a protective surface finish. Processes like electroless nickel plating are commonly used to fabricate precision components.

Electroplating

One of the most common techniques after CNC machining, surface finishes that have been electroplated offer an array of beneficial properties to parts. This finishing process protects components from corrosion and contaminants with a thin metallic layer. Not only is it functional, electroplating also improves a part’s appearance, resulting in a decorative surface finish. However, electroplating processes require considerable precision and involve significant energy and dangerous chemicals, making it less environmentally friendly. Additionally, this technique only works with conductive materials, limiting its usage.

Passivation

A common post-machining surface finishing technique, passivation improves both part performance and overall quality. After CNC machining, finishes involving passivation help resolve issues like microscopic roughness and impurities on a part’s exterior that can negatively affect performance. Typically used with stainless steel, it involves immersing the component in a passivation solution containing citric acid, nitric acid, or sometimes both. The acid then reacts with the part’s external features.

Passivation helps form a uniform oxide layer to prevent corrosion and aid resistance to other destructive elements. Passivation is also often a desirable method for machining finishes as it’s one of the surface finishing processes that can be made more ecologically friendly. This is done using only citric acid when creating the oxide layer forming the surface finish. In machining finishes like passivation, however, the process can be pretty time-consuming and take several hours to complete, depending upon how large and complex the part is.

Mechanical Finishes

After CNC machining, surface finishing may be done using mechanical rather than chemical methods. Mechanical machining finishes include as-machined, bead blasting, grinding, laser engraving, painting, polishing, powder coating, thread rolling, tumbling, and vibratory finishing.

As-Machined

Also referred to as “as-milled,” it’s a part that comes directly from CNC machining, with machining finishes that include slight blemishes and visible tool marks. However, with an average roughness of 3.2 microns after machining, as-machined surface finishes result in incredibly tight dimensional tolerances. Add to this the fact no additional processing is needed, and as-machined parts become a very affordable solution. As these components don’t have a protective coating, they can be easily damaged and less aesthetically pleasing.

Bead Blasting

This technique uses ceramic or glass beads propelled across the part’s surface. Finish processes like bead blasting produce a textured yet uniform surface that masks imperfections. This machining surface finish also removes contaminants and utilizes pressurized air to clean the part’s exterior. Bead blasting is often used before painting, electroplating, or similar machining finishes to help clear any leftover debris from previous fabrication processes.

Grinding

These methods for machining finishes use a coarse grinding wheel that removes material from the surface. Finish processes like grinding are often used to improve a part’s surface finish after CNC turning operations. Part manufacturers can achieve exceptionally tight tolerances in machining finishes with a grinding wheel. Grinding techniques for machining surface finishes for turned components can be done on the inside or outside diameter to make various features on a component’s exterior, such as flats, holes, or slots.

Two related machining finishes utilize a grinding process: 

• Honing: Using a rotating stone called a “hone,” along with a fine abrasive, this process works on the inside diameter to remove a thin layer of material that creates a polished surface finish; in machining precision parts, honing is essentially more like polishing than grinding.
• Lapping: This process is used on the outside diameter of turned components to attain tighter tolerances while improving the surface finish; processes involving lapping use abrasive paste applied to a lapping plate when removing a thin layer of material from the part’s exterior.

Grinding processes also depend on the component’s geometry, which can be centered or centerless.

Laser Engraving

This process uses a laser for machining finishes that include letters, logos, or numbers onto a component after CNC machining. Surface finishing with lasers typically occurs when the turning or milling cannot create specific fonts or geometries.

Painting

A versatile finishing method after CNC machining, painted surface finishes offer protection from corrosion while also making a part more attractive. However, paint tends to peel when exposed to harsh chemicals or due to abrasion, which then exposes the part’s interior underneath the surface finish. Processes typically involve paints that protect part exteriors over the long term while also allowing a variety of colors that can make a component more appealing.

Polishing

After machining, surface finishing via polishing usually uses a repetitive process that starts with the coarsest abrasives, gradually working the part’s exterior with progressively less coarse abrasives. This results in a smooth and aesthetically pleasing surface finish. One of the simpler surface finish processes, polishing takes considerable time, which can significantly reduce costs. It can also be complicated to finish components with complex geometries, the finishing of which often requires special equipment and skills.

Powder Coating

This is one of the most common finishing techniques done after CNC machining. Surface finishing for metallic components often involves powder coating. Unlike paint, powder coats help resist chips and scrapes while not fading over time, though they also offer similar color variation. The process is also more environmentally friendly as well. However, powder coat machining finishes require a significant investment in materials and equipment, so this method is normally only used for larger production runs. Powder coating also limits the materials and applications for which it can be used.

Thread Rolling

This surface finish in machining produces threads in precision parts, which can instead be produced with a CNC Swiss machine via a turning process. This technique is often used for making threaded fasteners for the aerospace industry, though threads tend to be rolled after machining. Surface finishing using thread rolling rather than a lathe uses a die with a shaped thread, which is then pressed into a machined blank, displacing material to form the threads.

Tumbling

Using media of different types and sizes, tumbling involves putting components within a rotating barrel to smooth their surface finish. In machining away burrs and other excess material, these surface finish processes also polish away machining markings while brightening the parts. The material used for these machining finishes differs depending upon the part’s geometry and application, along with the material from which it’s made. Ceramics, metals, and plastics are used as tumbling media and come in various shapes and sizes.

Vibratory Finishing

Like tumbling, machining surface finishes that work with vibratory media utilize a machine with a tub-like container. Parts are put into the tub, where media works to remove burrs and other marks on the surface. Finish processes augment components’ exterior appearance, with vibratory machining finishes preferred for many turned parts due to the need for tight tolerances throughout their complex geometries.

Precision Machining Surface Finishes from Staub

Staub Precision Machine is a turnkey manufacturer that utilizes cutting-edge automation technology to provide consistent and repeatable results. Our CNC machining finishing services include plating, deburring, and assembly so that they meet your final requirements when we ship them.

Our machining surface finishing services include: 

• Automated mechanical deburring:
• Bead blasting
• High-energy centrifugal deburring
• Magnetic spinning
• Thermal deburring
• Vibratory tumbling
• Complex sub-assembly of parts
• Manual deburring
• Precision grinding processes
• Simple assembly, including installation of helicoils, o-rings, or pins
• Various coating, painting, and plating surface finishes through trusted partners

Staub provides turnkey manufacturing of complex parts through our tailored machining and surface finishing of parts. To learn more about our finishing capabilities, contact the experts at Staub today.