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What Are Secondary Machining Processes?

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What Are Secondary Machining Processes?​

Depending on the type of part being manufactured, there are many steps in the process from raw material to finished part. Parts with various surface features or unique shapes often use multiple machines and machining processes. 

For example, a CNC machine might cut and form the general shape of the part. Then another machine may form threads on the parts. Then a grinder machine durburrs the part. And after that, the parts are washed and engraved.

Modern CNC machines have advanced capabilities, but they can’t do everything.

Secondary machining processes form the features that weren’t done in the initial machining of the part. This includes things like deburring, engraving, sub-assembly, and surface treatments like anodizing, heat treatment, and powder coating.

The secondary processes are less extensive than the initial machining. They’re used to add the specific final touches to the parts and components. 

Here are four examples of common secondary processes that we provide.


Parts are engraved for a variety of reasons. One of the most common reasons is to keep track of different lot numbers and help identify parts that look similar. Engraving parts can also help identify different metal alloys since they usually look the same.

After parts are machined, we use in-house CNC engravers and laser engravers to add part numbers, lot numbers, AISI numbers, and more to parts.

Laser engravers are more accurate and can be used for smaller text, or more complex designs like a QR code. The CNC engraver will provide a more durable engraving.


Deburring is a secondary process used to remove small imperfections from parts. Deburring isn’t always necessary, but sometimes the machining process creates small burrs on parts—especially when there are small slots, threads, or sharp edges.

During the deburring process, a spinning brush, grinding wheel, or belt is used to remove the burrs and smoothen out the part. 

A burr is a small protrusion on a part and they’re usually sharp. For certain parts, deburring is a necessary process for parts to function properly and ensure safe handling.


Sub-assembly is a process where two or more separate parts are attached together by the manufacturer. The term “sub-assembly” refers to components that are first assembled together and then integrated into a larger assembled unit. So, a sub-assembly isn’t the final product. Some more complex parts or components are machined as separate parts to speed up the production process, and then assembled.

In some cases, the assembled parts are two different materials, like attaching a rubber o-ring to a metal part.

When sub-assembly operations are outsourced to the manufacturer, it saves time and allows the customer to scale their operations.

Surface treatments

Various things can be done to the surface of the part. This includes heat treatment, anodizing, plating, and powder coating. These are usually to add durability to the part.

Different surface treatments are used for different types of parts and use cases. Metals that are more susceptible to rust or scratches are more likely to benefit from surface treatments.

Steel parts are the most common for plating, when a thin layer of zinc, nickel, or chromium is added to the part. Many steel parts are also heat treated, which makes the steel harder and stronger, but also more brittle.

Aluminum parts are often anodized, which adds a thin layer of oxidized aluminum so that its surface is no longer reactive. The layer of oxidation offers protection from scratches and mechanical wear, as well as chemical protection from water and oxygen.

Primary vs. Secondary Machining Processes

When we talk about machining, it’s crucial to differentiate between primary and secondary processes.

Primary Machining Processes

These processes form the foundation of the machining world. 

It’s where a raw material undergoes substantial transformations to emerge as a product resembling its final form. Think of this as sculpting a rough statue out of a marble block. Here, substantial material is removed, and the basic structure of the part is established. Primary processes include operations like turning, milling, and drilling.

  • Objective: The primary goal is to form the main structure of the part. This involves turning a raw material into a component that closely resembles the final product in shape and size.
  • Techniques: These often include processes like milling, turning, and drilling, where significant material removal occurs.
  • Machinery: Primary machining typically uses machines like CNC milling machines, lathes, and drill presses.
  • Outcome: At the end of this stage, the part will have its main shape but might lack finer details, specific finishes, or refined edges.

Secondary Machining Processes

Once the basic form is established, we shift to refining and enhancing.

The secondary processes are like the final touches to the statue, where intricate details are carved, rough edges are smoothed, and the piece is polished to perfection. These operations, which include deburring, engraving, and surface treatments, are crucial for functional efficacy, aesthetics, and safety.

  • Objective: The goal here is refinement. After primary machining gives the part its general shape, secondary processes enhance its function, appearance, and specificity.
  • Techniques: These can range from deburring (removing rough edges) to engraving specific codes or information on the part. Other processes can include heat treatment or surface finishing.
  • Machinery: This could involve more specialized equipment, such as grinders, engravers, or machines for specific surface treatments.
  • Outcome: The final product, ready for use or assembly, emerges after these processes. It meets all quality standards, functional specifications, and aesthetic requirements.

Secondary processes impact on project lead times

Understanding the timing of a project, especially when lead time is critical, requires a grasp of how secondary processes fit into the overall production timeline:

Integration with Primary Processes: In some cases, secondary processes can be integrated seamlessly with primary ones. For instance, a modern CNC machine might mill a component and then immediately engrave a serial number, all in one go. This reduces additional setup and handling time.

Complexity and Specificity: The more complex or specific the secondary process, the longer it might take. For example, a simple deburring process might add minimal time, while multi-step surface treatments could extend the lead time.

Batch Processing: Some secondary processes, especially those related to finishing, might be done in batches. If your project is part of a larger batch, it could influence the lead time.

Efficiency Enhancements: Many manufacturers, aware of the potential time added by secondary processes, invest in technologies or methodologies to speed these up. For instance, automation and robotics can drastically reduce the time taken for certain secondary operations.

Feedback and Revisions: If there’s a need for adjustments after the secondary processes (maybe a logo isn’t engraved clearly), this can add to the project’s duration. However, regular quality checks reduce these delays.

While secondary processes generally increase lead times, their impact varies based on the project, the processes involved, and the manufacturer’s capabilities. It’s always good to discuss lead time expectations upfront, especially if you’re working on a tight schedule.

Spex has served as a local manufacturer since 1946. We provide a wide range of manufacturing services. If you want to learn more about our precision machining capabilities or secondary machining services, reach out to one of our team members to get a custom quote.


Are there any materials that particularly benefit from secondary processes?

  • All materials can benefit from secondary processes depending on the application. For instance, steel parts often undergo surface treatments to prevent rust, while aluminum parts might be anodized to increase surface durability.

How do I know if my part requires secondary processes?

  • It depends on the design, application, and the desired finish of your part. If a part has sharp edges, requires specific markings, or needs additional surface treatments, it will probably require secondary processes.

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