10 Common Types of Turning Operations in Machining

Types of Turning Operations in Machining

Have you ever wondered how precise, cylindrical components in machines are shaped and finished? Turning operations, a crucial aspect of CNC machining, play a pivotal role in manufacturing everything from simple bolts to aerospace components with intricate geometries.

In this blog post, we will delve into the 10 types of turning operations. It will provide you with a clear understanding of each process and its application.

By the end of this article, you’ll not only be familiar with knurling, threading, and parting-off, but also equipped with knowledge that enhances your skills in CNC machining and engineering design. Get ready to turn your curiosity into expertise.

1. External Turning

External turning is a fundamental CNC machining process on a CNC lathe used to shape the outer diameter of a cylindrical workpiece. This operation involves a cutting tool removing material from the workpiece as it rotates on a lathe. The objective is to achieve the desired outer shape and surface finish, whether it’s a smooth cylinder, a tapered end, or a detailed profile. 

External turning is versatile and essential for creating components such as shafts, rods, and spindles, where precise external contours are critical. The operation’s efficiency and accuracy make it a staple in the manufacturing of various mechanical and structural parts.

2. Internal Turning/Boring


Internal turning, commonly known as boring, enlarges an existing hole or refines the internal diameter of a hollow workpiece. Unlike external turning, which shapes the outside of a part, internal turning focuses on the inside, using a single-point cutting tool to smooth out irregularities and achieve precise internal geometries.

The key difference between internal turning and boring lies in their specific applications: internal turning adjusts internal contours and diameters. While boring is primarily used to increase hole sizes with greater accuracy and to create tapered or contoured holes as required.

This distinction makes boring operations especially critical in applications requiring exceptionally precise fits, such as in the assembly of complex machinery.

3. Threading


Threading is a precise turning operation where a cutting tool carves helical grooves into a cylindrical workpiece. This process can create either external threads, like those seen on bolts, or internal threads, such as those within a nut.

The thread types can vary widely, each serving specific functions. The common types include metric, Unified Thread Standard (UTS), Acme, and trapezoidal threads.

Metric and UTS threads are generally used in fastening and mechanical assemblies requiring tight tolerances and strength. While Acme and trapezoidal threads are ideal for movements and load-bearing in machines like lathes and lead screws.

Each thread type adheres to specific standards to ensure compatibility and functionality across various applications.

4. Knurling


Knurling is a machining process that impresses a pattern onto the surface of a part to enhance grip and aesthetic appeal. It involves pressing a lathe tool with a specific pattern—often diamond-shaped or straight-lined—against a rotating workpiece to form raised ridges. This operation not only improves the handling characteristics of components like tool handles and knobs but also adds a decorative touch that can enhance visual interest and tactile feedback.

5. Drilling

Drilling in turning operations involves the use of a lathe to create cylindrical holes in a rotating workpiece. This process integrates the functionality of a drill press with the precision of a lathe, where drill bits are mounted on the tailstock or a turret to bore straight into the material. The choice of drill bits is crucial and varies based on the material hardness, hole size, and required finish. Drill bits in lathes are typically made from high-speed steel or carbide to handle the demands of various materials, ensuring efficient and accurate drilling.

6. Parting-Off

Parting-off, also known as cutoff, is a turning operation where you essentially separate a part of the workpiece from the main stock. Imagine slicing a roll of cookie dough to bake individual cookies—that’s what parting-off does in machining.

This process involves a narrow cutting tool that penetrates perpendicularly into the workpiece as it spins, cutting through until it parts the piece. This method is crucial for material separation, allowing manufacturers to efficiently produce individual components from a long stock of material. It’s a fundamental step in batch production, ensuring each segment is precisely cut for further detailing or use.

7. Reaming

Reaming is a machining process used to refine and finish holes made by drilling, ensuring the holes are perfectly round and of exact dimensions. When a drill bit creates a hole, the result might not be perfectly smooth or accurately sized.

A reamer, which is slightly larger than the drill bit, is then used to remove a small amount of material, enhancing the precision of the hole’s diameter and improving its surface finish.

This step is crucial for applications where tight tolerances and smooth interiors are necessary, such as in fitting bearing assemblies and other critical mechanical components.

8. Grooving


Grooving is a turning process that involves cutting narrow, recessed channels into a workpiece. This process is vital for creating specific features such as O-ring seats and snap ring grooves, which are crucial for assembling mechanical parts. 

The types of grooves typically include straight, face, and internal grooves, each serving different functional needs in components. Straight grooves are cut along the length of a cylindrical part, face grooves are cut perpendicular to the part’s axis, and internal grooves are machined inside a hollow part.

These grooves play essential roles in ensuring parts fit securely and perform their functions reliably.

9. Taper Turning

Tapping is a turning operation that involves creating internal threads within a pre-drilled hole in a workpiece. By using a tapping tool that moves axially, this process cuts precise threads to match the specifications of screws and bolts. The purpose of taper turning is to allow for components to be assembled by threading fasteners into these newly created internal threads.

This operation is crucial in manufacturing sectors where secure bolt connections are needed, such as in the automotive, aerospace, and construction industries. Tapping ensures that components can be easily assembled, disassembled, and securely reassembled without damaging the workpiece.

10. Facing


Facing is a machining operation used to adjust the length and clean up the end of a workpiece. It is particularly useful when the workpiece is initially longer than needed.

During facing, a cutting tool is employed to move radially along the workpiece, which rotates parallel to its axis. This process trims the end of the workpiece to the desired length and creates a smooth end surface.

Facing ensures the part ends are flat and square to the axis, essential for the part’s functionality and assembly in its final application.

How to Choose the Right CNC Turning Operations?

Choosing the right turning operations for your machining project is crucial for achieving high precision and efficiency. Here are some key points to consider:

Material Characteristics: Assess the properties of the material, including hardness, toughness, and machinability. Different materials might require specific turning operations to achieve optimal results.

Desired Finish and Tolerance: Consider what surface finish and tolerances are necessary for the component. Operations like fine turning or hard turning might be required for high precision and smooth finishes.

Complexity of the Part: Analyze the complexity of the part’s design. For intricate shapes or patterns, operations such as contour turning or threading may be necessary.

Production Volume: Determine the volume of parts needed. For high-volume production, operations that can be easily automated and repeated with precision, such as CNC turning, are more suitable.

Tool Availability and Capability: Check the availability and capability of your tools. Ensure that your equipment can handle the specific operations required, especially for specialized tasks like knurling or reaming.

By carefully evaluating these aspects, you can select the most appropriate turning operations to meet the demands of your project efficiently and cost-effectively.


In this blog, we’ve explored the ten fundamental turning operations used in CNC machining, from external turning and boring to more specialized processes like threading and knurling.

Each operation has its unique purpose and application, ensuring precision and efficiency in manufacturing parts with complex geometries and specific requirements. Understanding these operations provides a solid foundation for anyone interested in the technical aspects of CNC machining and the production of high-quality mechanical components.

If you’re looking to leverage high-precision CNC turning services for your projects, don’t hesitate to contact us. Our expertise and advanced technology are at your service to produce top-quality CNC turned parts tailored to your specifications.