CNC Threading: What is it and How Does it Work?

CNC threading refers to the process of creating threads (internal or external) on a workpiece using a CNC machine (Computer Numerical Control machine). CNC machines automate the threading process, offering high precision, repeatability, and efficiency compared to manual methods.

Threading can be done on both lathes and milling machines, but CNC lathes are the most common machines used for threading operations due to their ability to rotate the workpiece and precisely control the tool movement.

Types of CNC Threading:

1. External Threading: Creating threads on the outer surface of a part, such as bolts or screws.
2. Internal Threading: Creating threads inside a hole, like those found in nuts or threaded holes in components.

How CNC Threading Works:

The CNC threading process involves a series of steps to form threads accurately. Here's a breakdown of the process:

1. Tool Selection:

   • Threading Tools: A single-point cutting tool or a threading insert is used. For external threading, the tool moves along the surface of the workpiece to cut threads. For internal threading, a boring tool with the right profile is used.
   • Tool Geometry: The tool's geometry (angle, radius, etc.) is set based on the thread type (e.g., metric, imperial, fine, coarse, etc.).

2. Programming the CNC Machine:

   • The operator or programmer defines the thread specifications (e.g., pitch, diameter, and depth) using G-code or CNC programming software.
   • Thread Pitch: The distance between threads, typically specified in inches (for imperial threads) or millimeters (for metric threads).
   • Thread Depth: The amount of material to be removed from the surface to create the threads.
   • The machine’s control system then uses this information to guide the cutting tool along the appropriate path to form the thread.

3. Tool Movement and Thread Cutting:

   • For External Threading: The workpiece is mounted on the lathe, and a rotating tool moves along the length of the part, cutting a helical thread into the outer surface.
   • For Internal Threading: The tool is inserted into a pre-drilled hole and moves along the hole’s axis to cut the internal thread.
   • CNC threading usually uses multiple passes, especially when cutting deep threads, to prevent excessive tool wear and achieve a finer finish.

4. Thread Forming:

   • As the tool cuts, it progressively forms the shape of the thread by removing material from the workpiece.
   • In some cases, coolant is used to cool the cutting area and reduce friction.
   • The CNC system ensures that the tool cuts with the correct depth and angle to meet the specified thread dimensions.

5. Finishing Passes:

   • After the initial cuts, finishing passes may be made to smooth out the threads and achieve the required surface finish. This ensures that the threads are sharp, clear, and functional.

Key CNC Threading Parameters:

• Thread Pitch: Defines the distance between adjacent threads. It is one of the most important parameters for determining thread specifications.
• Thread Depth: The total depth of the cut made by the threading tool.
• Lead: In multi-lead threading, this is the distance the tool moves per revolution.
• Tool Geometry: The cutting angle, radius, and other aspects of the tool are critical in achieving the correct thread shape.

Threading Methods in CNC Machines:

1. CNC Lathe Threading:

   • The most common CNC threading method, especially for creating external threads on round parts.
   • The workpiece rotates, and a stationary tool (often a single-point tool or threading insert) moves along the length of the part.
   • The tool cuts the thread by following the helical path needed for the thread's profile.

2. CNC Milling Machine Threading:

   • Though less common than lathe threading, CNC milling machines can also be used to cut threads, especially for non-round parts or when threading internal holes.
   • A rotary tool is typically used in combination with precise positioning to create the thread.

CNC Threading Advantages:

• High Precision: CNC threading machines are extremely accurate, ensuring that threads conform to tight tolerances, which is critical for parts that need to fit precisely.
• Consistency: CNC machines can repeat the threading operation with the same quality and precision, ideal for mass production.
• Automation: The process is largely automated, reducing human error and increasing production speed.
• Complex Threads: CNC systems can produce various types of threads, including custom or non-standard threads, using simple programming adjustments.
• Tool Wear Management: CNC machines can optimize tool paths and manage tool wear effectively, resulting in better thread quality and longer tool life.

Common Thread Types Manufactured Using CNC Threading:

• Unified Thread Standard (UTS): Common in the U.S. for screws and bolts (e.g., UNC, UNF).
• Metric Threads (M): Standard in most of the world for industrial and automotive uses (e.g., M6, M12).
• Acme Threads: Used in applications requiring a trapezoidal thread form, like lead screws.
• BSP (British Standard Pipe) Threads: Commonly used for plumbing and hydraulic fittings.
• NPT (National Pipe Thread): A type of thread used in pipe fittings, with a tapered profile for a tight seal.

Tips for Successful CNC Threading:

• Correct Tool Selection: Choose the right threading tool (single-point tool, insert, or tap) based on the material and thread specifications.
• Proper Speed and Feed: Use appropriate cutting speeds and feed rates to ensure clean, sharp threads without tool damage or excessive wear.
• Use Coolant: For materials that generate significant heat during threading, such as stainless steel, use coolant to improve tool life and reduce thermal damage.
• Thread Verification: After threading, check the threads using a thread gauge or micrometer to ensure they meet the required specifications.
• Avoid Overcutting: Do not cut too deep in a single pass, as it can cause tool deflection and damage. Multiple shallow cuts are preferred.

Conclusion:

CNC threading is an automated, high-precision process for creating threads on a workpiece, widely used in industries like automotive, aerospace, and manufacturing. It leverages advanced CNC technology to produce both internal and external threads with high accuracy, repeatability, and efficiency. By controlling the threading tool’s movement and optimizing the cutting parameters, CNC threading can produce high-quality threads in a variety of materials and configurations.