Introduction to Threading Methods

Welcome to the first article in my special series, focusing on all things threading. In this article, we'll look at the intricacies of both the plastic-working and metal-cutting methods and provide insights into how threads are produced.

Threads are essential components in various everyday products, and understanding how they're made involves exploring the main manufacturing methods, such as plastic-working and metal cutting. Let's take a closer look at these methods.

Plastic Working: The Thread-Rolling Method

Plastic working is the predominant method used for producing threads, in which we rely on the thread-rolling method. This technique is widely used for traditional bolts, screws, and increasingly for their corresponding nuts.

Thread rolling is a forming process where the material is squeezed and forced through a set of dies with the 'negative thread profile' on them, similar to a mould for forging or casting. When the threads are formed, the dies push into the material to form the minor diameter and the root radius. More than 50% of threads produced in the industry are produced using this method.

Metal Cutting: The Methods

On the other hand, metal cutting utilises various methods, including taps, dies, milling, grinding and turning. When cutting or grinding a thread form, the starting diameter of the workpiece is the major diameter of the thread. Subsequently, material is subtractively removed during the cutting or grinding process to achieve the final thread form. Let’s explore each one of them in more detail.

• Taps: Tapping is the process of creating threads in a workpiece using a tapping tool that enters an existing hole axially. The tap size matches the hole's dimensions. The workpiece rotates at a low speed, and the tap, fixed in a synchronised holder, supports the threading process to produce a thread.

• Dies: A threading die is a tool for external threading on a workpiece. It rotates around the workpiece, cutting threads into it. The die size corresponds to the desired thread dimensions. The workpiece remains stationary, and the threading die, secured in a holder, creates threads as it rotates.

• Thread Milling: Thread milling differs from tapping as it doesn't directly link the thread geometry to the tool. The thread shape results from the rotation of the milling cutter and the simultaneous helical interpolation movement of the machine. Using a solid thread milling cutter allows the production of various thread diameters with the same pitch, suitable for both left and right-handed threads.

• Thread Grinding: This precision machining process employs a grinding wheel to create threads on a workpiece, ensuring high accuracy and tight tolerances. This operation is typically used for threads used in taps, measuring tools, and thread gauges.

• Thread Turning: A common lathe operation, thread turning utilises a cutting tool to cut threads onto a workpiece's outer surface. The tool moves alongside the workpiece, producing the thread with specific pitches and lengths. Deeper threads may require multiple passes in which a carbide insert shaped like a thread is used for the task. Success typically depends on precise machine setup, especially when initiating the helix from the same starting point in multiple passes.

While thread-cutting operations represent a smaller part of machining, they hold significant importance, as incorrectly produced threads can lead to the rejection of costly pre-machined components. However, with proper guidance, these challenges can be avoided, maintaining cost-effectiveness and reducing the rejection rates in your machine shops.

In my upcoming article, our focus shifts to “Thread forms and their functions” where we discuss the different forms and how they are grouped into four categories based on their main functions. Until then, keep threading and stay tuned for more insights.