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Thread forming: Producing threads without chip removal
If you're looking to create a thread, the first decision you'll face is which method to use. While tapping is a common and often preferred choice, it's not the only option available. You can also mill, roll, turn, or form threads. In this post, we’ll take a closer look at the last method: thread forming. Understanding the differences between each threading technique will help you make an informed decision based on your specific needs.
Thread forming is a chipless process, meaning no material is removed during the operation. Instead of cutting, the thread is pressed into the workpiece using a forming tap. This cold-forming method occurs below the recrystallization temperature of the material, which helps improve the pull-out strength of the final thread. The process involves displacing material rather than cutting it, resulting in a stronger and more durable thread.
Compared to traditional thread cutting, forming taps have a different design. They don’t have sharp cutting edges and are usually polygonal in shape. Since no chips are produced, they may not require flutes for chip removal. Some forming taps feature oil grooves to ensure proper lubrication. Instead of a chamfer, they have a forming cone that gradually increases the diameter of the thread as it forms.
When you examine a formed thread closely, you might notice small hollows or "claws" at the tips. These are natural byproducts of the forming process. More importantly, the fibers within the material remain intact, unlike in cutting methods where the material is severed. This continuous fiber flow contributes to better mechanical properties and durability.
One advantage of thread forming is the ability to achieve deeper threads compared to machining methods, limited only by the tool length and proper lubrication. Additionally, thread forming allows for higher rotational speeds, making it more efficient in certain applications. However, due to the high friction involved, using the right cooling lubricant is crucial. The torque required for forming is also higher than in cutting, depending on factors like material type, diameter, pitch, and coating.
Thread forming is best suited for materials that are highly formable at room temperature, such as light metals, nickel and cobalt alloys, titanium, non-ferrous metals, and certain steels. It’s not ideal for harder or less malleable materials. Also, the pitch is limited—typically up to 3–4 mm—so very coarse threads may not be achievable through this method.
When using a thread forming tap, it's essential to calculate the correct core hole diameter. Unlike with cutting taps, the core hole must be slightly larger but with tighter tolerances. The formula for determining the core hole size is Dk = D – (0.5 × P) + 0.05, where D is the nominal diameter and P is the pitch. Tables are also available to simplify this calculation.
High friction forces during forming mean that proper lubrication is critical. Oil grooves or internal lubricant systems can help maintain performance. Additionally, tight tolerances on the borehole require precision and care. Finally, thread forming taps are not suitable for manual use—they require specialized equipment.
Despite its limitations, thread forming offers several benefits. No chips are generated, reducing cleanup and potential damage. The threads produced are strong, smooth, and have excellent surface quality. Forming taps also tend to last longer than standard taps, especially for deep threads.
However, the method has some drawbacks. Its application is limited to specific materials, and the high heat and friction require careful handling. Proper workshop tools are necessary, and the process demands precise execution.
If you’re interested in thread forming taps or compatible cutting oils, check out the online shop of BAER, a trusted tap manufacturer. Explore their selection and reach out if you need assistance—we’re always here to help!