What you should learn about screw thread

  • Screw thread refers to a continuously raised portion of a helical shape and of a specific cross-section made on the surface of a cylindrical or conical base. It is a shape with uniform spiral bulge on the external or internal surface of the products. It has the functions of tightening, connection and sealing, suitable for mostly used screw products.

    Classification of screw threads

    Threads can be divided into cylindrical threads and conical threads according to their parent shape.

    According to its position in the parent body, it is divided into external thread and internal thread.

    According to its cross-sectional shape (tooth type), it is divided into triangular thread, rectangular thread, trapezoidal thread, serrated thread, and other special shape threads.


    1. After the first industrial revolution, the British invented the screw-rod lathe, screw die, and taps, which laid the technical foundation for the mass production of threads.

    2. In 1841, the British Whitworth proposed the world’s first thread standard (characteristic code BSP, BSW, BSF). As a result, the screw thread standard technical system was born.

    3. In 1905, British Taylor invented the principle of thread gauge design (Taylor principle). The United Kingdom became the first country in the world to fully master the technology of thread processing and testing, and the inch thread standard was the first to be recognized worldwide.

    4. After the Second World War, the American thread was transformed into a unified thread standard (characteristic code UN) used by the Allies in World War II. The American pipe thread standard is independently developed by the Americans, and together with the British pipe thread, it constitutes the two major standards of the pipe thread in the world today.

    5. The metric common thread (code M) standard is the standard formed after the American thread. Metric thread standards are widely used around the world and are incorporated into ISO standards. When the metric system of units was determined as the international legal unit of measurement, the status of the metric common thread in the international standard was further improved.

    Machine Screw → Self Tapping Screw → Self Drilling Tapping Screw

    Machine thread: When assembling, first drill and tap teeth on the assembly and the internal thread to be tapped should be the same as the external thread of the screw, and use a small torque for assembly.

    Self-tapping thread: When assembling, drill holes on the assembly first, without tapping the inner teeth, and use a large torque to assemble.

    Self-drilling tapping thread: used directly on the assembly, the screw drilling and tapping are formed at one time.

    Screw thread processing method:

    The thread processing method was mass-produced after the British invented the screw-rod lathe, screw dies, and taps. Later, the processing methods of threads became more, such as thread turning, thread milling, thread rolling, etc.

    screw thread

    Screw thread turning

    The tools for thread turning are cost-effective and the machining techniques are easy to master. However, the processing efficiency is relatively low, and it can only be applied to the processing of threaded parts in small and medium batches.

    Thread milling is a new machining method after thread turning.

    Advantages of screw thread milling

    Thread milling is suitable for machining special-shaped parts that are not suitable for rotation, and most of them are used in machine tools and compound machining machines with milling functions. It can not only ensure high-precision thread tolerances but also complete the machining of parts in one clamping.

    Disadvantages of screw thread milling

    The processing efficiency is low, the processing cost is high, and it is not suitable for the processing of batch parts.

    Thread rolling

    Rolling processing is suitable for the production of large batches of standard parts. Roll forming has some specific advantages over cutting threads. It is a processing method that uses a roll-forming die to plastically deform the workpiece to obtain a thread. According to the different rolling dies, thread rolling can be divided into two types: thread rolling plate processing and thread rolling roller processing.