Thread refers to the continuous protruding parts with a specific cross-section, shaped like a spiral, made on the surface of a cylindrical or conical parent body. Threads are classified based on the shape of the parent body into cylindrical threads and conical threads; based on their position on the parent body into external threads and internal threads; and based on their cross-sectional shape (thread form) into triangular threads, rectangular threads, trapezoidal threads, serrated threads, and other special-shaped threads.

On the surface of a cylinder or cone, the continuous protrusions with a specified thread form are formed along the spiral. The protrusions refer to the solid parts on both sides of the thread, also known as teeth.

In machining, threads are cut on a cylindrical shaft (or the surface of an internal hole) using a tool or grinding wheel. At this time, the workpiece rotates once, and the tool moves a certain distance along the axial direction of the workpiece. The marks left by the tool on the workpiece are the threads. Threads formed on the outer surface are called external threads, while those formed on the inner surface are called internal threads. The foundation of threads is the helix on the surface of a circular shaft. Typically, if the cross-section of the thread is triangular, it is called a triangular thread; if the cross-section is trapezoidal, it is called a trapezoidal thread; if the cross-section is serrated, it is called a serrated thread; if the cross-section is square, it is called a square thread; if the cross-section is arc-shaped, it is called an arc-shaped thread, etc.

Main Geometric Parameters of Cylindrical Threads

1. Major Diameter: The diameter of an imaginary cylinder that coincides with the crest of an external thread or the root of an internal thread. The nominal diameter of the thread is the major diameter.

2. Minor Diameter: The diameter of an imaginary cylinder that coincides with the root of an external thread or the crest of an internal thread.

3. Pitch Diameter: The diameter of an imaginary cylinder where the width of the protrusions and grooves of the thread form are equal.

4. Pitch: The axial distance between corresponding points on adjacent teeth along the pitch line.

5. Lead: The axial distance between corresponding points on adjacent teeth along the same helix.

6. Thread Angle: The angle between adjacent thread flanks on the thread form.

7. Helix Angle: The angle between the tangent to the helix on the pitch cylinder and a plane perpendicular to the thread axis.

8. Working Height: The distance between the overlapping parts of the thread forms of two mating threads in a direction perpendicular to the thread axis, etc.

The nominal diameter of threads, except for pipe threads, is the major diameter. Pipe threads use the inner diameter of the pipe as the nominal diameter. Threads are standardized into metric (SI) and imperial systems. The international standard adopts the metric system, and China also uses the metric system.

Threads with a helix angle smaller than the friction angle do not loosen under axial force and are called self-locking, though their transmission efficiency is relatively low.

Among cylindrical threads, triangular threads have good self-locking properties. They are divided into coarse and fine threads, with coarse threads generally used for connections. Fine threads have a smaller pitch and helix angle, providing better self-locking performance. They are often used in small parts, thin-walled tubes, connections subject to vibration or variable loads, and fine-tuning devices.

Conical threads have a triangular thread form and rely mainly on the deformation of the teeth to ensure the tightness of the thread pair, making them commonly used in pipe fittings.

Threads are classified by cross-sectional shape (thread form) into triangular threads, rectangular threads, trapezoidal threads, and serrated threads. Among these, triangular threads are mainly used for connections (see thread connections), while rectangular, trapezoidal, and serrated threads are mainly used for transmission. Threads on the outer surface of the parent body are called external threads, and those on the inner surface are called internal threads. Threads formed on a cylindrical parent body are called cylindrical threads, and those formed on a conical parent body are called conical threads. Threads are classified by the direction of the helix into left-hand and right-hand threads, with right-hand threads being the most common. Threads can be single-start or multi-start. Those used for connections are mostly single-start, while those used for transmission, where rapid advancement or high efficiency is required, use double-start or multi-start threads, though generally not more than four starts.

Triangular threads are mainly used for connections, while rectangular, trapezoidal, and serrated threads are mainly used for transmission. Threads are classified by helix direction into left-hand and right-hand threads, with right-hand threads being the most common. They are also classified by the number of helix starts into single-start, double-start, and multi-start threads. Threads for connections are mostly single-start, while those for transmission use double-start or multi-start threads. Based on tooth size, threads are classified into coarse and fine threads. Depending on the application and function, they can be categorized as fastening threads, pipe threads, transmission threads, and special-purpose threads.

In cylindrical threads, triangular threads have good self-locking properties. They are divided into coarse and fine threads, with coarse threads generally used for connections. Fine threads have a smaller pitch and helix angle, providing better self-locking performance. They are often used in small parts, thin-walled tubes, connections subject to vibration or variable loads, and fine-tuning devices. Pipe threads are used for tight connections in pipe fittings. Rectangular threads have high efficiency but are difficult to grind, and the alignment of internal and external threads is challenging, so they are often replaced by trapezoidal threads. Serrated threads have a working flank close to a rectangular straight edge and are mostly used to withstand unidirectional axial forces.

Conical threads have a triangular thread form and rely mainly on the deformation of the teeth to ensure the tightness of the thread pair, making them commonly used in pipe fittings.

Threads can also be classified into sealing threads and non-sealing threads based on sealing performance.

Mold
A method of directly machining threads using a mold.

Rolling
A processing method where a forming rolling mold plastically deforms the workpiece to obtain threads. Thread rolling is generally performed on thread rolling machines, thread rolling machines, or automatic lathes equipped with automatic opening and closing thread rolling heads. It is suitable for mass production of standard fasteners and other threaded connectors with external threads. The outer diameter of rolled threads generally does not exceed 25 mm, and the length does not exceed 100 mm. Thread accuracy can reach Grade 2 (GB197-63), and the diameter of the blank is approximately equal to the pitch diameter of the thread being processed. Rolling generally cannot process internal threads, but for workpieces with softer materials, internal threads can be cold-extruded using a slotless tap (maximum diameter up to about 30 mm), with a working principle similar to tapping. The torque required for cold extruding internal threads is about twice that of tapping, and the processing accuracy and surface quality are slightly higher than tapping.

The advantages of thread rolling are: surface roughness is lower than turning, milling, and grinding; the thread surface is cold-worked and hardened, improving strength and hardness; high material utilization; productivity is multiplied compared to cutting processing, and it is easy to automate; rolling mold life is very long. However, thread rolling requires the hardness of the workpiece material not to exceed HRC 40; the dimensional accuracy of the blank is required to be high; the accuracy and hardness of the rolling mold are also high, and manufacturing the mold is relatively difficult; it is not suitable for rolling threads with asymmetric tooth forms.

Depending on the rolling mold, thread rolling can be divided into thread rolling and thread rolling.

Thread rolling: Two thread rolling plates with thread forms are offset by 1/2 pitch and arranged opposite each other. The static plate is fixed, and the moving plate reciprocates linearly parallel to the static plate. When the workpiece is fed between the two plates, the moving plate advances and rolls the workpiece, causing plastic deformation on its surface to form threads.

Thread rolling can be divided into radial thread rolling, tangential thread rolling, and thread rolling head rolling.

Radial thread rolling: Two (or three) thread rolling wheels with thread forms are installed on parallel shafts, and the workpiece is placed on a support between the two wheels. The two wheels rotate in the same direction at the same speed, and one of them also performs a radial feed motion. The workpiece rotates driven by the thread rolling wheels, and the surface is radially squeezed to form threads. For some lead screws with low accuracy requirements, a similar method can be used for rolling forming.

Tangential thread rolling: Also known as planetary thread rolling, the rolling tool consists of a rotating central thread rolling wheel and three fixed arc-shaped thread plates. During thread rolling, the workpiece can be continuously fed, so productivity is higher than thread rolling and radial thread rolling.

Thread rolling head rolling: Performed on an automatic lathe, generally used to process short threads on workpieces. The rolling head has 3 to 4 thread rolling wheels evenly distributed around the workpiece. During thread rolling, the workpiece rotates, and the rolling head feeds axially to roll the threads onto the workpiece.

Cutting
Refers to the method of processing threads on a workpiece using forming tools or grinding tools.

Thread milling: Milling is performed on a thread milling machine using a disc milling cutter or a comb milling cutter. Disc milling cutters are mainly used for milling trapezoidal external threads on workpieces such as lead screws and worms. Comb milling cutters are used for milling internal and external general threads and taper threads. Since a multi-edge milling cutter is used and the working part length is greater than the length of the thread being processed, the workpiece only needs to rotate 1.25 to 1.5 turns to complete the processing, resulting in high productivity. The pitch accuracy of thread milling can generally reach Grade 8 to 9, with a surface roughness of R 5 to 0.63 micrometers. This method is suitable for batch production of general-precision thread workpieces or rough machining before grinding.

In today's technologically advanced world, machining centers have become an irreplaceable tool for production enterprises, so thread processing is increasingly done by milling, which offers high efficiency, simplified steps, and high precision, thereby bringing greater benefits to enterprises. To meet this demand, many companies have emerged to provide professional solutions for threads with special requirements.

Thread grinding: Mainly used for processing precision threads on hardened workpieces on thread grinding machines. Depending on the cross-sectional shape of the grinding wheel, it can be divided into single-line grinding wheel grinding and multi-line grinding wheel grinding. Single-line grinding wheel grinding can achieve pitch accuracy of Grade 5 to 6 and surface roughness of R 1.25 to 0.08 micrometers, and the grinding wheel dressing is relatively convenient. This method is suitable for grinding precision lead screws, thread gauges, worms, small batches of thread workpieces, and sharpening precision hobs. Multi-line grinding wheel grinding is further divided into longitudinal grinding and plunge grinding. In longitudinal grinding, the width of the grinding wheel is less than the length of the thread being ground, and the grinding wheel moves longitudinally once or several times to grind the thread to the final size. In plunge grinding, the width of the grinding wheel is greater than the length of the thread being ground, and the grinding wheel radially plunges into the workpiece surface. The workpiece rotates about 1.25 turns to complete the grinding, resulting in higher productivity but slightly lower accuracy, and the grinding wheel dressing is more complex. Plunge grinding is suitable for sharpening large batches of taps and grinding some threads for fastening.

Thread lapping: Using a softer material such as cast iron to make a nut-type or screw-type thread lapping tool, the parts of the processed thread on the workpiece with pitch errors are lapped by rotating forward and reverse to improve pitch accuracy. Hardened internal threads are often lapped to eliminate deformation and improve accuracy.

Tapping and threading: Tapping involves rotating a tap into a pre-drilled hole in the workpiece with a certain torque to process internal threads. Threading involves using a die to cut external threads on a rod (or pipe) workpiece. The processing accuracy of tapping or threading depends on the accuracy of the tap or die. Although there are many methods for processing internal and external threads, small-diameter internal threads can only be processed by tapping. Tapping and threading can be done manually or using a lathe, drilling machine, tapping machine, or threading machine.

Turning
Precautions for turning threads: Consider the expansion of the thread form during thread processing. The major diameter (nominal diameter d) of external threads should generally be turned 0.2~0.4mm smaller than the basic size (about 0.13P), ensuring that after turning the thread, the crest has a width of 0.125P (P is the pitch). When boring the bottom hole of an internal thread, ensure the bottom hole diameter is the nominal diameter - P.

Thread cutting should ensure sufficient acceleration feed section δ1 and deceleration retraction section δ2 at both ends to remove the non-standard pitch thread segments caused by speed changes at both ends. Similarly, during thread cutting, the feed rate override function and feed pause function are invalid; if the feed pause button is pressed at this time, the tool will stop only after completing the thread segment. The feed amount for thread processing can refer to the thread minor diameter, which is the final feed position of the thread tool. The minor diameter of the thread is: major diameter - 1.2 times the pitch; the feed amount for thread processing should be continuously reduced, and the specific feed amount should be selected based on the tool and workpiece material, but the last feed should not be less than 0.1mm. After thread processing is completed, the thread quality can be judged by observing the thread form, and timely measures can be taken. However, it should be noted that for external threads, when the thread crest is not sharp, increasing the tool's cutting amount will instead increase the major diameter of the thread, and the increase depends on the plasticity of the material. When the crest has been sharpened, increasing the tool's cutting amount will proportionally reduce the major diameter. Based on this characteristic, the cutting amount for threads should be correctly handled to prevent scrap. For general standard threads, thread ring gauges or plug gauges are used for measurement. When measuring external threads, if the "go" ring gauge (go gauge) just screws in and the "no-go" ring gauge (no-go gauge) does not screw in, it means the processed thread meets the requirements; otherwise, it is unqualified. When measuring internal threads, a thread plug gauge is used, and the same method is applied. In addition to thread ring gauges or plug gauges, other measuring tools can be used for measurement, such as a thread micrometer to measure the pitch diameter of the thread, etc.