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Author: Site Editor Publish Time: 2025-02-05 Origin: Site
The secrets of thread processing: 8 methods are fully understood at once
The English word corresponding to screw is Screw. The meaning of this word has changed significantly in the past few hundred years.
The application of the principle of thread can be traced back to the spiral water-lifting tool created by the Greek scholar Archimedes in 220 BC. In the 4th century AD, countries along the Mediterranean coast began to apply the principle of bolts and nuts on presses used for brewing. At that time, the external threads were all wrapped around a cylindrical bar with a rope and then engraved according to this mark, while the internal threads were often wrapped around the external threads with softer materials and hammered into shape.
Around 1500, the Italian Leonardo da Vinci drew a sketch of a thread processing device, which had the idea of using a female screw and an exchange gear to process threads of different pitches. Since then, the method of mechanical thread cutting has developed in the European watchmaking industry.
In 1760, the British brothers J. Wyatt and W. Wyatt obtained a patent for cutting wood screws with a special device. In 1778, the Englishman J. Ramsden manufactured a thread cutting device driven by a worm gear pair, which could process long threads with high precision. In 1797, the Englishman H. Maudslay used a female lead screw and an exchange gear to turn metal threads of different pitches on a lathe improved by him, laying the foundation for the basic method of turning threads.
In the 1820s, Maudslay manufactured the first batch of taps and dies for thread processing. In the early 20th century, the development of the automobile industry further promoted the standardization of threads and the development of various precision and efficient thread processing methods. Various automatic opening die heads and automatic shrinking taps were invented one after another, and thread milling began to be used. In the early 1930s, thread grinding appeared.
Although thread rolling technology was patented in the early 19th century, it developed very slowly due to the difficulty in mold manufacturing. It was not until the Second World War (1942-1945) that it developed rapidly due to the needs of arms production and the development of thread grinding technology to solve the problem of mold manufacturing precision.
Threads are mainly divided into connecting threads and transmission threads.
For connecting threads, the main processing methods are: tapping, threading, threading, thread rolling, thread rolling, etc.
For transmission threads, the main processing methods are: rough and fine turning---grinding, whirlwind milling---rough and fine turning, etc.
Thread cutting generally refers to the method of processing threads on workpieces with forming tools or grinding tools, mainly turning, milling, tapping, thread grinding, grinding and whirlwind cutting. When turning, milling and grinding threads, the transmission chain of the machine tool ensures that the turning tool, milling cutter or grinding wheel moves one lead accurately and evenly along the axial direction of the workpiece for every rotation of the workpiece. When tapping or threading, the tool (tap or die) and the workpiece rotate relative to each other, and the first formed thread groove guides the tool (or workpiece) to move axially.
Forming turning tools or thread combing tools can be used to turn threads on lathes.
Using a forming turning tool to turn threads is a common method for single-piece and small-batch production of threaded workpieces due to its simple tool structure; using a thread combing tool to turn threads has high production efficiency, but the tool structure is complex and is only suitable for turning short threaded workpieces with fine teeth in medium and large-scale production.
The pitch accuracy of trapezoidal threads turned by ordinary lathes can generally only reach 8 to 9 levels (JB2886-81, the same below); processing threads on specialized thread lathes can significantly improve productivity or accuracy.
Milling with a disc milling cutter or a comb milling cutter on a thread milling machine.
Disc milling cutters are mainly used for milling trapezoidal external threads on workpieces such as screws and worms.
Comb milling cutters are used to mill internal and external ordinary threads and tapered threads. Since they are milled with multi-blade milling cutters and the length of their working parts is greater than the length of the processed thread, the workpiece only needs to rotate 1.25 to 1.5 turns to complete the processing, and the productivity is very high.
The pitch accuracy of thread milling can generally reach 8 to 9 levels, and the surface roughness is R5 to 0.63 microns. This method is suitable for batch production of threaded workpieces with general precision or rough processing before grinding.
It is mainly used to process precision threads of hardened workpieces on thread grinders. According to the different cross-sectional shapes of the grinding wheel, it is divided into single-line grinding wheel and multi-line grinding wheel grinding.
The pitch accuracy that can be achieved by single-line grinding wheel grinding is 5 to 6 levels, the surface roughness is R1.25 to 0.08 microns, and the grinding wheel dressing is more convenient. This method is suitable for grinding precision screws, thread gauges, worms, small batches of threaded workpieces and relief grinding precision hobs.
Multi-line grinding wheel grinding is divided into longitudinal grinding and cut-in grinding. The grinding wheel width of the longitudinal grinding method is less than the length of the thread to be ground, and the grinding wheel can grind the thread to the final size by moving it longitudinally once or several times. The grinding wheel width of the plunge grinding method is greater than the length of the thread being ground. The grinding wheel radially cuts into the surface of the workpiece. The workpiece can be ground after about 1.25 turns. The productivity is high, but the accuracy is slightly lower, and the grinding wheel dressing is relatively complicated. The plunge grinding method is suitable for grinding large batches of taps and grinding certain fastening threads.
A nut-type or screw-type thread grinding tool is made of softer materials such as cast iron. The parts of the processed threads on the workpiece with pitch errors are rotated forward and backward to improve the pitch accuracy. Hardened internal threads are usually also ground to eliminate deformation and improve accuracy.
Tapping: The tap is screwed into the pre-drilled bottom hole on the workpiece with a certain torque to process the internal thread.
Threading: The external thread is cut on the bar (or tube) workpiece with a die. 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 taps. Tapping and threading can be done manually or by lathe, drilling machine, tapping machine and threading machine.
A processing method that uses a forming rolling die to make the workpiece plastically deform to obtain threads. Thread rolling is generally performed on a thread rolling machine or an automatic lathe equipped with an automatic opening and closing thread rolling head. It is suitable for mass production of external threads of standard fasteners and other threaded connectors.
The outer diameter of the rolled thread is generally not more than 25 mm, the length is not more than 100 mm, the thread accuracy can reach level 2 (GB197-63), and the diameter of the blank used is roughly equal to the median diameter of the thread being processed.
Rolling generally cannot process internal threads, but for workpieces with softer materials, a slotless extrusion tap can be used to cold extrude internal threads (the maximum diameter can reach about 30 mm), and the working principle is similar to tapping.
The torque required for cold extrusion of internal threads is about 1 times greater than that of tapping, and the processing accuracy and surface quality are slightly higher than tapping.
① The surface roughness is less than that of turning, milling and grinding;
② The thread surface after rolling can improve strength and hardness due to cold hardening;
③ High material utilization rate;
④ The productivity is doubled compared with cutting processing, and it is easy to realize automation;
⑤ The life of rolling die is very long.
However, the hardness of the workpiece material required for rolling thread does not exceed HRC40; the requirements for the precision of the blank size are high; the requirements for the precision and hardness of the rolling die are also high, and the manufacturing of the die is relatively difficult; it is not suitable for rolling threads with asymmetric tooth shapes.
According to the different rolling dies, thread rolling can be divided into two categories: thread rolling and thread rolling.
Two thread rolling plates with thread tooth shapes are arranged relative to each other with a staggered pitch of 1/2, the static plate is fixed, and the dynamic plate makes a reciprocating linear motion parallel to the static plate. When the workpiece is fed between the two plates, the dynamic plate moves forward and presses the workpiece, causing its surface to plastically deform and form a thread.
There are three types of thread rolling: radial thread rolling, tangential thread rolling and rolling head thread rolling.
① Radial thread rolling: 2 (or 3) thread rolling wheels with threaded tooth profiles are installed on parallel shafts, and the workpiece is placed on the support between the two wheels. The two wheels rotate in the same direction and at the same speed, and one of the wheels also performs radial feed motion. The workpiece rotates under the drive of the thread rolling wheel, and the surface is radially extruded to form threads. For some screws with low precision requirements, similar methods can also be used for rolling.
② Tangential thread rolling: also known as planetary thread rolling, the rolling tool consists of a rotating central thread rolling wheel and 3 fixed arc-shaped thread plates. During thread rolling, the workpiece can be fed continuously, so the productivity is higher than thread rolling and radial thread rolling.
③ Thread rolling head thread rolling: It is carried out on an automatic lathe and is generally used to process short threads on the workpiece. There are 3 to 4 thread rolling wheels evenly distributed on the outer circumference of the workpiece in the rolling head. During thread rolling, the workpiece rotates, and the rolling head feeds axially to roll the workpiece into threads.
Ordinary threads are usually processed using machining centers or tapping equipment and tools, and sometimes they can also be tapped manually.
However, in some special cases, the above methods are not easy to obtain good processing results, such as the need to process threads after heat treatment of parts due to negligence, or due to material factors, such as the need to tap directly on carbide workpieces. At this time, it is necessary to consider the EDM processing method.
Compared with the machining method, the sequence of EDM is the same. Both need to drill a bottom hole first, and the bottom hole diameter needs to be determined according to the working conditions. The electrode needs to be processed into a thread shape, and the electrode needs to be able to rotate during the processing.
