Summary of eight processing methods of thread, you must know when doing machining

Summary of eight processing methods of thread, you must know when doing machining
The English word corresponding to screw is Screw. The meaning of this word has changed a lot in recent hundreds of years. At least in 1725, it means “mating”.
The application of the thread principle can be traced back to the spiral water-lifting tool created by the Greek scholar Archimedes in 220 BC.
In the 4th century AD, the Mediterranean countries began to apply the principle of bolts and nuts to the presses used in winemaking. At that time, the external thread was wound with a rope to a cylindrical bar, and then carved according to this mark, while the internal thread was often formed by hammering the external thread with a softer material.
Around 1500, in the sketch of the thread processing device drawn by the Italian Leonardo da Vinci, there was an idea of ​​using the female screw and the exchange gear to process threads with different pitches. Since then, the method of mechanically cutting threads 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 British J. Ramsden once manufactured a thread cutting device driven by a worm gear pair, which can process long threads with high precision. In 1797, the Englishman H. Maudsley used the female screw and exchange gear to turn metal threads of different pitches on his improved lathe, which laid the basic method of turning threads.
In the 1820s, Maudsley produced the first taps and dies for threading.
At the beginning of the 20th century, the development of the automobile industry further promoted the standardization of threads and the development of various precise 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 applied.
In the early 1930s, thread grinding appeared.
Although the thread rolling technology was patented in the early 19th century, due to the difficulty of mold manufacturing, the development was very slow until the Second World War (1942-1945), due to the needs of arms production and the development of thread grinding technology. The precision problem of mold manufacturing has developed rapidly. cnc turning part
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, whirl milling—coarse and fine turning, etc.
The first category: thread cutting
It generally refers to the method of machining threads on workpieces with forming tools or abrasive tools, mainly including turning, milling, tapping and thread grinding, grinding and whirling cutting. When turning, milling and grinding threads, the drive chain of the machine tool ensures that the turning tool, milling cutter or grinding wheel moves exactly and evenly one lead along the axis of the workpiece for each revolution of the workpiece. When tapping or threading, the tool (tap or die) and the workpiece rotate relative to each other, and the tool (or workpiece) is guided by the previously formed thread groove to move axially.
1. Thread turning
Thread turning on a lathe can be done with a forming turning tool or a thread comb. Turning threads with a forming turning tool is a common method for single-piece and small batch production of threaded workpieces due to the simple tool structure; turning threads with a thread combing tool has high production efficiency, but the tool structure is complex, only suitable for medium and large batch production. Turning short thread workpieces with fine pitch. The pitch accuracy of ordinary lathes for turning trapezoidal threads can generally only reach 8 to 9 grades (JB2886-81, the same below); machining threads on specialized thread lathes can significantly improve productivity or accuracy.

2. Thread milling
Milling with a disc or comb cutter on a thread mill.
Disc milling cutters are mainly used for milling trapezoidal external threads on workpieces such as screw and worm. Comb-shaped milling cutter is used for milling internal and external common threads and tapered threads. Since it is milled with a multi-blade milling cutter and the length of its working part is greater than the length of the thread to be processed, the workpiece only needs to be rotated 1.25 to 1.5 turns to be processed. Done with high productivity. The pitch accuracy of thread milling can generally reach 8 to 9 grades, and the surface roughness is R5 to 0.63 microns. This method is suitable for mass production of threaded workpieces of general precision or for roughing before grinding.

Thread milling cutter for machining internal threads
3. Thread grinding
It is mainly used to process precision threads of hardened workpieces on thread grinding machines. According to the shape of the cross-section of the grinding wheel, it can be divided into two types: single-line grinding wheel and multi-line grinding wheel. The pitch accuracy that can be achieved by single-line grinding wheel grinding is 5 to 6 grades, and the surface roughness is R1.25 to 0.08 microns, which is more convenient for grinding wheel dressing. 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 method and plunge grinding method. In the longitudinal grinding method, the width of the grinding wheel is smaller than the length of the thread to be ground, and the grinding wheel moves longitudinally once or several times to grind the thread to the final size. The width of the grinding wheel of the plunge grinding method is larger than the length of the thread to be ground. The grinding wheel is cut radially into the surface of the workpiece, and the workpiece can be ground well after about 1.25 revolutions. The productivity is high, but the precision is slightly lower, and the grinding wheel dressing is more complicated. Plunge grinding is suitable for relief grinding of large batches of taps and for grinding certain threads for fastening.  aluminum extrusion parts

4. Thread grinding

The nut-type or screw-type thread grinder is made of soft materials such as cast iron, and the parts where the thread has a pitch error on the workpiece are subjected to forward and reverse rotation grinding to improve the pitch accuracy. Hardened internal threads are usually ground to eliminate deformation and improve accuracy.
5. Tapping and threading
Tapping
It is to screw the tap into the pre-drilled bottom hole on the workpiece with a certain torque to process the internal thread.

Thread
It is to cut the external thread on the bar (or pipe) workpiece with die. The machining accuracy of tapping or threading depends on the accuracy of the tap or die. aluminum parts
Although there are many ways to process internal and external threads, small-diameter internal threads can only be processed by taps. Tapping and threading can be performed by hand, as well as by lathes, drill presses, tapping machines and threading machines.

The second category: thread rolling
The processing method of plastically deforming the workpiece with a forming rolling die to obtain a thread. The thread rolling is generally performed on a thread rolling machine or an automatic lathe with an automatic opening and closing thread rolling head. External threads for mass production of standard fasteners and other threaded couplings. 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 pitch diameter of the processed thread. Rolling generally cannot process internal threads, but for workpieces with softer materials, a grooveless extrusion tap can be used to cold-extrude internal threads (the maximum diameter can reach about 30 mm). The working principle is similar to that of tapping. The torque required for cold extrusion of internal threads is about 1 times larger than that of tapping, and the machining accuracy and surface quality are slightly higher than those of tapping.
Advantages of thread rolling: ①The surface roughness is smaller than that of turning, milling and grinding; ②The surface of the thread after rolling can improve the strength and hardness due to cold work hardening; ③The material utilization rate is high; ④The productivity is doubled compared to cutting , and it is easy to realize automation; ⑤ The life of the rolling die is very long. However, rolling thread requires that the hardness of the workpiece material does not exceed HRC40; the dimensional accuracy of the blank is high; the precision and hardness of the rolling die are also high, and it is difficult to manufacture the die; it is not suitable for rolling threads with asymmetric tooth shape.
According to the different rolling dies, thread rolling can be divided into two types: thread rolling and thread rolling.
6. Thread rolling
Two thread rolling plates with threaded tooth shape are arranged opposite to each other with a 1/2 pitch, the static plate is fixed, and the moving plate moves in a reciprocating linear motion parallel to the static plate. When the workpiece is sent between the two plates, the moving plate moves forward and rubs the workpiece to plastically deform the surface to form a thread (Figure 6 [Screwing]).
7. Thread rolling
There are three types of radial thread rolling, tangential thread rolling and rolling head thread rolling.
①Radial thread rolling: 2 (or 3) thread rolling wheels with thread profile are installed on mutually parallel shafts, the workpiece is placed on the support between the two wheels, and the two wheels rotate in the same direction and at the same speed (Figure 7). [Radial thread rolling]), one of the rounds also performs radial feed motion. The workpiece is rotated by the thread rolling wheel, and the surface is radially extruded to form threads. For some lead screws that do not require high precision, a similar method can also be used for roll forming.
②Tangential 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 (Fig. 8 [Tangential thread rolling]). During thread rolling, the workpiece can be continuously fed, so the productivity is higher than that of thread rolling and radial thread rolling.
③ Thread rolling head: 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 periphery of the workpiece in the rolling head (Fig. 9 [Thread rolling head rolling]). During thread rolling, the workpiece rotates and the rolling head feeds axially to roll the workpiece out of the thread.
8. EDM threading
The processing of ordinary threads generally uses machining centers or tapping equipment and tools, and sometimes manual tapping is also possible. However, in some special cases, the above method is not easy to obtain good processing results, such as the need to machine threads after heat treatment of parts due to negligence, or due to material constraints, such as the need to tap directly on carbide workpieces. At this time, it is necessary to consider the processing method of EDM.
Compared with the machining method, the EDM process is in the same order, and the bottom hole needs to be drilled first, and the diameter of the bottom hole should be determined according to the working conditions. The electrode needs to be machined into a thread shape, and the electrode needs to be able to rotate during the machining process.

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