The processing methods of various threads are really better than one!
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The application of the thread principle can be traced back to 220 BC, when the Greek scholar Archimedes created the spiral water lifting tool.
In the 4th century A.D., the principle of bolts and nuts began to be applied to the press for brewing along the Mediterranean coast. At that time, the external threads were all wrapped around a cylindrical bar with a rope, and then carved according to this mark, while the internal threads were often formed by hammering with soft materials wrapped around the external threads.
Around 1500, in the sketch of thread processing device drawn by Italian Leonardo da Vinci, there was an idea of using mother screw and exchange gear to process threads with different pitches. Since then, the method of mechanical thread cutting has developed in European watch manufacturing industry.
In 1760, the British brothers J. Wyatt and W. Wyatt obtained a patent for cutting wooden screws with a special device. In 1778, an Englishman, J. ramsden, made a thread cutting device driven by a worm gear pair, which could process long threads with high precision. In 1797, H. Maudslay, an Englishman, turned metal threads with different pitches on the lathe improved by him, which laid the basic method for turning threads.
In the 1820s, Maudslay produced the first batch of taps and dies for thread processing.
At the beginning of the 20th century, the development of 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, it developed slowly because of the difficulty in mold manufacturing. It was not until the Second World War (1942 ~ 1945) that the demand of ammunition production and the development of thread grinding technology solved the precision problem of mold manufacturing, and it achieved rapid development.
Threads are mainly divided into connection threads and transmission threads.
For connecting threads, the main processing methods are tapping, threading, turning, rolling and rubbing.
For transmission thread, its processing methods are mainly: rough turning-grinding, cyclone milling-rough turning, etc.
Today, let’s take a look at the various processing methods of threads with Xiao Bang and see which method is better.
01
thread cutting
Generally speaking, it refers to the methods of machining threads on workpieces with forming tools or abrasives, mainly including turning, milling, tapping, threading, grinding, grinding and cyclone 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 a lead accurately and evenly along the axial direction of the workpiece every time the workpiece rotates. When tapping or threading, the tool (tap or die) and the workpiece make relative rotation movement, and the tool (or workpiece) is guided to make axial movement by the pre-formed thread groove.
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Forming lathe tool or thread comb tool can be used for turning threads on lathe (see thread processing tool). Turning thread with forming turning tool is a common method to produce threaded workpiece in single piece and small batch because of its simple structure. Using thread comb cutter to turn threads has high production efficiency, but the cutter structure is complex, which is only suitable for turning short thread workpieces with fine teeth in medium and large batch production. Generally, the pitch accuracy of trapezoidal thread turning by ordinary lathe can only reach 8~9 grades (JB 2886-81, the same below); Machining threads on specialized threading lathe can significantly improve productivity or accuracy.
02
Thread milling
Milling with a disc cutter or a comb cutter on a thread milling machine. Disc milling cutter is mainly used for milling trapezoidal external threads on screw rods, worms and other workpieces. Comb milling cutter is used for milling internal and external common threads and taper threads. Because it is milled by multi-edge milling cutter, and the length of its working part is longer than the length of the processed thread, the workpiece can be processed only by rotating 1.25~1.5 revolutions, and the productivity is very high. The pitch accuracy of thread milling can generally reach 8~9 grades. This method is suitable for mass production of threads with general accuracy or rough machining before grinding.
03
Thread grinding
It is mainly used to process the precision thread of hardened workpiece on the thread grinder.
Thread grinding can be divided into single-line grinding wheel and multi-line grinding wheel according to the different cross-sectional shapes of grinding wheels. Single-wire grinding wheel grinding can achieve pitch accuracy of 5~6 grades and surface roughness of Ra1.25~0.08 microns, which is convenient for grinding wheel dressing.
This method is suitable for grinding precision screw, thread gauge, worm, small batch of threaded workpiece and shovel grinding precision hob. Multi-line grinding wheel grinding is divided into longitudinal grinding method and cut-in grinding method. The width of the grinding wheel in longitudinal grinding method is smaller than the length of the ground thread, and the grinding wheel can be reversely threaded to the compared size by moving it longitudinally for one or several strokes. The width of the grinding wheel in the plunge grinding method is larger than the length of the thread to be ground. When the grinding wheel radially plunges into the surface of the workpiece, the workpiece can be ground in about 1.25 revolutions. The productivity is higher, but the accuracy is slightly lower, and the dressing of the grinding wheel is more complicated. Cut-in grinding method is suitable for relieving and grinding taps in large quantities and grinding some fastening threads.
04
Thread grinding
The nut-type or screw-type thread grinding tool is made of soft materials such as cast iron, and the parts of the processed thread with pitch error on the workpiece are rotated forward and backward to improve the pitch accuracy. Hardened internal threads are usually ground to eliminate changes and improve accuracy.
05
Tapping and threading
Tapping is to screw the tap into the pre-drilled bottom hole on the workpiece with a certain torque to process internal threads.
The threading is to cut the external thread on the bar (or tube) workpiece with a die. The machining accuracy of tapping or threading depends on the accuracy of tap or die. Although there are many methods to process internal and external threads, small-diameter internal threads can only be processed by taps. Tapping and threading can be done by hand, or by lathe, drilling machine, tapping machine and threading machine.
06
Thread rolling
A machining method of plastic deformation of workpiece by forming and rolling die to obtain thread. Thread rolling is generally carried out on thread rolling machine, thread rolling machine or automatic lathe with automatic thread rolling head, which is suitable for mass production of external threads of standard fasteners and other threaded connectors.
Generally, the outer diameter of the rolled thread is not more than 25mm, the length is not more than 100mm, the thread accuracy can reach Grade 2 (GB 197-63), and the diameters of all blanks are roughly equal to the middle diameter of the processed thread. Generally, internal thread can’t be processed by rolling, but the internal thread can be cold-extruded by groove extrusion tap for soft workpiece (the larger diameter can reach about 30 mm), and the working principle is similar to tapping. The torque required for cold extrusion of internal thread is about 1 times larger than that of tapping, and the machining accuracy and surface quality are slightly higher than that of tapping.
Thread rolling point
1. The surface roughness is less than turning, milling and grinding;
2. The strength and hardness of the rolled thread surface can be improved due to cold work hardening;
3. The material utilization rate is high, the productivity is doubled than that of cutting, and it is easy to be automated;
4. The service life of the rolling die is very long, but the hardness of the workpiece material is required not to exceed HRC40 when rolling the thread;
5. It requires high dimensional accuracy of the blank;
6. The precision and hardness of the rolling die are also high, and it is difficult to manufacture the die;
7. It is not suitable for rolling threads with asymmetrical teeth.
According to the different rolling dies, thread rolling can be divided into two types: thread rolling and thread rolling.
Thread rolling: Two thread-tooth-shaped thread rolling plates are arranged oppositely with 1/2 pitch staggered, the static plate is fixed, and the moving plate makes reciprocating linear motion parallel to the static plate. When the workpiece is fed between the two plates, the moving plate moves forward to rub and press the workpiece, so that its surface is plastically deformed into threads.
Thread rolling: there are three types of thread rolling: radial thread rolling, tangential thread rolling and rolling head thread rolling.
Radial thread rolling: two (or three) thread-toothed thread rolling wheels are installed on parallel shafts, and the workpiece is placed on the support between the two wheels, and the two wheels rotate at the same speed in the same direction.
One of the wheels also moves radially. The workpiece is driven by the thread roller to rotate, and the surface is radially extruded to form threads. For some lead screws with low precision requirements, a similar method can also be used for rolling forming.
Tangential wire rolling: also known as planetary wire rolling, the rolling tool consists of a rotating wire rolling wheel and three fixed arc wire plates.
During wire rolling, the workpiece can be continuously fed, so the productivity is higher than that of wire rolling and radial wire rolling.
Rolling head thread rolling: it is carried out on an automatic lathe and is generally used to process short threads on workpieces. The rolling head has 3~4 wire rollers evenly distributed on the periphery of the workpiece.
During wire rolling, the workpiece rotates, and the rolling head feeds axially to roll the workpiece into threads.