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 A.D., Mediterranean countries began to apply the principle of bolts and nuts to wine-making presses. At that time, the external threads were all wrapped around a cylindrical rod with a rope and then carved according to this mark. The internal thread is usually formed by hammering with soft material wrapped around the external thread. /kloc-about 0/500, Leonardo, Italy? In the sketch of thread processing device drawn by Leonardo da Vinci, there is an idea of processing threads with different pitches with female screws and exchange gears. Since then, the method of mechanical thread cutting has been developed in European watch manufacturing industry. 1760, British brothers J. Wyatt and W. Wyatt obtained a patent for cutting wooden screws with a special device. 1778, an Englishman J. ramsden made a thread cutting device driven by a worm gear pair, which can process long threads with high precision. 1797, an Englishman Maudslay H. turned metal threads with different pitches by using mother screw and exchange gear on his improved lathe, which laid the basic method for turning threads. 1In the 1920s, Maudslay manufactured 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 thread and the development of various precise and efficient thread processing methods. Various automatic die opening heads and automatic tap shrinking were invented one after another, and thread milling began to be applied. In the early 1930s, thread grinding appeared. Although the wire rolling technology was patented at the beginning of 19 century, it developed slowly because of the difficulty in mold manufacturing. It was not until World War II (1942 ~ 1945) that the precision problem of die manufacturing was solved due to the need of munitions production and the development of thread grinding technology.
1) thread cutting generally refers to the method of machining threads on workpieces with forming tools or abrasives, mainly including turning, milling, tapping, tapping, 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 tapping, the cutter (tap or die) moves relative to the workpiece, and the cutter (or workpiece) is guided by the pre-formed thread groove for axial movement. Thread turning (thread turning) To turn threads on a lathe, you can use a forming lathe tool or a thread comb tool (see thread processing tool). Because of its simple structure, forming turning tool is a common method to produce threaded workpieces in small batches. Using thread comb cutter to turn threads has high production efficiency, but the cutter structure is complex, which is only suitable for turning fine teeth and short threads in medium and large-scale production. Generally, the pitch accuracy of trapezoidal thread turning by ordinary lathe can only reach 8 ~ 9 grades (JB2886-8 1, the same below); Machining threads on a special thread lathe can significantly improve productivity or accuracy.
2) Thread milling: Milling with a disc milling cutter or a comb milling cutter on a thread milling machine. Disc milling cutter is mainly used for milling trapezoidal external thread on screw, worm and other workpieces. Comb milling cutter is used for milling internal and external common threads and tapered threads. Because it is milled by a multi-edge milling cutter, the length of its working part is longer than the length of the thread to be machined, so the workpiece can be machined 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, and the surface roughness R 5 ~ 0.63 microns. This method is suitable for mass production of threaded workpieces with average precision or rough machining before grinding.
3) Thread grinding is mainly used to process the precise thread of hardened workpiece on the thread grinding machine, which 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 can achieve pitch accuracy of 5 ~ 6 and surface roughness of r 1.25 ~ 0.08 micron, which is convenient for grinding wheel dressing. This method is suitable for grinding precision screws, thread gauges, worms, small batches of threaded workpieces and shovel grinding precision hobs. Multi-line grinding wheel grinding is divided into longitudinal grinding method and cut-in grinding method. In the longitudinal grinding method, the width of the grinding wheel is smaller than the length of the ground thread, and the thread can be ground to the final size by moving the grinding wheel longitudinally for one or several strokes. The width of grinding wheel in plunge grinding method is greater than the length of grinding thread. When the grinding wheel cuts into the surface of the workpiece radially, the workpiece can be ground in about 65438 0.25 revolutions, with high productivity, but slightly lower accuracy, and the grinding wheel dressing is more complicated. The cut-in grinding method is suitable for large-scale shovel grinding of taps and grinding of some fastening threads.
4) Thread grinding Nut-type or screw-type thread grinding tools are made of soft materials such as cast iron, and the parts of the workpiece with pitch errors are ground by forward and backward rotation to improve the pitch accuracy. Hardened internal threads are usually ground to eliminate deformation and improve accuracy.
5) Tapping Tapping (tapping with a tap) is to screw the tap into the pre-drilled bottom hole on the workpiece with a certain torque to process internal threads. Set of threads (set of threads with die) is to cut the external thread on the bar (or pipe) workpiece with a die. The machining accuracy of tapping or tapping depends on the accuracy of tap or die. Although there are many ways to process internal and external threads, small-diameter internal threads can only be processed with taps. Tapping and tapping can be done by hand, or by lathe, drilling machine, tapping machine and tapping machine.
6) thread rolling: the workpiece is plastically deformed by a forming and rolling die to obtain threads. Wire rolling is generally carried out on a wire rolling machine. The thread rolling machine is carried out on an automatic lathe with an 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 diameter of the blank used is roughly equal to the middle diameter of the processed thread. Generally, the internal thread cannot be processed by rolling, but for soft workpieces (the maximum diameter can reach about 30 mm), the internal thread can be cold extruded by slotless extrusion tap, and the working principle is similar to tapping. The torque required for cold extrusion of internal thread is about 1 times that required for tapping, and the machining accuracy and surface quality are slightly higher than tapping.
The advantages of wire rolling are: the surface roughness is less than that of turning, milling and grinding; Due to cold working hardening, the strength and hardness of the rolled thread surface can be improved; High material utilization rate; The productivity is twice as high as that of cutting, and it is easy to realize automation; The rolling die has long service life. However, rolling thread requires that the hardness of workpiece material should not exceed HRC40, which requires high dimensional accuracy of blank; The precision and hardness of the rolling die are also high, and it is difficult to manufacture the die; Not suitable for thread with asymmetric hobbing.
According to the different rolling dies, rolling threads can be divided into thread rolling and thread rolling.
Two thread-shaped wire rolling plates are arranged oppositely, and the staggered spacing is 1/2. The static plate is fixed, and the moving plate does reciprocating linear motion parallel to the static plate. When a workpiece is fed between two plates, the moving plate moves forward to rub and press the workpiece, so that its surface is plastically deformed into a thread.
There are three kinds of thread rolling: radial thread rolling, tangential thread rolling and head thread rolling:
Radial thread rolling: two (or three) thread rolling wheels are installed on parallel shafts, and the workpiece is placed on the bracket between the two wheels. The two wheels rotate at the same speed (radial thread rolling), and one of them also performs radial feed movement. The workpiece is driven by the threaded 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 roll forming.
Tangential thread rolling: also known as planetary thread rolling, the rolling tool consists of 1 rotation center.