Thread Content
There are two main methods for processing internal and external threads on workpieces: cutting and rolling. 1) Thread cutting generally refers to the method of processing threads on the workpiece using forming tools or grinding tools. It mainly includes turning, milling, tapping, threading, grinding, grinding and cyclone cutting. When turning, milling and grinding threads, every time the workpiece rotates, 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. When tapping or threading, the tool (tap or die) and the workpiece perform relative rotational motion, and the previously formed thread groove guides the tool (or workpiece) to move axially. Thread turning Thread turning on a lathe can use a forming turning tool or a thread comb (see thread processing tools). Turning threads with a forming turning tool is a common method for single-piece and small batch production of threaded workpieces due to its simple tool structure. Turning threads with a thread cutter has high production efficiency, but the tool structure is complex and is only suitable for turning short threaded workpieces with fine threads in medium and large batch production. The pitch accuracy of trapezoidal thread turning with ordinary lathes can generally only reach level 8 to 9 (JB2886-81, the same below). When threads are processed on a specialized thread lathe, productivity or accuracy can be significantly improved. 2) Thread milling is performed on a thread milling machine with a disc milling cutter or a comb milling cutter. Disc milling cutters are mainly used for milling trapezoidal external threads on screw rods, worms and other workpieces. Comb-shaped milling cutters are used for milling internal and external ordinary threads and tapered threads. Since they are milled with a multi-edge milling cutter, and the length of the working part is greater than the length of the thread being processed, the workpiece only needs to be rotated 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 level 8 to 9, and the surface roughness is R 5 to 0.63 microns. This method is suitable for batch production of threaded workpieces with general precision or rough machining before grinding. 3) Thread grinding is mainly used to process precision threads of hardened workpieces on thread grinders. According to the different cross-sectional shapes of the grinding wheels, there are two types of grinding: single-line grinding wheels and multi-line grinding wheels. The pitch accuracy that can be achieved by single-line grinding wheel grinding is 5 to 6 levels, and the surface roughness is R 1.25 to 0.08 microns. The grinding wheel is more convenient to dress. 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 two types: longitudinal grinding and plunge grinding. The width of the grinding wheel in the longitudinal grinding method is smaller than the length of the thread to be ground. The grinding wheel can be moved longitudinally once or several times to grind the thread to the final size. The width of the grinding wheel in the plunge grinding method is larger than the length of the thread to be ground. The grinding wheel radially cuts into the surface of the workpiece. The workpiece can be ground by turning about 1.25 revolutions. The productivity is high, but the accuracy is slightly lower, and the grinding wheel dressing is more complicated. The plunge grinding method is suitable for relieving large batches of taps and grinding certain fastening threads. 4) Thread grinding uses a nut-type or screw-type thread grinding tool made of soft materials such as cast iron, and performs forward and reverse rotation grinding on the parts of the workpiece where the thread has a pitch error to improve the pitch accuracy. Hardened internal threads are usually ground to eliminate deformation and improve accuracy. 5) Tapping and thread tapping. It uses a certain torque to screw the tap into the pre-drilled bottom hole on the workpiece to process the internal thread. Threading is to use a die to cut external threads on the bar (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 with taps. Tapping and threading can be done manually, or lathes, drill presses, tapping machines and threading machines can be used. 6) Thread rolling is a processing method that uses a forming rolling die to plastically deform the workpiece to obtain threads. Thread rolling is usually performed on a thread rolling machine. Thread rolling machine or automatic lathe equipped with automatic opening and closing thread rolling head, suitable for mass production of external threads of standard fasteners and other threaded connectors. The outer diameter of rolled threads generally does not exceed 25 mm, and the length does not exceed 100 mm. The thread accuracy can reach level 2 (GB197-63). The diameter of the blank used is roughly equal to the medium diameter of the thread being processed. Rolling generally cannot process internal threads, but for softer workpieces, grooveless extrusion taps can be used to cold-extrude internal threads (the maximum diameter can reach about 30 mm). The working principle is similar to tapping. The torque required for cold-extruded internal threads is about twice as large as tapping, and the processing accuracy and surface quality are slightly higher than tapping. The advantages of thread rolling are: the surface roughness is smaller than that of turning, milling and grinding; the strength and hardness of the thread surface after rolling can be improved due to cold work hardening; the material utilization rate is high; the productivity is doubled compared with cutting processing, and it is easy to realize automation; the rolling mold has a long life. However, rolling threads 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 mold are also high; it is difficult to manufacture the mold; and it is not suitable for rolling threads with asymmetrical tooth shapes. According to different rolling dies, thread rolling can be divided into two types: thread rolling and thread rolling. Two thread rolling plates with threaded tooth profiles are arranged relative to each other and staggered by 1/2 pitch. The static plate is fixed and the movable plate makes a reciprocating linear motion parallel to the static plate. When the workpiece is fed between the two plates, the moving plate moves forward to rub the workpiece, causing its surface to plastically deform to form thread rolling. There are three types of thread rolling: radial rolling, tangential rolling and rolling head rolling. Radial thread rolling: 2 (or 3) threaded thread rolling wheels are installed on mutually parallel axes. The workpiece is placed on the support between the two wheels. The two wheels rotate in the same direction at a constant speed, and one of the wheels also performs radial feed motion. The workpiece rotates under the driving of the rolling wheel, and the surface is radially extruded to form threads. For some screws that do not require high precision, similar methods can also be used to roll forming. Tangential thread rolling: Also known as planetary thread rolling, the rolling tool consists of a rotating central rolling wheel and three fixed arc-shaped thread plates. When rolling, the workpiece can be continuously fed, so the productivity is higher than thread rolling and radial thread rolling. Thread rolling head: It is performed on an automatic lathe and is generally used to process short threads on the workpiece. When the rolling head has 3 to 4 rolling wheels evenly distributed around the workpiece, the workpiece rotates, the rolling head feeds axially, and the workpiece is rolled out of threads.