Thread cutting
Generally refers to the method of processing threads on the workpiece using forming tools or abrasive tools, mainly including turning, milling, tapping, threading, grinding, grinding and whirlwind 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 make relative rotational motion, and the previously formed thread groove guides the tool (or workpiece) to move axially.
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Threads can be turned on a lathe using a forming tool or a thread comb (see Threading 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 comb tool has high production efficiency, but the tool structure is complex and is only suitable for medium and large batch production. Turning fine threaded workpieces with short threads. The pitch accuracy of trapezoidal thread turning with ordinary lathes can generally only reach level 8 to 9 (JB 2886-81, the same below); when threads are processed on a specialized thread lathe, productivity or accuracy can be significantly improved.
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Thread milling
Milling is performed on a thread milling machine with a disc or comb cutter. Disc milling cutters are mainly used for milling trapezoidal external threads on screw rods, worms and other workpieces. The comb milling cutter is used for milling internal and external ordinary threads and tapered threads. Since it is milled with a multi-edged milling cutter and the length of the working part is greater than the length of the thread to be processed, the workpiece only needs to be rotated 1.25~1.5 turns to process. Done, very productive. The pitch accuracy of thread milling can generally reach level 8~9. This method is suitable for batch production of thread work with general precision or for rough machining before grinding.
Thread machining cyclone milling cutter
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Thread grinding
Mainly used for processing precision threads of hardened workpieces on thread grinders.
Thread grinding is divided into two types: single-line grinding wheel and multi-line grinding wheel grinding according to the different cross-sectional shapes of the grinding wheel. The pitch accuracy that can be achieved by single-line grinding wheel is 5~6, the surface roughness is Ra1.25~0.08 micron, 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 of 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 being ground. The grinding wheel can be moved longitudinally once or several times to grind the thread back to the final size. The width of the grinding wheel in the plunge grinding method is greater than the length of the thread to be ground. The grinding wheel cuts into the surface of the workpiece radially. The workpiece can be ground in about 1.25 turns. 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.
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Thread grinding
Nut-type or screw-type thread grinding tools are made of soft materials such as cast iron. The parts of the processed threads on the workpiece that have pitch errors are ground in forward and reverse rotation to improve the pitch accuracy. Hardened internal threads are also usually ground to eliminate variations and improve accuracy.
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Tapping and threading
Tapping is to use a certain torque to screw a tap into a pre-drilled bottom hole on the workpiece to process internal threads.
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 performed manually, or lathes, drill presses, tapping machines and threading machines can be used.
God-like tapping technology
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Thread rolling
A processing method that uses a forming rolling die to plastically deform the workpiece to obtain threads. Thread rolling is generally performed on a thread rolling machine, 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 connections.
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 (GB 197-63). The diameter of all blanks is roughly equal to the pitch 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 extrusion of internal threads is about twice as large as tapping, and the processing accuracy and surface quality are slightly higher than tapping.
External thread rolling processing
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Advantages of thread rolling
1. Surface roughness is smaller than that of turning, milling and grinding;
2. The strength and hardness of the thread surface after rolling can be improved due to cold work hardening;
3. The material utilization rate is high, the productivity is doubled compared with cutting processing, and it is easy to realize automation;
4. The rolling die has a long life, but thread rolling requires the hardness of the workpiece material not to exceed HRC40;
5. The requirements for blank dimensional accuracy are relatively high;
6. The precision and hardness requirements of rolling molds are also high, and it is difficult to manufacture molds;
7. Not suitable for rolling threads with asymmetric profile.
According to different rolling dies, thread rolling can be divided into two types: thread rolling and thread rolling.
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 sent between the two plates, the moving plate advances and rubs the workpiece, causing its surface to plastically deform to form threads.
Thread rolling: There are three types: radial thread rolling, tangential thread rolling and rolling head thread rolling.
Radial thread rolling: 2 (or 3) thread rolling wheels are installed on parallel axes. The workpiece is placed on the support between the two wheels, and the two wheels rotate in the same direction and at a constant speed.
One of the rounds also performs radial feed motion. The workpiece rotates driven by 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 wire plates.
During thread rolling, the workpiece can be fed continuously, so the productivity is higher than that of thread rolling and radial thread rolling.
Thread rolling with rolling head: It is performed on an automatic lathe and is generally used to process short threads on the workpiece. The rolling head has 3 to 4 rolling wheels evenly distributed around the periphery of the workpiece.
During thread rolling, the workpiece rotates and the rolling head feeds axially to roll the workpiece out of the thread.
