Thread machining is a method of machining various internal and external threads using thread machining tools.
1. Thread cutting
Thread machining usually refers to the process of creating threads on the surface of a workpiece using special tools or dies. This process covers a variety of technologies, such as turning, milling, tapping, threading, grinding, grinding, and whirlwind cutting. During turning, milling and grinding operations, every time the workpiece rotates one circle, the machine tool's transmission system ensures that the tool (whether it is a turning tool, milling cutter or grinding wheel) accurately and evenly advances a lead distance along the axial direction of the workpiece. When tapping or threading, there will be a relative rotation between the tool (i.e., tap or die) and the workpiece, and the initially formed thread groove is used as a guide to drive the tool (or the workpiece itself) to move in the axial direction.
2. Thread turning
When machining threads on a lathe, you can choose to use a forming turning tool or a thread combing tool to complete the turning process. Using a forming turning tool to turn threads is a common method, especially suitable for single-piece or small-batch production, because it has the advantage of a simple tool structure. Although the use of thread comb cutters to turn threads can improve production efficiency, due to its relatively complex tool structure, it is more suitable for medium and large-scale production, especially for turning fine-tooth and short-length threaded workpieces. For ordinary lathes, the pitch accuracy that can be achieved when turning trapezoidal threads is usually limited to 8 to 9 levels. However, if a specialized thread lathe is used for processing, it can not only significantly improve the production efficiency of the thread, but also achieve greater improvement in accuracy.
3. Thread milling
In the thread milling process, disc milling cutters and comb milling cutters are two commonly used tools on thread milling machines. Disc milling cutters are often used to mill trapezoidal external threads on workpieces such as screws and worms. The comb milling cutter is more versatile and can mill internal and external ordinary threads and tapered threads. Since the comb milling cutter adopts a multi-blade design and its working length exceeds the length of the processed thread, the entire milling process can be completed by rotating the workpiece 1.25 to 1.5 times, which greatly improves production efficiency. The pitch accuracy that can be achieved by thread milling can generally reach 8 to 9 levels, and the surface roughness can be controlled between R5 and 0.63 microns. This method is very suitable for batch production of threaded workpieces with general precision requirements, or as a rough processing stage before grinding.
4. Thread Grinding
Thread grinding is mainly used for precision thread processing of hardened workpieces on thread grinders. According to the different shapes of the grinding wheel cross-section, it can be divided into two methods: single-line grinding wheel grinding and multi-line grinding wheel grinding. The pitch accuracy that can be achieved by single-line grinding wheel grinding is as high as 5 to 6 levels, and the surface roughness can be controlled within the range of R1.25 to 0.08 microns, and the dressing of the grinding wheel is relatively simple. This method is very suitable for grinding threaded workpieces with high precision requirements such as precision screws, thread gauges, worms, etc., and is also suitable for small batches of threaded workpieces and precision hobs. Multi-line grinding wheel grinding is further subdivided into longitudinal grinding and plunge grinding. The longitudinal grinding method uses a grinding wheel with a width less than the length of the thread to be ground. The thread can be ground to the final size by moving the grinding wheel longitudinally once or multiple times. The plunge grinding method uses a grinding wheel with a width greater than the length of the thread to be ground. The grinding wheel radially cuts into the surface of the workpiece, and the workpiece only needs to rotate about 1.25 turns to complete the grinding, so the productivity is higher. However, the precision of the plunge grinding method is slightly lower, and the dressing process of the grinding wheel is relatively complicated. This method is more suitable for grinding a large number of taps and grinding certain fastening threads.
5. Thread grinding
Using relatively soft materials such as cast iron, nut-type or screw-type thread grinding tools can be made. These tools are used to perform forward and reverse rotation grinding operations on the threaded parts of the workpiece that have been processed and have pitch errors. The purpose of this is to improve the pitch accuracy of the thread. For hardened internal threads, grinding can also be used to eliminate possible deformations, thereby further improving their accuracy.
6. Tapping and Threading
Tapping is the process of turning a tap into a pre-drilled bottom hole on a workpiece with a certain torque to produce an internal thread.
Threading is a process that uses a die to cut an external thread on a bar (or tube). The accuracy of this processing method mainly depends on the accuracy of the tap or die used. Although there are many methods for processing internal and external threads, taps are the only processing tools for small diameter internal threads. Both tapping and threading operations can be completed manually or with the help of mechanical equipment such as lathes, drilling machines, tapping machines or threading machines.
7. Thread Rolling
Thread rolling is a processing technology that applies pressure to the workpiece through a forming rolling die to cause it to undergo plastic deformation and thus form a thread. This process is usually carried out 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 very suitable for mass production of standard fasteners and other external parts that require threaded connections.
The size limit of rolled threads is usually no more than 25 mm in outer diameter and no more than 100 mm in length, but the obtained thread accuracy can reach a very high level 2 standard. The diameter of the blank for rolling is usually close to the mean diameter of the thread to be processed.
It is worth noting that although rolling technology is mainly used for the processing of external threads, for workpieces with softer materials, a slotless extrusion tap can still be used for cold extrusion of internal threads (maximum diameter of about 30 mm). The working principle of this cold extruded internal thread is similar to tapping, but the torque required is about 1 times that of tapping, and the precision and surface quality of the processing are also slightly higher than tapping.
The advantages of thread rolling are:
① The surface roughness is less than that of turning, milling and grinding;
② The surface of the rolled thread can improve strength and hardness due to cold work hardening;
③ High material utilization;
④ The productivity is doubled compared to cutting processing, and it is easy to automate;
⑤ The rolling die has a long life. However, thread rolling requires that the hardness of the workpiece material does not exceed HRC40; the roughcast size accuracy requirements are high; the rolling die accuracy and hardness requirements are also high, and the mold manufacturing is difficult; it is not suitable for rolling threads with asymmetric tooth shapes.
According to the different rolling dies, thread rolling can be divided into two categories: thread rolling and thread rolling.
The thread rolling process uses two thread rolling plates with thread tooth shapes, which are arranged relative to each other in a half-pitch offset manner. Among them, one plate remains stationary as a static plate; and the other plate performs reciprocating linear motion parallel to the static plate as a dynamic plate. When the workpiece to be processed is fed between the two thread rolling plates, the dynamic plate will move forward and exert pressure on the workpiece, forming threads on the workpiece surface by plastic deformation.
There are three types of thread rolling: radial thread rolling, tangential thread rolling and rolling head thread rolling.
① Radial thread rolling: 2 (or 3) thread rolling wheels with threaded tooth profiles are installed on parallel shafts, the workpiece is placed on the support between the two wheels, the two wheels rotate in the same direction at the same speed, and one of the wheels also performs radial feed motion. The workpiece rotates under the drive of the thread rolling wheel, and the surface is radially extruded to form threads. For some screws with low precision requirements, similar methods can also be used for rolling.
② Tangential thread rolling: also known as planetary thread rolling, the rolling tool consists of a rotating central thread rolling wheel and 3 fixed arc-shaped thread plates. When rolling, the workpiece can be fed continuously, so the productivity is higher than thread rolling and radial thread rolling.
③ Thread rolling head thread rolling: 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 circumference of the workpiece in the rolling head. When rolling, the workpiece rotates, and the rolling head feeds axially to roll the workpiece into threads.(来源：UG学习堂小胥收徒)
