Threads are widely used for fastening and connecting mechanical parts and transmitting torque. According to their uses, they can be divided into three categories:
- Fastening threads: used for fastening and connecting parts, the main requirements are screwability and connection reliability. Ordinary threads and MJ threads are more common in the selection of thread types.
- Transmission threads: used for transmitting force and displacement such as machine tool screws, requiring smooth transmission and reliable transmission force. Trapezoidal threads are more common in the selection of thread types.
- Tight threads: used for sealing, requiring a tight connection, no water leakage, no air leakage, and no oil leakage. 55° and 60° tapered pipe threads are more common in the selection of thread types.
Since threads are interchangeable and universal, their applications involve various industries. Therefore, thread processing is also crucial.
Thread Processing Technology
Thread refers to continuous protrusions and grooves with the same cross-section formed along the spiral line on the surface of a cylinder or cone. In actual production and processing, there are two main methods for processing internal and external threads: cutting and rolling.
Thread Cutting
Cutting is the most efficient and economical processing method for processing threaded parts. It generally refers to the method of processing threads on workpieces with forming tools or grinding tools, mainly turning, milling, tapping, threading, etc. When turning and milling threads, the machine tool’s transmission chain ensures that the turning tool and milling cutter move one lead accurately and evenly along the axial direction of the workpiece for each rotation of the workpiece: when tapping or threading, the tool (tap or die) and the workpiece rotate relative to each other, and the first formed thread groove guides the tool (or workpiece) to move axially.
Thread Turning
Thread turning is divided into thread processing by ordinary lathe and thread processing by CNC lathe.
In order to cut the required thread on the lathe, it is necessary to ensure that the spindle rotates one circle and the tool holder drives the turning tool to move a pitch of the processed thread. This strict relationship between the spindle and the tool holder of the ordinary lathe is realized by the internal connection transmission chain, that is, by the total transmission ratio of all transmission pairs between the spindle and the lead screw. When processing different thread pitches, it is only necessary to adjust the total transmission ratio of all transmission pairs between the spindle and the lead screw. Since its value is affected by the transmission pair in the transmission chain, there are only a few limited values, so the thread pitch of the turning is limited.
The rapid development of science and technology has put forward high-precision and high-complexity requirements for mechanical products, and the product modification is frequent. This not only puts forward requirements for precision and efficiency for machine tools and equipment, but also puts forward requirements for versatility and flexibility. Especially in industrial sectors such as aviation, shipbuilding, and weapons production, most of the components that need to be processed have the characteristics of high precision and complex shapes. Using ordinary machine tools to process such parts is not only labor-intensive and inefficient, but also difficult to ensure accuracy. Some parts cannot even be processed. In view of this, CNC machine tools will become the mainstream of mechanical processing equipment.
CNC lathe thread processing is based on the detected spindle rotation signal to control the feed of the stepper motor, realize the proportional relationship required for thread turning, and cut out threads that meet the requirements. Its thread processing is completed by combining the control system, spindle system, and drive system. It solves the problems existing in CNC lathe thread turning through the connection of incremental photoelectric encoders, detection, calculation and processing of CNC systems. The pitch compensation calculation method eliminates the pitch processing error. The processed parts are not only high-precision, but also can reduce the labor intensity of the operator and improve processing efficiency. The inevitable trend of CNC lathe thread turning is suitable for processing large quantities of parts with complex shapes.
Thread Milling
Thread milling is generally performed on a special thread milling machine. According to the different structures of the milling cutter used, it can be divided into two methods.
- Milling with a disc-shaped thread milling cutter. This method generally processes large-sized transmission threads. Due to the low processing accuracy, it is usually only used for rough processing, and then fine processing is performed by turning.
- Milling with a comb-shaped thread milling cutter. It is generally used to process short triangular internal and external threads with small pitch. The processing efficiency is high but the accuracy is low.
There are three kinds of movements in the processing process, namely: rapid rotation of the milling cutter; slow rotation of the workpiece. Longitudinal movement of the milling cutter or the workpiece. In batch and mass production, milling is widely used to process threads.
Tapping and Threading
Tapping is to use a certain torque to screw the tap into the pre-drilled bottom hole on the workpiece to process the internal thread. For small-sized internal threads, tapping is almost the only effective processing method.
In single-piece small-batch production, tapping can be done manually with a tap. When the batch is large, it should be processed with a machine tap on a lathe, drilling machine or tapping machine, and the processing efficiency is higher than turning the thread. For example, in the processing of some commonly used nuts, the turner can first drill the bottom hole and cut it off, and then use the tap to process the threaded part, which not only reduces the operator’s processing intensity but also improves product quality and labor efficiency.
Threading is to use a die to cut out an external thread on a bar (or tube) workpiece. The thread diameter of the threading is generally not more than 16mm. This processing method is suitable for processing parts with smaller batches.
The disadvantage of tapping and threading is that the processing accuracy is low, so it is mainly used for ordinary threads with low processing accuracy.
Thread Grinding
It is often used for the processing of hardened threads, such as taps, thread gauges, thread rolling wheels and threads on precision transmission screws. In order to correct the deformation caused by heat treatment and improve the processing accuracy, grinding is necessary. Thread grinding is generally performed on a special thread grinder. Before grinding, the thread can be rough-processed by turning, milling, etc. For small-sized precision threads, they can also be directly cut. The processing range involved is relatively small.
Rolling Thread
Thread rolling is a method of using rolling tools to squeeze and plastically deform metal to form threads. The metal fibers of rolled threads are continuous, while the metal fibers of cut threads are disconnected. From the microstructure analysis, rolled threads have unique advantages:
- Rolling threads can improve surface finish and are not easy to produce waste. 、
- It improves the hardness and strength of the surface metal of the workpiece being processed, especially the surface hardness of the bottom of the tooth has greatly increased, which can improve the tensile strength and shear strength of the thread compared to the turned thread, thereby increasing the wear resistance and service life of the thread.
- It can reduce the consumption of tools, and the production efficiency is several to dozens of times higher than that of cutting threads, and it is easier to realize automation.
- Rolling is a chipless processing that can save materials.
Thread rolling is generally 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 suitable for mass production of external threads of standard fasteners and other threaded connections. The rolling process ensures that the material fibers are not cut, the direction of the metal flow lines is reasonable, there is a cold work hardening effect, and residual stress is left on the surface. The strength of the rolled thread is 30~40% higher than the fatigue strength of the cut thread. If the thread is rolled after heat treatment, its fatigue strength can be increased by 70~100%.
The outer diameter of the rolled thread is generally not more than 25mm, and the length is not more than 100mm. The diameter of the blank used is roughly equal to the median diameter of the processed thread. The hardness of the material cannot exceed the concentration, causing premature fatigue damage. Rolling generally cannot process internal threads. According to the different rolling dies, thread rolling can be divided into thread rolling and thread rolling.
Rolling Thread
Rolling thread is to use two thread rolling plates with thread grooves to squeeze the workpiece so that the required thread is formed on the surface of the workpiece.
When rolling the thread, one of the two thread rolling plates is fixed on the machine tool and the other moves linearly with the machine tool slide. In this way, the workpiece is driven to rotate in a clockwise direction. One stroke of the movable thread rolling plate is just enough to complete the thread extrusion of the workpiece.
Thread Rolling
In thread rolling, two thread rolling wheels are installed on the main shafts on both sides of the thread rolling machine, which are equal to each other. The main shafts drive them to rotate in the same direction and gradually approach each other. According to the structure of general thread rolling machines, one main shaft is fixed and does not move (called fixed main shaft), and the other main shaft can move in the horizontal direction (called movable main shaft).
This processing method needs to be used on the thread rolling machine, and the thread to be processed must be completed by a thread rolling wheel with this thread specification. The operator adjusts the installation of the thread rolling wheel to place the workpiece between the two thread rolling wheels along the feeding direction of the thread rolling wheel, and gently holds it with his hand at the end. The workpiece is between the thread rolling shafts and is driven by them to rotate in the opposite direction. In this way, the thread on the surface of the thread rolling wheel is pressed into the workpiece under the action of radial force, so that the thread is formed on the surface of the workpiece.
The thread processed by thread rolling not only has good surface quality, but also greatly enhances the fatigue strength of the thread, and is suitable for large-scale thread processing in batch production.
Factors to be Considered in Thread Processing
Like other types of surfaces, threads also have certain requirements for dimensional accuracy, shape and position accuracy, and surface quality. Due to their different uses and usage requirements, the technical requirements are also different. Through the analysis of the above multi-thread processing methods, the factors to be considered comprehensively when processing a certain part are.
Workpiece Shape
If you encounter a relatively small internal thread workpiece, the thread processing is generally completed by tapping on a lathe or drilling machine. Bolts and screws are generally rolled on a thread rolling machine. If the workpiece is large, the thread is generally completed on a lathe.
Thread Size Accuracy
If the thread size accuracy requirement is low, you can choose to turn the thread or use a tap and die to process the thread, or mill the thread, such as general bolts and screws. On the contrary, high-precision threads need to be rolled to meet the part accuracy requirements, such as precision bolts.
Workpiece Material and Heat Treatment
Affected by the material of the workpiece and the strength after heat treatment, the thread after heat treatment is difficult to machine. When the thread structure is suitable, rolling is usually used to complete the external thread processing, which is not only easy to process but also can improve the strength of the part.
Production Type, Conditions and Technical Requirements
Sometimes, although bolt-type structural parts can be rolled, because rolling the thread should be the first choice according to the thread strength and surface quality, considering the small batch and only single-piece production, the thread is machined. At the same time, it must also be based on the drawing requirements. Some part drawings require the thread to be rolled. Finally, it is necessary to meet the production conditions of the unit and use the existing tooling equipment of the unit to complete the thread processing as much as possible.
The above mainly discusses the thread processing methods and analyzes the main factors to be considered in thread processing. After understanding several common thread processing methods, we should consider them comprehensively when compiling process instructions, including production type, process equipment, assembly and technical requirements, thread size accuracy selection, heat treatment and other factors. According to the process characteristics of each processing method, we should select the appropriate manufacturing process and previous and subsequent processes to meet the assembly requirements and drawing requirements finally. In this way, we can select the appropriate processing method for the threads in actual production, and strive to complete the thread processing in the simplest, most economical, and most efficient way.
