Bevel gears are important components that can transmit power at a stable transmission ratio. They are widely used in mechanical products such as automobiles and tractors due to their high transmission efficiency, high load-carrying capacity, smooth transmission, wear resistance, and compact structure. In order to avoid the involute tooth surface of bevel gears from being damaged and to reduce noise and impact, it is necessary to perform top trimming on the bevel gears.
Introduction
In the processing of bevel gears, due to the different modules at both ends of the gear teeth, the tooth thickness and tooth height are also different. Therefore, the current gear cutting technology cannot form the top trimming like cylindrical gears during the cutting process. In the manufacturing process of bevel gears, the processing quality of the tooth surface is usually judged by adjusting the position, size, and shape of the contact area of the meshing gears, and the tooth surface detection is relatively less. In practical applications, the top part of the tooth does not participate in the meshing, and the force and motion are mainly transmitted by the middle part of the gear teeth. For the above reasons, the top trimming technology of bevel gears has not been widely applied in the gear processing industry in the past. However, with the increase of the gear load and linear velocity during meshing, the top trimming technology of bevel gears has gradually begun to be used.
Problems with the Current Top Trimming Technology for Bevel Gears
Inability to Perform Top Trimming During Gear Cutting
During the processing of bevel gears, it is impossible to achieve top trimming simultaneously with gear cutting due to the different modules at both ends of the gear teeth and the varying tooth thickness and height. In the manufacturing process of bevel gears, the quality of the tooth surface is typically judged by adjusting the position, size, and shape of the contact area of the meshing gears, with less emphasis on tooth surface detection. In practical applications, the top part of the tooth does not primarily engage in meshing, and the force and motion are mainly transmitted through the middle section of the gear teeth. Due to these reasons, the top trimming technology for bevel gears was not widely used in the gear processing industry in the past. However, as the load and linear velocity of the gears during meshing have increased, the top trimming technology for bevel gears has gradually gained more attention and application.
Manual Grinding for Top Trimming
Currently, due to technological limitations, it is not possible to achieve top trimming on spiral bevel gears during the gear cutting process. The common method for top trimming of spiral bevel gears is manual grinding. Using an angle grinder as a tool to perform 钳工修缘 on the tooth top of the spiral bevel gear, as shown in Figure 1. However, manual grinding has several drawbacks. Firstly, the consistency and uniformity of the trimming size are poor, as it relies on experience. Secondly, the efficiency and accuracy are low, and the labor intensity is high, making it unable to meet the requirements of mass production. Additionally, manual grinding presents the following issues: 1. During the process, the grinding wheel and iron filings produce dust, which affects the normal breathing of the operator. 2. The friction between the grinding wheel and the workpiece generates a significant amount of heat, resulting in a strong odor and an unpleasant smell at the site. 3. Generally, pneumatic (electric) grinding wheels are used in manual trimming, which causes loud noise and poses safety hazards.
| Problem | Description |
|---|---|
| Inability to Perform Top Trimming During Gear Cutting | Due to the different modules, tooth thickness, and tooth height at both ends of the bevel gear teeth, the current gear cutting technology cannot achieve top trimming simultaneously. |
| Manual Grinding for Top Trimming | Common for spiral bevel gears, but has issues such as poor consistency, low efficiency, high labor intensity, dust, heat, odor, noise, and safety hazards. |
Technical Solution for Top Trimming of Bevel Gears
Technical Difficulties
There are several difficulties in performing top trimming on bevel gears under traditional conditions:
- Tooth Shape Issue: The parts that require top trimming are spiral bevel gears with an involute tooth profile. It is challenging for the cutting tool to follow the involute shape during the trimming process.
- Machining Program Issue: Direct programming on the machining center is extremely difficult because there is a certain error between the machined spiral bevel gear part and the theoretical model. Standard parameters cannot be directly used in the machining program. Therefore, the program needs to be generated in a computer and undergo simulation.
- Three-Dimensional Model Issue: There are many varieties of bevel gear parts, and individually modeling each one takes a long time. Therefore, a method is needed where by inputting various parameters, the target model can be directly generated.
Technical Principle
Through secondary development in the NX software, interfaces are established for various bevel gears. By inputting relevant parameters, models of various left-handed and right-handed bevel gears can be generated. Using the machining simulation function of the NX software to simulate the tool path and generate the machining program, which is then imported into the vertical machining center. Chamfering processing is carried out using five-axis and four-linkage. The technical route adopted in the project is shown in Figure 2.
| Step | Description |
|---|---|
| Research | Analyze the gear trimming technology and the product structure of the gear factory. |
| Select Typical Parts | Choose representative bevel gear parts for the top trimming experiment. |
| Software Secondary Development | Develop a small plugin in the NX software to input the adjustment parameters of the actual machining machine for the spiral bevel gear and obtain different models. |
| Machining Center Technology Research | Study the capabilities and limitations of the machining center for the top trimming process. |
| Establish Three-Dimensional Model | Create 3D models of the selected bevel gear parts using the NX software. |
| Tooling Design and Manufacturing | Design and manufacture two sets of hydraulic fixtures for the experiment, ensuring accurate positioning and clamping of the workpieces. |
| Generate Machining Program | Input chamfering parameters and milling cutter parameters in the NX software to obtain the machining G code for top trimming. |
| Tool Preparation | Select and prepare the appropriate cutting tools for the experiment. |
| Typical Parts Experiment | Conduct the top trimming experiment on the selected bevel gear parts. |
| Experiment Data Analysis | Analyze the data from the experiment to evaluate the performance of the top trimming process. |
| Improvement | Make improvements based on the analysis of the experiment data. |
| Form Achievement | Finalize the top trimming technology and its application process. |
Experimental Preparation
Software Preparation
A small plugin is designed based on the NX software. By inputting the actual machining adjustment parameters of the spiral bevel gear, different models can be obtained, as shown in Figure 3. Three-coordinate data is collected for a certain tooth surface of the actual product and compared with the built model. The error is found to be within 0.1 mm. By continuing to input the chamfering parameters and milling cutter parameters, and with simple human-computer interaction operations, the machining G code for the top trimming of the spiral bevel gear can be obtained. The parameters that need to be input include the number of teeth of the large and small wheels, the module, the pressure angle, the spiral angle, the tooth width, and the gear blank, which can all be directly obtained from the drawing, and the NX software directly reads and calculates the results.
| Software | Function |
|---|---|
| NX Software with Plugin | Generate models based on input parameters, compare with actual product data, and generate machining G code. |
Hardware Preparation
Rotary Table and Installation
The rotary table has two CNC axes: the swing axis and the rotation axis. Only the swing axis is linked with the machine tool. To ensure machining accuracy and efficiency, in the bevel gear trimming scheme, the rotation axis performs indexing motion and is not linked with the machine tool. The swing axis is linked with the machine tool to ensure that the cutting point is always on a plane, as shown in Figure 4.
| Hardware | Description |
|---|---|
| Rotary Table | Has two CNC axes (swing axis and rotation axis). The swing axis is linked with the machine tool to ensure the cutting point is on a plane during machining. |
Fixture Design and Manufacturing
Based on the conditions of the machine tool, rotary table, and parts, two sets of hydraulic fixtures for the experiment are manufactured. The fixtures are equipped with positioning pins to determine the position of the tooth slot of the part, as shown in Figure 5. The hydraulic fixtures can automatically clamp the workpiece, eliminate the gap between the fixture and the workpiece, and ensure accurate positioning, thereby guaranteeing that the workpiece is clamped in place and achieving good consistency in processing the parts.
| Hardware | Description |
|---|---|
| Fixture | Two sets of hydraulic fixtures with positioning pins to ensure accurate positioning and clamping of the workpieces. |
Tool Selection
Two types of cutting tools are selected for the experimental machining: one is a finger-shaped chamfering cutter with a taper of 45° and a diameter of ϕ16 mm; the other is a two-edge ball-end carbide milling cutter with a diameter of ϕ12 mm, which is a standard ball-end milling cutter, as shown in Figure 6.
| Hardware | Description |
|---|---|
| Cutting Tools | Two types of cutters: finger-shaped chamfering cutter and two-edge ball-end carbide milling cutter. |
Top Trimming Experiment
Part Installation and Positioning
The CNC rotary table is installed in the middle of the workbench of the vertical machining center, the fixture is installed on the rotary table, and the part is installed on the fixture. The positioning pin on the fixture is aligned with one tooth slot of the part, and then the part is clamped, and the positioning pin is withdrawn.
Tool Setting
The machining program generated by the secondary development of the NX software is imported into the vertical machining center, and the tool is set according to the tool setting method of the CNC machine tool. For the right-handed driving bevel gear, the concave tooth surface is used as the reference, and the angle formed by the center of the tool and the line connecting the center of the face cone when cutting the convex tooth surface is compared with the angle generated by the model to adjust the milling position of the convex tooth surface. For the left-handed driven bevel gear, the convex tooth surface is used as the reference during tool setting, and the tool setting angle of the machine tool is compared with that of the model to correct the milling position of the concave tooth surface, while other parameters remain unchanged.
Debugging
The program is run for trial cutting, and the machining parameters are adjusted based on the surface roughness of the trial-cut part and the wear of the tool. Other error compensations are made according to the dimensional error: the spiral angle is corrected by modifying the machine code; the size of the chamfer is determined by modifying the offset of the A-axis and the Z-axis, or by modifying the tool length compensation.
The entire process consists of tool engagement – cutting – tool disengagement – indexing, and it is carried out in a cycle. During cutting, the positions of the y-axis and z-axis of the machine tool remain unchanged, and only the x-axis moves, while the rotary table rotates the A-axis. When the tool engages and disengages, the spindle moves in the z-axis direction. During the indexing of the rotary table, the position of the spindle remains unchanged. The top trimming experiment is shown in Figure 7.
| Experiment Step | Description |
|---|---|
| Part Installation and Positioning | Install the rotary table, fixture, and part on the machining center, ensuring accurate positioning. |
| Tool Setting | Import the machining program and set the tool based on the characteristics of the bevel gear. |
| Debugging | Adjust machining parameters based on trial cutting results and make error compensations. |
Conclusion
By performing the top trimming of bevel gears on a five-axis and four-linkage vertical machining center, the tool path and CNC program are automatically generated, reducing the difficulty of manual programming and improving work efficiency. The trimming error is controlled within 0.3 mm, and the machining effect is good. This technology is not only applicable to the typical parts in this project but can also be extended to other gear products, having certain promotion and application value in the gear manufacturing industry.
In future research, we can further explore the optimization of the top trimming process to improve the quality and performance of bevel gears. Additionally, the development of more advanced machining technologies and equipment can also enhance the efficiency and accuracy of the top trimming process. Furthermore, the study of the relationship between the top trimming parameters and the performance of the gear transmission system can provide more theoretical support for the design and manufacturing of bevel gears.