Steering knuckles, important automotive components, are commonly produced in cast or forged blanks. They are typically manufactured using a decentralized processing method, employing vertical machining, multiple processes and fixtures, and standard cutting tools for mass production, as shown in Figure 1. Long-term use has revealed that this process has drawbacks, including the need for multiple clamping operations, high personnel and equipment requirements, large floor space, high energy consumption, and high maintenance costs.
Analysis suggests that the machining of the steering knuckle is characterized by multiple spatial orientations and strict positional requirements between the holes. Figure 1 is a schematic diagram of a typical automotive steering knuckle machining area (the dark area is the part to be machined). To solve the machining difficulties, how can the existing vertical machining + multi-process multi-fixture + standard tooling process be improved separately? These will be discussed in detail below.
Part 1. Process Direction Selection
To address the drawbacks of fragmented processing methods, high efficiency, high automation, and reduced manpower have become essential choices. The integrated application of multi-spindle machine tools, highly complex cutting tools, and five-axis rotary tables is currently the optimal choice for efficient, multi-directional machining of steering knuckle products. Furthermore, achieving single-clamping and centralized processing of steering knuckles is a crucial means of ensuring product consistency.
Part 2. Determination of Equipment, Cutting Tools, and Fixtures
Equipment, cutting tools, and fixtures support and influence each other, and a comprehensive consideration is needed to achieve a given process route.
1. Equipment: Multi-spindle equipment solution
Using a dual-exchange CNC rotary table eliminates the impact of loading and unloading time on cycle time, improving equipment utilization and reducing machining cycle time. In addition to the standard X, Y, and Z axes, two more rotary axes, A and B, are added to the CNC rotary table to achieve multi-directional indexing, solving the problem of multi-space orientation machining. Multi-spindle equipment (Figure 3) is equipped with the same number of fixtures and tools during machining; for a dual-table setup, the number of fixtures should be twice the number of spindles.
2. Tooling system requirements: Use thermally expanded tool holders.
The primary requirement for the tooling system is adaptability to high speeds. The structural characteristics of ordinary BT toolholders cannot meet high precision requirements; therefore, thermally expanded toolholders are more suitable. Simultaneously, tooling can be combined to reduce the number of tools and tool changes, and shorten the toolpath length. Figure 4 shows a double-disc end mill used for simultaneously milling two sides of a certain thickness, and Figure 5 shows a compound boring and milling cutter, enabling simultaneous rough boring and planar scraping of three hole sizes. Taking the production of steering knuckles using a distributed process as an example, a total of 24 tools are required. Under the same conditions, using compound tools can reduce the number of tools to 18.
Figure 4 Double-disc end mill
3. Fixture Design: The core is the selection of positioning points.
For cast or forged blanks, the selection should focus on areas where the casting or forging process easily meets the dimensional requirements, or areas less prone to deformation due to the process. Secondly, the selection of clamping points and clamping forces is crucial. The clamping force and supporting force should be aligned to avoid clamping torque. In the scheme shown in Figure 6, the clamping positions are all directly above the Z-axis movement limit. During finishing, due to the relatively small cutting force, the clamping force can be reduced through secondary pressure application via hydraulic system adjustments, thereby minimizing clamping deformation and improving product machining accuracy.
Using concurrent engineering during product development, and setting necessary process limit points and clamping points, can ideally solve the problems of positioning and clamping point selection. In fixtures that complete all machining operations in one clamping, there are significant cutting forces during the roughing stage; therefore, the rigidity design needs to be carefully calculated and a certain margin should be allowed. Figure 7 shows the digital model and physical object of a fixture designed for completing all parts of a steering knuckle in one sequence using a three-spindle machine.
4. Interference inspection of fixtures, cutting tools and equipment
After the fixtures and cutting tools are designed, the interference between the workpiece, fixtures and cutting tools under the cutting state of the workpiece needs to be verified based on the digital model to ensure that problems are found and solved to the greatest extent possible during the design phase.
Part 3. Process Validation
The verification process of a process method involves comprehensive testing of equipment, cutting tools, fixtures, and software. The rationality of these elements is ultimately reflected in the selection range of various process parameters during machining. For a production line, cycle time is another crucial consideration. When quality requirements increase, it is often necessary to adjust process parameters to reduce feed rate or cutting allowance, which typically increases cycle time. Therefore, ensuring that the parameters of the entire system are selected at a reasonable level while meeting product quality requirements, taking into account cycle time requirements, and guaranteeing that tool wear is within a normal range is key to adjusting process parameters.
Part 4 Steering Knuckle Design
For clamping irregularly shaped parts, the main considerations are centering and clamping stability; contour clamping is the best approach. Steering knuckle clamping solution for passenger cars (sedans):
Process steering knuckle OP-10
Conclusion
For automotive steering knuckles, the aforementioned multi-spindle process integration scheme is a relatively ideal choice. However, the machining of each product requires theoretical analysis of various aspects such as equipment, fixtures, and cutting tools, and the corresponding results must be obtained after extensive experimental verification. The process design of this steering knuckle also has reference value for other types of mass-produced products.
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