The implementation of the “2mm project†in the automotive industry, the establishment of a data-based quality control system, the realization of high-quality body manufacturing, and ultimately the improvement of the mating accuracy of the entire vehicle have been considered as a measure of whether automakers are controlling the body size. Meet the international advanced standards.
In modern automobile manufacturing, the Continuous Improvement Indicator (CII) of body manufacturing is commonly used to control the manufacturing quality of the vehicle body, ie "2mm project." This error index is not the actual deviation of the measurement data of the body manufacturing quality, but a comprehensive evaluation of the dimensional stability indicators of the body manufacturing, not only can be applied to vehicle manufacturing, but also can be applied to component manufacturing. "2mm engineering" applied to the automotive industry can not only achieve economical automobile manufacturing, but also a comprehensive reflection of a country's manufacturing technology level.
The essence of “2mm project†is to establish a data-based manufacturing quality control system, identify manufacturing errors by using manufacturing data modeling and analysis, ensure the stability of the vehicle body manufacturing process, and ultimately improve the accuracy of vehicle assembly. Its core is to adopt advanced body measurement technology and establish a coordinated, consistent, and efficient measurement system from the stamping process, processing equipment to the assembly process. Through data analysis and accumulation, human experience management is upgraded to a scientific management level.
Influence Factors and Control Measures of Body Accuracy
In the process of implementing the “2mm project†by auto manufacturers, the matching of auto bodies has always been the main influencing factor. If the auto body mating accuracy exceeds the specified value, it will affect the auto manufacturing quality, production cycle and product cost. The automobile body matching error mainly involves four reasons (Figure 1), and corresponding measures must be taken to effectively control it.
1, body design quality
In the process of a product from scratch, the quality of the original design will have a decisive influence on the inherent quality of the product. Therefore, the following measures must be taken:
â–¡ Ensure the consistency between car body design benchmarks and process benchmarks.
The design datum should be consistent with the assembly datum and the processing datum, which is conducive to the stability of the quality of the car body matching welding.
â–¡ Use integral stampings.
For the structural parts that are not easy to guarantee the structural size or size requirements, the overall stamping parts should be used as much as possible to reduce the assembly error and welding distortion when the vehicle body is matched.
â–¡ Reasonably consider the vehicle body matching welding method.
Welded joint designs use resistance welding methods as much as possible to reduce welding distortion.
â–¡ Tack welds have a reasonable position and number.
The location and quantity of the tack welds influence whether the body stiffness of the process can guarantee the body quality of the subsequent process. Especially in the transformation of the production line, pay attention to the impact of solder joint migration on the accuracy of the body.
2, the impact of welding fixture
Figure 2 and Figure 3 show the side body assembly fixtures and the roof assembly fixtures, respectively.
Figure 2 side assembly fixture
Figure 3 Canopy assembly fixture
â–¡ The welding fixture's positioning datum should be coincident with the product design datum and the assembly datum. This can eliminate the dimensional error due to different datums, simplify the assembly coordination, and improve the assembly accuracy of the welding fixture.
â–¡ The fixture manufacturing accuracy should meet the design requirements. Before the trial production of the vehicle body, the fixture shall be debugged, whether the inspection fixture can ensure the welding precision, the coordination of the fixture and the parts, and the assessment of the matching quality of the parts.
â–¡ The dimensions of the parts related to the vehicle body matching should be ensured. In particular, the dimensions of the assembly holes, assembly surfaces, and process holes must be strictly controlled because the deformation during the welding of the vehicle body often results in the dimensional deviation of these parts.
â–¡ In the premise that the fixture manufacturing accuracy is ensured, it is also necessary to consider the heat treatment and production maintenance of important positioning pins and surfaces.
3, control the welding deformation
â–¡ Using resistance welding can reduce body welding distortion. Since the resistance welding is an internal heat source, the heating concentration, heat affected zone is small, and high-quality joints are easily obtained.
â–¡ Use advanced welding equipment to reduce body welding distortion. Robot welding can not only make the solder joints even when the body is welded, the spot welding sequence is stable, but also the welding speed can be increased.
â–¡ In strict accordance with the welding process to control the body welding deformation. In the process of product production, a clear welding procedure document must be established and strictly controlled in accordance with the requirements of the welding procedure document.
□ With the development of the “lightweight†direction of the car body, high-strength steel plates have been widely used. This material is very sensitive to manufacturing precision. The deviation of part accuracy not only affects the accuracy of the white car body, but also greatly affects the strength of the car body. influences.
4, the accuracy of parts and the impact of the operation process
â–¡ Improve the manufacturing quality of parts. Due to the fact that many of the components contained in the vehicle are provided by different suppliers, there are differences in the technical level, which can cause cumulative errors in the welding process, affecting the accuracy of the vehicle body and the quality of the vehicle. In addition, the parts produced by the press shop will have a large amount of springback due to the process or mold, which will adversely affect the accuracy of the vehicle body. Therefore, controlling the accuracy of parts is the primary issue for improving the accuracy of the body.
â–¡ Improve the parts and components packaging and logistics methods of all supporting plants. Changes in the status of components in the logistics process are included in the scope of quality management to prevent collisions between components in the transportation process.
â–¡ The impact of the operation process includes the transfer of parts between the fixtures in the production process, as well as manual operation. If the operation is not done properly, the positioning holes and surfaces will be deformed, which will affect the positioning of the subsequent processes and the matching precision between the parts. .
Ideas and Methods to Improve Body Accuracy
1, strengthen equipment management
â–¡ Strengthen the management of welding fixtures, gages, molds, coordinate measuring machines, and robots. During robot teaching, bad welding posture will directly lead to deformation of the parts near the solder joints.
â–¡ Each type of welding shop has nearly 60 to 80 sets of fixtures. Therefore, high-precision and stable fixtures are the basic guarantee for realizing high-precision bodies. In the fixture management process, a fixture management database should be established to facilitate a good grasp of the state of each fixture. In addition, special attention should be paid to the relative management of fixture positioning pins. Some fixtures only analyze the position of a single positioning pin. Accuracy and wear are not enough and must be considered from the perspective of correlation, otherwise high-precision white body cannot be guaranteed. In addition, the maintenance of the fixture should be strengthened, and the metal residue that spatters on the positioning pin and the positioning surface during the welding process should be cleaned in time.
□ Appropriate use of "fixture fixtures". The purpose of the "fixture inspection tool" is to replace the functions of some inspection tools with high-precision fixtures, so as to truly monitor the quality of components in the production process. The premise of “fixture fixtures†is high-precision fixtures, which allows production employees to understand the quality of parts flowing into the process in time, which is a new idea that not only saves costs, but also fully allows all employees to participate in quality management.
2, grasp the process control
Process quality control is an important part of quality management. The main task of process control is to control the quality characteristic value within the specified fluctuation range and make the process under control. Statistical quality control (SPC) is an important method of process quality control. SPC is to randomly sample the overall quality characteristics of the production process and make statistical inferences on the whole. It analyzes and evaluates the production process through the use of control charts, based on feedback information. The system problems are discovered in a timely manner and the process is maintained in a controlled state that is only affected by random factors to achieve the purpose of quality control.
SPC establishes a process-centric quality management system based on graphs or numerical values. The quality of data is very important. Accuracy and reliability of data directly affect whether or not we take appropriate actions when appropriate. There are two main factors:
â–¡ The impact of the measurement system;
â–¡ Record data, calculations and other human influences.
The key to process quality control lies in finding the control point. The quality information of the control point is an important part of the production site quality information, which is managed centrally by workshop quality engineers. The control charts and data records at each control point contain many valuable quality information. The workshop quality engineers collect and summarize them in time and perform statistical analysis with the craftsmen as an important basis for guiding these processes.
3, systematic and scientific analysis of body data
The body data obtained through the three-coordinate measurement must be systematically and scientifically analyzed, and left-right-difference management and correlation management should be used to find out the existing laws among these data. For example, assuming that the deviation is managed by ±1.0 mm, as shown in Fig. 4, when the left side is deviated by -0.8 mm in the X direction and the right side is deviated by 0.8 mm in the X direction, theoretically the left and right side deviations are calculated. All are within the acceptable range, but there is a change of 1.6mm from the left and right side-to-side difference, which inevitably results in the close integration of the front end of the hood and the front fender.
4, TQM strengthens the implementation
TQM (Total Quality Management) is a customer-oriented management method. It emphasizes the effective use of human resources and measurement methods, and improves product and service quality through continuous improvement (Figure 5). TQM is an idea, a set of methods, means and techniques. Through the participation of all employees, improvement of processes, products, services and company culture, TQM can produce 100% of qualified products in 100% time to meet customer needs. The fundamental difference between the basic principles of total quality management and other concepts is that it emphasizes that management must begin by identifying the quality requirements of the customer for the product, and finally the customer is satisfied with the quality of the product. Total quality management is to guide the coordinated activities of people, machines, and information in order to achieve this goal. An important feature of total quality management is the control of quality from the source. For example, it promotes and establishes his sense of responsibility for product quality by measuring his own performance by the operator.
Figure 5 Total Quality Management emphasizes continuous improvement to improve product quality and service quality
Similarly, improving the accuracy of the body alone is not enough to rely on the efforts of quality management personnel. This is a systematic project. It must rely on the concerted efforts of all employees: from the design stage to the production stage, from the parts suppliers to the vehicle manufacturers. The four major workshops, from the quality of the parts and parts to the precision of the body parts, from the process control of each process to the comprehensive analysis of the assembly, require full participation to reach the final goal.
Conclusion
The "2mm project" is not a single product quality evaluation system, but it promotes companies to adapt to global market competition through the evaluation of the effectiveness of the company's "2mm project." Since the vehicle body matching is closely related to the safety and reliability of the entire vehicle, and the vehicle body is also a matching carrier for automobile parts, the quality of its manufacturing has a great influence on the overall vehicle quality.