A machine tool fixture is a device for mounting a workpiece on a machine tool. Its function is to make the workpiece have a correct position relative to the machine tool and the tool, and to keep this position unchanged during the processing. In processing production, whether it is to ensure the quality of processed products, improve production efficiency, or to reduce the labor intensity of workers and ensure safe production, fixtures play a vital role. However, with the continuous development of modern processing technology, the challenges faced by fixture research and development are also increasing .
In the entire production cycle of the fixture, the time spent on fixture design and inspection accounts for a large proportion. Therefore, shortening the time spent on fixture design and inspection and enhancing the reliability of fixture design are the top priorities of fixture research and development. Rapid Prototyping (RP) is an advanced manufacturing technology developed in the late 1990s. It can directly accept product 3D data without using traditional processing tools (such as cutting tools, fixtures, etc.), quickly, directly and accurately convert virtual data models into solid models with certain functions, and realize the manufacturing of new products with complex shapes. It allows designers to experience the design feeling in an unprecedented intuitive way and can quickly verify the rationality, assemblability and aesthetics of product structure, so as to timely discover problems in design and modify and improve the designed products .
In this paper, rapid prototyping technology is used in the research and development of fixtures. By taking advantage of the convenience of rapid prototyping technology, the CAD data of products at any stage of research and development can be materialized for the design, clamping inspection and structural optimization of its supporting fixtures, thus shortening the research and development cycle of fixtures and reducing research and development costs.
Defects in fixture design at this stage
With the rapid development of microelectronics technology, computer technology and automatic control technology, the manufacturing industry is now shifting towards high speed, high precision, high flexibility and intelligence. The concept of parallel engineering and the widespread application of CNC processing technology have greatly improved product quality and shortened the development cycle. However, due to the improvement of processing accuracy, speed and process concentration, the requirements for fixtures are higher.
1. Defects of concurrent engineering fixture design
For concurrent engineering, it is an integrated, concurrent systematic method for designing products and their related processes (including manufacturing processes and support processes). It takes full consideration of all factors in the product design cycle and strives for success at one time, while designing subsequent sub-processes. Sub-processes usually include process design, tooling design, etc..
Fixture design is an important sub-process in the concurrent design process of products. In order to better complete the design work of this sub-process, computer-aided fixture design (CAFD) and the CE-CAFD system design based on PDM proposed by Chen Weifang et al. have played an irreplaceable role. However, concurrent engineering only crosses the original sequential work in time and space, and its subsequent processes are still based on upstream theoretical data. For example, the fixture design of a certain process in the middle is limited to drawings or graphics on the computer, even in concurrent engineering, without reference to prototypes.
2.Defects in fixture design for CNC machining
For CNC machining, as an important part of the modern manufacturing system, especially the machining center, it is a flexible machine tool that integrates milling, drilling, reaming, tapping, boring, etc. It can automatically change tools and perform one-time processing on the workpiece. CNC machine tools that can process multiple processes after clamping . Due to the concentration of processes and automatic tool changing, the time spent on auxiliary operations such as workpiece clamping, measurement and machine tool adjustment is reduced, so that the processing time of the machine tool reaches about 80% of its startup time (only 15% to 20% for ordinary machine tools) %); At the same time, it also reduces the time for disassembly, transportation and placement of workpieces between each process, shortens the product production cycle, and significantly improves economic efficiency .
However, due to the concentration of processes and multi-axis linkage, the motion trajectory during processing is more complex, as shown in Figures 1 and 2, which makes the requirements for the fixture even more stringent. Not only must it adapt to the characteristics of CNC multi-faceted machining, but it must also avoid the interference of the fixture structure, including the components on the fixture, on the tool’s motion trajectory. The fixture structure must not hinder the tool’s multi-sided processing of various parts of the workpiece .
In order to solve these problems, observation method, assembly simulation and processing simulation method are currently mainly used. The so-called observation method relies on the designer’s experience to make judgments. Assembly simulation and processing simulation use some software’s assembly, motion modules and processing modules to simulate. To judge the rationality of the fixture design. However, a suitable fixture needs to be continuously revised in conjunction with actual production to fully meet the needs of actual processing. The above method is carried out in a virtual environment and is inspected in an ideal state. It cannot fully consider various situations during processing. To carry out actual clamping and processing inspection, you must wait for the prototype of the test piece to be processed. A lot of time wasted.
Application of rapid prototyping technology in fixture design
Whether it is concurrent engineering or fixture design for CNC machine tools, the fundamental reason for its defects is the lack of test piece prototypes. Rapid prototyping technology can quickly and directly process the data information of any processing stage of the product into entities, and use this entity as a test piece prototype for related fixture design, clamping inspection and processing inspection, which can make up for the shortcomings in the above fixture design.
Rapid prototyping technology is used in the development of fixtures for parts to be processed, and its main process is shown in Figure 3. In concurrent engineering, the embarrassment of designing fixtures based on drawings or graphics on computers without reference to physical models is avoided. In this way, information exchange between various design stages is realized, and physical models of each stage can be quickly obtained, which is conducive to reducing unnecessary rework in the fixture design process, greatly improving the possibility of successful design and development at one time, and giving full play to the advantages of concurrent engineering.
For CNC machining with complex motion, rapid prototyping technology is used for fixture development and processing of solid models. In addition to facilitating fixture design, it is more important for clamping inspection and processing inspection, because CNC machine tools are mainly used to process complex parts, and their supporting fixtures are generally more complex, which is easy to cause interference between fixture components or between fixture components and cutting tools. With a solid model, actual clamping and processing inspection can be carried out, instead of relying solely on related software for virtual operations. Theory and practice are combined to further ensure the feasibility and rationality of the developed fixture.
Rapid prototyping of models
As shown in Figures 4 and 5, these are the three-view and modeling drawings of a semi-finished part. According to the traditional method, when designing the fixture for the next process, the designer can only rely on his experience and use these three-view and modeling drawings as references, which has many limitations. Using rapid prototyping to produce samples, as shown in Figure 6, can provide designers with intuitive references.
The rapid prototyping machine used is called melt extrusion rapid prototyping machine, model MEM320A, molding material is ABS B601, and support material is ABS S301. The rapid prototyping machine processing process is shown in Table 1.
Using traditional design methods, in order to verify the rationality of the fixture design and produce the prototype of the part at this stage, it is necessary to go through casting blanks, necessary heat treatment, drilling, rough milling and other processes, which takes about several weeks or even longer. However, using rapid prototyping technology, it only takes an hour and a half, which will greatly reduce waiting time, speed up the development and inspection of supporting fixtures, allow parallel engineering to give full play to its advantages, and avoid unnecessary losses caused by interference during CNC machining.
Conclusion
Rapid prototyping technology has been widely used in the fields of electrical appliances, transportation, aerospace, industrial design, medical care, etc. due to its rapidity, high flexibility and high integration. Researchers in the fields of art and architecture have also begun to use RP equipment. In terms of industrial design, it is mainly used for evaluation, review and structural inspection in the product design stage, and for the design and manufacture of molds. When RP technology is used for fixture design, although some CAD surface information will be lost when the three-dimensional model is processed into a surface, that is, the model surface is approximated by a number of plane triangles and converted into the standard file format—STL format, which will cause errors, this does not affect its use in fixture design. RP technology has opened up a shortcut in fixture research and development. This design and inspection based on solid models is more reliable and more convincing.