Table of Contents
Why do we need fixtures?
Whether in traditional manufacturing or modern flexible manufacturing systems, fixtures play a vital role in the manufacturing and processing process. Its main function is to fix and position the workpiece so that it remains stable and precise during processing. Specifically, there are several important reasons for using fixtures:
1. Ensure processing accuracy and quality
Precise positioning: The fixture can accurately position the workpiece to ensure the accuracy of each processing position during processing. This helps to keep the size and shape of the workpiece consistent and ensure the quality of the final product.
Stable clamping: The fixture provides a stable clamping force to prevent the workpiece from moving or vibrating during processing, thereby reducing processing errors and surface defects.
2. Improve production efficiency
Reduce clamping time: The use of fixtures can greatly reduce the clamping and adjustment time of the workpiece and improve production efficiency. For example, the combination fixture can be quickly adjusted to meet the processing requirements of different workpieces.
Increase processing speed: Since the fixture can stably fix the workpiece, the machine can process at a higher speed, thereby improving productivity.
3. Improve worker safety
Avoid holding the workpiece: Holding the workpiece is unsafe during high-speed cutting or heavy processing. The fixture can securely fix the workpiece, reducing the risk of workers directly contacting tools and workpieces.
Reduce worker fatigue: The use of fixtures reduces the need for workers to manually adjust and clamp the workpiece, thereby reducing labor intensity and fatigue.
4. Improve workpiece consistency
Repeat accuracy: The fixture makes the processing position and method of each workpiece consistent, thereby ensuring the consistency of processing. This is particularly important for mass production, which can ensure that the specifications and performance of each batch of products are the same.
Standardized production: The fixture can help to achieve the standardization of workpiece clamping and processing, thereby simplifying the production process and reducing operational errors and the generation of defective products.
5. Save manufacturing costs
Reduce scrap rate: Since the fixture can effectively improve the processing accuracy and consistency, it reduces the scrap caused by inaccurate positioning or unstable clamping, and reduces production costs.
Reduce processing time: Reduce the clamping and adjustment time of the workpiece, increase the processing speed, thereby shortening the production cycle and saving manufacturing costs.
6. Adapt to multi-variety and small batch production
Flexibility: Flexible fixtures and combination fixtures can be quickly adjusted to adapt to the processing needs of different workpieces, which is particularly important for enterprises with multi-variety and small batch production.
Reduce changeover time: Fixtures can be quickly replaced and adjusted, thereby reducing the changeover time between different workpieces and improving the flexibility and response speed of the production line.
7. Enhance the controllability of the production process
Process control: Fixtures can ensure consistency and repeatability during the processing, making the process more controllable, easy to monitor and adjust.
Quality traceability: By using fixtures to fix the workpiece, it is easier to track the quality of the workpiece at each link in the production process, which facilitates quality management and control.
Since a large number of processing operations require clamping, fixture design is very important in the manufacturing system, which directly affects the processing quality, production efficiency and manufacturing cost.
What is a flexible fixture?
A flexible fixture is a workpiece fixing device designed to adapt to multi-variety and small batch production. It can be flexibly adjusted and reconfigured to adapt to the shapes and sizes of different workpieces. The design concept of flexible fixtures is to improve production efficiency, reduce costs and reduce equipment adjustment time, so as to adapt to the needs of modern manufacturing for flexibility and rapid response.
Flexible fixtures came into being with the development of flexible manufacturing systems and computer integrated manufacturing systems (CIMS). With the introduction of CIMS, multi-variety small batch production has received increasing attention, and flexible fixtures that adapt to the needs of such product and production changes are even more indispensable.
Basic steps of fixture design
Fixture design generally includes the following steps:
Installation planning: confirm the positioning, number of clamping times, positioning method of the workpiece in each clamping and processing surface during processing.
Fixture planning: confirm the positioning surface, positioning point and clamping surface and clamping point on the workpiece during processing.
Fixture configuration design: select fixture components and assemble them into a fixture that can clamp the workpiece.
Fixture configuration design has become an important issue in the field of CAFD. With the application of more and more CNC machine tools and machining centers, many processes can be completed in one clamping, which requires reliable fixture configuration design.
Research and development of flexible fixtures
1. Overview of the development of flexible fixtures
The process equipment of the manufacturing process includes the selection and design of tools, fixtures, testing equipment and molds. To reduce the cost of clamping and production preparation time, flexible clamping is essential.
Generally speaking, a flexible fixture refers to the ability of the fixture to adapt changes and continue to be used after the shape and size of the workpiece have changed to a certain extent.
However, the change of the workpiece can be in a small range, that is, in a range of similar shapes and sizes with little change, or in a large range, that is, the shape of the parts is completely different and the size changes are also large.
Therefore, the definition of flexible fixtures is still vague, without clear definitions and boundaries. Generally speaking, it refers to fixtures that are used in conjunction with machine tools and machining centers and have the ability to clamp a variety of different workpieces.
Flexible fixtures include combined fixtures, programmable fixtures, general flexible fixtures, phase change material flexible fixtures and other fixtures.
Various types of fixtures can be obtained by using different phase change materials.
The clamping force in mechanical fixtures is only applied to a very small area, even point contact, while phase change material flexible fixtures are suitable for clamping workpieces with complex surfaces and weak rigidity that are easy to deform.
1.1 Innovative flexible fixtures
Including three types
1) Authentic phase-change fixtures and pseudo-phase change flexible fixtures.
This type of fixture uses the physical properties of the material to make it change from liquid phase to solid phase and then back to liquid phase. The specific application is that when the material is in liquid phase, the workpiece is placed in a container containing the material, and then external stimulation is applied to make the material quickly change to solid phase, clamp the workpiece, and after processing, the phase change material is changed from solid phase to liquid phase, and the workpiece is taken out;
Pseudo-phase change material is relative to the true phase change material. This is to simulate the bidirectional properties of phase change material with a particle fluidized bed, that is, to use a certain physical method to force the material to change between “solid state” and “flow state”, which is called pseudo-phase change, that is, it looks like phase change, but actually no phase change occurs.
The basic principle is that the fluidized bed is filled with fine metal particles, and there are air inlets at the bottom of the bed. Standard fixture components are placed in the bed, and the components are buried in the particles. The workpiece is clamped by gravity and hydraulic chucks. After processing, gas is introduced from the air inlet to loosen the particles. The workpiece is taken out to complete the processing.
2) Adaptable fixtures, which refers to fixtures whose clamping elements can automatically adapt to the shape of the workpiece, that is, they can change their shape when clamping to adapt to the changes of the workpiece.
3) Modular program-controlled fixtures, which are driven by a servo system. The positioning, clamping and supporting elements are installed on a bidirectional movable guide rail, and different fixture configurations are designed through CNC programs.
1.2 Innovation of traditional fixtures
Traditional fixtures can be divided into adjustable fixtures and combined fixtures.
Adjustable fixtures include general adjustable fixtures and special adjustable fixtures. The latter mainly uses grouping technology as a technical tool.
Combined fixtures are a new type of process equipment developed on the basis of component standardization.
2. Combination fixture
This fixture is a new type of process equipment developed on the basis of parts standardization. The combination fixture system contains a variety of components, and can be used for various purposes, such as vertical or horizontal positioning and clamping.
According to the different occasions they are used, combination fixtures can be divided into:
lathe fixtures, drilling machine fixtures, boring machine fixtures and bed fixtures.
According to different uses, it can be divided into eight categories, namely: basic parts, support parts, positioning parts, guide parts, clamping parts, fasteners, auxiliary parts and assembly parts.
Combination fixtures can shorten the production preparation cycle, ensure product quality and improve economic benefits, and are very suitable for multi-variety and small batch production types.
With the rapid development of flexible equipment and processing systems in recent years, highly flexible combination fixtures with fully standardized components have become an essential auxiliary means for manufacturing systems.
There are generally two types of combination fixture systems: T-slot-based and pin-based (also called hole-based) systems.
The combination fixture based on T-slot has been developed and used in manufacturing for more than 50 years. It realizes the precise positioning of the workpiece through the mutually perpendicular and parallel T-slots on the base. In this system, when the fixture components are fastened on the same row of T-slots, the assembly order of the fixture components should be arranged reasonably.
There are many combination fixture systems based on T-slots in the world, and the more famous ones are: Wharton Unitool (England), YCT (Russia), CATIC (China), Halder (Germany) systems.
The combination fixture based on pins mainly realizes the precise positioning and fastening of the fixture components through the positioning holes on the base. The main systems are: BlucoTechnik (Germany, the United States), Kipp (Germany), Qu-Co (the United States), Stevens (the United States), CarLane (the United States), SAFE (the United States), TJMGS (China), CPI (Russia).
Modular fixtures are flexible clamping methods with great development prospects in both FMS and CIMS. A key technology for the application of modular fixtures is computer-aided fixture technology (including CAFD, fixture design verification and fixture management).
To adapt to agile manufacturing, a new fixture concept and clamping system has emerged, namely agile fixtures, which are the extension and development of traditional modular fixtures and flexible fixtures.
It is increasingly valued by the fixture industry due to its good reconfigurability, good scalability, good openness, strong adaptability, fast response to new products and reconstruction speed.
It is of great significance to improve the ability of manufacturing systems to quickly respond to product changes, shorten product design and manufacturing cycles, increase the flexibility of manufacturing systems, reduce costs, and improve product quality.
Computer-aided fixture design (CAFD)
1. Development of CAFD
In the past decade, the manufacturing research community has focused on the development and improvement of aspects such as computer-aided design/computer-aided manufacturing (CAD/CAM) and computer-aided process planning (CAPP).
Only in the past 20 years has CAFD developed into an important component of CAD/CAM integrated technology and an important aspect of CAPP. It is the link between design and manufacturing in the CIMS environment.
With the establishment of CAD/CAM systems in industry, CAFD is naturally applied to fixture design. And because the fixture has a great impact on manufacturing costs and manufacturing cycles, researchers began to notice the important role of CAFD and regarded shortening the processing preparation cycle as an important goal of CAFD.
At this stage, Fuh et al. introduced an interactive CAFD software, which has a fixture component library and a fixture assembly sequence selected by menu. This is the first generation of interactive CAFD (I-CAFD) fixture design system that cooperates with CAD software, uses CAD’s ability to process graphics, and is connected to the user’s experience.
The purpose of developing I-CAFD is:
1) More designers can interactively design fixtures to complete more complex fixture design tasks;
2) Shorten the development cycle by providing geometric operation tools;
The second generation of CAFD developed in the mid-1980s produced two types of CAFD systems based on variant and generative methods: group technology (GT) and knowledge (KB).
In the variant fixture design method, workpieces belonging to the same component family are considered to have similar processing features, similar operations and installation sequences, so designers can obtain the fixture design for another similar workpiece by slightly changing the existing design.
Through the coding system of group technology, the original similar fixture elements in the fixture library are adjusted accordingly to adapt to the processing requirements of the new workpiece.
When it is impossible to restore a similar fixture, the generative fixture design method can be used to generate a new fixture design scheme, and the newly generated fixture code can be added to the fixture library for reference for the next design.
The CAFD developed after the 1990s is the third generation. The characteristic of CAFD in this era is that it has turned to the software design of fixtures with the purpose of product fixture structure and actual production application as the orientation.
At present, the development of CAFD has greatly reduced the manufacturing development cycle, optimized the manufacturing process, verified the manufacturing process flow, and is playing an increasingly important role in FMS and CIMS.
Computer-aided fixture design is mostly focused on fixture planning research, including the algorithm proposed by DeMeter for selecting positioning and clamping positions: Menasa, DeVries studied fixture planning based on kinematic analysis;
The rule-based system developed by European scholars Pham, de Sam Lazaro, etc., which is used to design combined fixtures for prismatic workpieces: Hargrove SK, Kusiak A made a comprehensive review of the development of contemporary CAFD in the literature and proposed future development trends and needs.
Knowledge-based CAFD is to organize the knowledge of experts into rules and obtain various decisions based on the reasoning mechanism of the expert system.
Since rule application is not very realistic, ShuHuang Sun and JaHau Lewis Chen proposed an intelligent fixture design system based on case-based reasoning (CBR) to address the problems that rule-based expert systems cannot solve:
1) The knowledge in the design is too complex to be simplified into rules. Rules are accumulated from similar situations in the past, while design is carried out for specific cases. It is difficult to complete the design of specific cases through rules;
2) Experts cannot express their knowledge clearly;
3) For knowledge-based expert systems, knowledge acquisition is still an obstacle. The proposed system stores experience as cases. When it faces new problems, it finds the most similar case from previous cases and modifies the case to meet the new situation.
After the problem is solved, this new solution is stored as a case in the case library and can be applied again when similar situations arise.
2. Main research areas in the field of CAFD
1) Classification methods based on group technology and case-based reasoning in fixture design;
2) Determination of positioning points and clamping points through kinematic analysis;
3) Selection of positioning surfaces and clamping surfaces using knowledge-based expert systems;
4) Fixture planning based on geometric analysis;
5) Precision relationship analysis for positioning reference selection;
6) Configuration design of combined fixtures.
Development of Automated Fixture Design (AFD)
In recent years, the design of modular fixture systems has received widespread attention in the fixture industry, and the latest developments in this field have been reviewed in some literature.
Tao, Kumar, and Nee et al. verified the problem of force locking in fixture configuration by geometric calculation methods, and proposed a geometric reasoning method in determining the optimal clamping point and clamping sequence.
This method is very simple and effective in determining the optimal clamping point from the candidate clamping point layout after considering the force locking.
UtpalRoy and JianminLiao discussed the geometric reasoning mechanism in the automatic fixture design system, and introduced a method based on qualitative and quantitative reasoning. Through deformation analysis (one is the contact deformation caused by clamping, and the other is the bending deformation of the workpiece caused by cutting force), the support and clamping positions are rearranged as required to design the best support, positioning and clamping positions on a given workpiece, and complete the function of firmly and accurately clamping the workpiece during processing. Such a reasoning mechanism was implemented in the automatic fixture design prototype system.
The AFD system provides an intelligent automatic fixture design environment, which consists of 4 main modules:
1) fully informationized product model;
2) knowledge base;
3) reasoning mechanism;
4) final fixture configuration.
According to the degree of automation, fixture design systems are divided into: interactive, semi-automatic and automated.
An interactive fixture design system is a system in which the computer provides an informationized user interface for users, and assists users in selecting suitable fixture components based on the designer’s knowledge. Since the system requires users to select clamping surfaces, clamping points and fixture components according to the geometry and processing requirements of the workpiece, it is very time-consuming and has not fully developed the functions of the computer;
The semi-automatic fixture design system is improved on the basis of the interactive system, which reduces the requirements for the designer’s professional knowledge. The automated fixture design system is used to further improve the efficiency and quality of the fixture design process, and can automatically determine the clamping points and automatically select accurate clamping points from a series of candidate point lists.
Kumar, Fuh, Kow, etc. developed an automatic modular fixture design system based on the application of advanced CAD and AI technology in modular fixtures, using CAD methods and 3-DCADICAM software technology.
The system is integrated with an interactive and semi-automatic fixture design system. In addition to automatically determining the clamping point, it can also use a machining interference detection submodule to prevent interference between the cutting tool and the fixture during machining.
Hu and Rong proposed a fast interference detection algorithm, which simplifies the fixture component model into a two-dimensional wheel model with high information. It is different from the detection process using filled entities and is more effective in fixture verification.
Conclusion
Looking back at the development of the fixture industry and the latest research results, despite more than a decade of research, there is still no mature commercial fixture software in the world.
Therefore, the development of practical fixture CAD software should be the top priority in the current research of manufacturing software. A comprehensive and comprehensive automatic fixture design system that integrates CAD/CAMI/CAE technology with CNC programs and can detect interference between cutting tools and combined fixtures is the future development trend.
Due to the complexity of fixture design and the ever-changing actual situation, it is difficult for AFD research to make a major breakthrough in a short period of time. There is still a lot of work to be done to achieve application in actual production.