Computer-Aided Fixture Design: An Update
This powerful tool may help industry reduce cost and optimize production-line development
John W. Higgins Professor of Mechanical Engineering
H. Wang and H. Li
Research Associates, CAM-Lab
Worcester Polytechnic Institute
All fixtures are designed to hold a workpiece in position firmly and accurately during a manufacturing process. A company buys a machine tool and uses it for 15–20 years without much change or redesign, except for changes to the programming required to process different workpieces. Fixtures, however, are among the tooling components (with cutting tools, coolant, inspection tools, etc.) that manufacturing engineers (actually even product and production development engineers) must deal with in daily production, to achieve production quality, lowest production cost, and optimum efficiency. Skillfully executed fixture design contributes significantly to product quality, reduced cycle time, and minimum production cost.
Different workshops may use different types of fixtures. Generally speaking, dedicated fixtures are specially designed for highly efficient operations, and are used in mass production lines. Standard fixture components/devices (e.g., vises, chucks, and clamps) are used in the job-shop for producing a variety of products without preparing special fixtures, even though on-machine adjustment time may be necessary to ensure workpiece quality. Modular fixtures are the most practical flexible fixtures, and use standard components to build different configurations for different jobs. Modular fixtures are mostly used in flexible manufacturing systems, small batch production, and new product/production development processes. To accomplish a balance between flexibility and high efficiency, modularized fixtures have been developed for part families.
Today, efficient fixture design depends heavily on experience. On one hand, fixture design is very important to quality and operational efficiency. On the other, fixture design isn't completely based on scientific principles, and doesn't necessarily create a unique solution, particularly when the operational requirements are considered, such as ease of operation.
As CAD/CAM techniques developed, manufacturing engineers sought to use computer technology to assist fixture design. Currently, CAD technology has been widely used in many industries for many purposes, including fixture design. But the general CAD functions are most like to help in engineering drawing, solid model generation, and data management, including creation of CAD models of fixture components, which are usually provided by the fixture vendors. There is no fixture-specific design knowledge, or special packages for fixture design, available in the market.
The computer-aided Fixture Design (CAFD) concept was first proposed by a McDonnell-Douglas (St. Louis) engineer in 1983, with simple functions such as a fixturecomponent database and customized CAD interface manual, basically to save time and effort to conduct a fixture design. Since that time CAFD has developed into quite comprehensive technologies, including
- Fixture planning to determine setups when producing a specific part and locating datum selection within a setup;
- CAD-based fixture-configuration design to design fixture structures using the fixture-component database, and assemble the components into a fixture; and
- Fixturing analysis to verify the quality of a fixture design, for example by determining locating accuracy and fixturing stability.
In particular, CAFD technology has gone through several remarkable stages of development, such as Group Technology (GT) based and knowledge (rule) based CAFD to reuse the knowledge embedded in previous fixture design by using similarity and kinematics analysis to determine and optimize the fixturing positions. In addition, CAD-based design automation, Case-based Reasoning (CBR) technology, and variable fixture design have emerged. Most current research on CAFD is on modular fixture design for machining operations.
Although several generations of CAFD techniques and systems have developed in academia, and have been presented in many publications, industrial application of CAFD is very limited. Generally speaking, industry needs CAFD.
The driving force behind CAFD should come from commercial operations and technical developments, rather than academic research. However, at least in part because of the economic downturn, manufacturing outsourcing, and cost cutting in most industrial operations, investment in CAFD application is quite low. In spite of this situation, the potential benefit from using CAFD in industry is significant.
However, fixture design influences not only the fixture, but also manufacturing system design. For example, many new production lines are designed and implemented every year to make components for new car models in the automotive industry. When a production line is designed, fixtures play essential roles in both cycle-time planning and process-quality validation. To optimize production-line design, many alternative solutions may need to be generated and compared with each other.
Whether a setup design is feasible or optimal most likely depends on the realization of an efficient fixture design. It may involve tolerance stack-up analysis based on locating datum selection, cycle-time estimation for a balanced workload among workstations according to workpiece layout, and process parameter determination. In process parameter determination, the conceptual fixture design and fixturing stiffness should be considered, processing tool accessibility evaluated against the fixture design, and workpiece deformation analyzed under cutting force and relative to a fixture configuration. Therefore fixture design, together with related analysis functions, is important to develop a high-quality production system.
If CAFD technology were fully implemented, it would be much easier to perform such evaluation work in the production planning stage, and production system development time could be much reduced before launching a new production system. It's also possible that the CAFD technology is mature enough to replace the use of testing fixtures in the mass-production ramp-up stage, which will certainly offer significant economic benefit.
Particularly in manufacturing based on a global supply chain, production planning is often conducted at one location and the production line is operated in another. In such a case, computer simulation becomes necessary to ensure optimal results in production system development. Fixture design is part of production line design, and should be done digitally at an early stage of production planning. If any potential mistake can be identified during the early stage, it will lead to significant cost saving, when compared to finding the mistake later on in the production stage.
The common practice, however, is that the fixture design is usually done by second or third-tier suppliers. The production line developer, i.e., the tier-one supplier, needs to specify the fixture design requirement to the fixture- designing companies. But there may not be any scientific way to help them, except to rely on previous experience. On the other hand, the fixture designer may need an analysis or simulation result to convince their customer of the quality of their design, and avoid blame for any quality problems in production. To avoid problems, over-design is very common in fixture design. CAFD technology may help both the suppliers and the OEM optimize fixture design and production line design.
Another industrial practice in supplier-based manufacturing is quoting. CAFD technology may contribute to the rapid generation of conceptual production line design (including conceptual fixture design), with relatively accurate estimation of production cycle time and cost, while satisfying quality requirements. This rapid production-system design at the conceptual level is useful in the business-quoting stage, before going into the more costly details of the processes.
Fixture design is also part of manufacturing process development. In a large aerospace engineering company, a fairly accurate machining-process model and machine-tool structural dynamics model were developed separately. When doing real machining tests, however, the experimental data were quite different from data derived from the computer simulation. The reason was that the fixturing stiffness was the weakest link in the system. The fixture structure cannot be modeled in the planning stage if it was not yet designed and constructed. More surprisingly, the fixturing stiffness might vary under different fastening forces. Again, the CAFD technology can help to model and analyze fixturing stiffness for manufacturing-process optimization. Accessibility is another area of process verification, where the simulation should be integrated with the fixture design model to detect possible interferences.
Fixture design analysis is as important as the design itself. Once a fixture is designed with the CAFD technology, its quality needs to be evaluated. Much research has been done in academia on fixture design analysis. The following functions have been studied and developed.
1. Geometric models have been developed for
- Fixturing constraint analysis. When the fixturing contact points are determined or extracted from a design, the locating characteristics can be checked by simply calculating rank of the locating matrix. Therefore the full constraint, and under or over-locating can be identified. Furthermore, if the fixture is under or over-locating, the problematic direction can be also identified.
- Fixturing accuracy analysis. When the locating point variation (tolerance) is given for each locator in a fixture design, the tolerance stack-up effect on the machined features can be evaluated in terms of tolerance definitions such as parallelism and true position. Inversely, if the machining accuracy of a specific feature is given, the locating tolerance requirement for each locator can be determined based on a sensitivity analysis in the tolerance stack-up analysis. It should be noted that there is no unique solution for the tolerance assignment problem, and some rules may be applied, such as the concept of economic accuracy grade.
2. Mechanics models have been developed for
- Clamping stability analysis. This is to determine whether improper clamping design may destroy the locating contacts when the clamping force is applied. The calculation may not be as easy and straightforward as usually expected when friction force is considered, and the friction force cannot be ignored in most cases of fixturing.
- Dynamic stability analysis. Quasi-static models have been developed to check fixturing stability during machining. When toolpath and cutting-force information are given as a function of time, the locating reaction forces are evaluated for non-negative solutions. The dynamic response of the machining system with a fixture has not been the subject of much study. One challenge is estimation of the dynamic stiffness of the fixture, which has become a separate research topic.
3. Fixturing accessibility was evaluated for
- Fixturing surface accessibility analysis. When a workpiece surface is selected as a locating or clamping surface by using CAFD, it's necessary to check if the locator or clamp can be placed on the surface, and how easily that can be done. A CAD-based evaluation method has been developed to identify the accessible area and obstruction quantitatively for different fixturing functions.
- Loading and unloading accessibility analysis. How easily can the workpiece be loaded into and unloaded from the fixture? A CAD-based simulation method was used to evaluate the time required to load the workpiece into the fixture if the fixture design is varied.
4. Fixturing stiffness was studied on
- Workpiece deformation under clamping and machining force. The fixture design provided boundary conditions that were used to estimate the deformation of the workpiece subjected to the machining force. The results can be used for locating/clamping position optimization.
- Fixture structural stiffness. The fixture stiffness is highly nonlinear due to the contact interfaces between fixture components and the workpiece. A FEA model was established based on the assumed contact stiffness and gap elements, and an experimental method was developed to identify the normal and tangential contact stiffness. It's hoped that the fixture-stiffness information can be obtained digitally when a fixture is designed using CAFD, so process verification can be conducted before production is launched.
Operational requirements are difficult to model in CAFD. Such requirements are important measures of fixture design quality, and CAFD demands that users formulate the requirements as mathematically objective functions, knowledge-based rules, and/or design guidelines to be implemented. Many examples can be given in industrial practice, such as requirements for pre-locating, anti chip-shedding, error proofing, and minimal clamping time. It can be difficult to obtain generalized results in different applications.
CAFD technology has not been widely used in industry. Resolving the following challenges may help overcome the barrier to wide industrial application of CAFD
- Enhance the functions of CAFD with more built-in fixture design knowledge, CAD manipulation tools and a user-friendly interface.
- Integrate CAFD with product design and production information management systems to serve production planning and process verification activities, particularly in production-line development.
- Expand CAFD from being used mainly for machining operations to other processes such as welding, assembly, inspection, forming, and even heat-treatment processes. Expand from mechanical product production to other industrial applications such as electronic product production. Expand the research from an emphasis on modular fixture design to more dedicated fixture functions.
- Develop a viable business model to put CAFD into commercial operations, including workforce training.
CAFD is not a new topic but remains a missing link in CAD/CAM applications.
This article was first published in the April 2010 edition of Manufacturing Engineering magazine.