Setup Planning and Tolerance Analysis
6.1 Introduction
To help realise manufacturing planning for mass customisation, a CAD/CAM platform is needed and the major issues must be recognised and resolved. The main goal of CAMP for mass customisation is to help design feasible and optimal manufacturing plans quickly. Integrated information models are needed to prescribe the relationships between product design and manufacturing. Thus, changes in product design will prompt corresponding changes in manufacturing plans quickly and automatically. Since flexible manufacturing resources are used in mass customisation, such as multi-part fixtures, multi-axis machines and combination cutters, manufacturing planning systems must be designed to deal with these flexible manufacturing resources.
A framework of a comprehensive CAMP system is introduced. It is supported by an automated setup planning method and an information modelling technique, which are presented in the next two sections of this chapter. Fixture design is an important component of the system, but relatively independent in technique. It will be addressed separately. CAMP for rotational parts is not included in this chapter.
6.1.1 Current State-of-the-Art
CAMP can be divided into part information modelling, feature manufacturing strategy, manufacturing resource capability analysis, setup planning, and fixture design. Significant advances in research have been made during the past 3 decades.
Part information modelling. Part information includes geometry information and design specifications (tolerance, surface finish, etc.), which are defined in CAD models or neutral files (STEP, IGES, etc.). Feature technology is widely recognised as a useful tool for representing part information [6.2]. By the use of graph theory, part information can be represented by a feature-tolerance relationship graph (FTG), where parts are composed of features with design specifications described by relationships between these features [6.3]. The remaining challenge is to design a comprehensive part information modelling system that would allow new features to be added without programming effort.
Feature manufacturing strategy. Feature manufacturing method needs to be specified to link candidate manufacturing processes to features. It can be represented in two models. One is associating a list of candidate processes to a feature type. The other one is associating all product features that can be produced by a process type to the process type [6.4]. Both representations are needed in general to define the relationship between features and processes, including cutters and machine tools used in these processes. If a new feature type or process type is added, all the pre- defined feature manufacturing methods have to be updated. The maintenance effort involved in this updating procedure is huge and tedious.
Manufacturing resource capability analysis. Manufacturing resources include machine tools, cutting tools and fixtures [6.5]. In the mass-customisation environment, the challenge is how to evaluate the capabilities of candidate manufacturing resources and derive an optimal process design based on that. No practical solution is yet available to properly cover the enormous varieties of manufacturing resources in the marketplace.
Setup planning. The objective of setup planning is to determine the number of setups needed, the orientation of the workpiece and the machining process sequence
in each setup. The existing research has been focused on the following aspects:
• Setup constraint modelling. Geometric, manufacturing and kinematic constraints have been considered in setup planning [6.6]. Geometric constraints, including feature orientation [6.7][6.8] and tolerance analysis [6.7][6.3] for both rotational and non-rotational parts, have been studied.
Manufacturing constraints have been modelled by manufacturing knowledge such as operation precedence constraints [6.7][6.8] and best practices in industry [6.9].
• Decision-making strategies. Various techniques such as knowledge-based expert systems [6.10], neural networks [6.11], and graph-based analysis [6.3]
have been developed to aid setup planning. The existing research, however, has only taken into account limited manufacturing resource capabilities [6.7][6.11]. None will generate setup plans for mass customisation when multi-axis CNC machines, flexible fixtures and complicated cutting tools are involved.
• Inter-setup tolerance modelling. Currently, graph-based representation has been recognised as an effective tool to describe the relationship between datum and machining surfaces [6.3][6.12][6.13]. By the use of graphs, it is easy to track the tolerance stackup relationships among manufacturing processes.
• Information integration with other modules in manufacturing planning systems. Setup planning has a close relationship with tolerance analysis, fixture design and manufacturing planning. An information exchange standard needs to be established to make the integration more reliable and flexible.
Fixture design. The objective of fixture design is to generate fixture configurations to hold workpieces firmly and accurately during manufacturing processes. Previous work has been focused on an automatic modular fixture design, using standard fixture components to construct different fixture configurations [6.8], dedicated fixture design with predefined fixture component types [6.14], variation of fixture design for part families [6.15], and fixture design verifications [6.16]. In the mass-production or mass-customisation environment, multi-part fixtures are commonly used to achieve optimal cycle time. Optimisation of multi-part fixture design for mass customisation, however, is not well developed. More research is needed for fixture base selection/design, part layout and orientation, and multi-part fixture configuration. Furthermore, manufacturing planning needs to determine an optimal process sequence for all machining features, and to determine the optimal tool path to machine these features as presented in the fixture, based on the process sequence. Currently, no research work is being done in this area.
In summary, the current state-of-the-art technology involves several major limitations that can be discussed on three levels.
1. On the feature level, features and their manufacturing methods are restricted to a predefined format. Adding new features and new processes requires reprogramming.
2. On the part level, setup planning lacks a mechanism to consider both product feature tolerance relationships and flexible manufacturing resource capabilities. Without such a mechanism, realistic setup plans cannot be generated.
3. On the machine level, no research has been conducted with multi-part fixture design and the corresponding global tool-path generation.
A comprehensive CAMP system for mass customisation will be presented. The major contributions of this research are that: (1) new features, processes and manufacturing resources can be added and utilised without extra programming work due to the use of a comprehensive feature, setup, and manufacturing information model, and (2) the best manufacturing practices for a part family are organised on the three distinct levels described above. The manufacturing planning system is therefore modular and expandable so that manufacturing plans for new parts can be generated easily based on existing plans in the part families.