Feature Based Process Planning
ME 459 Advanced Topics in Design and Manufacturing
(Extract from online paper by Otto Willem Salomons )
1. Process planning
Process planning involves the preparation for the manufacture of products. Process planning deals with the selection and definition of the processes that have to be performed in order to transform raw material into a given shape [Ham88]. Process planning in part manufacturing includes [Kals 89]:
- the interpretation of the product model. - the selection of machine tools. - the selection of tool sets. - the determination of set-ups. - the design of fixtures. - the determination of machining methods. - the selection of cutting tools. - the determination of machining sequences. - the calculation of tool paths. - the calculation of cutting conditions. - the generation of NC programs. - capacity planning.Until recently these tasks have mainly been performed by human experts. This work - as in design - involves both creativity and skill. When creativity and skill are not sufficient to solve a process planning problem, the process planner will usually communicate the problem with the designer in order to try to solve the problem. During this communication, the (design) object is the basis for communication and the focus of attention; the design process and the design process knowledge, may have to be consulted during the communication.
The degree of detail of a process plan varies from industry to industry. The job-shop type of manufacturing environment usually requires the most detailed process plans since the design of tools, jigs and fixtures and manufacturing sequence etc. are dictated directly by the process plan [Chang 90].
When addressing process planning, in this thesis a small batch manufacturing environment is assumed. In such an environment, the production facilities are subjected to relatively small changes over time. This means that processes, machine tools, cutting tools, measuring machines etc. are not subjected to great changes over time. Therefore, designers must take into account these manufacturing constraints. In mass production, the manufacturing process is designed concurrently with product design. In such a case, the constraints that are imposed by the existing manufacturing environment are less stringent.
A number of reasons can be identified for the advance of computer aided process planning (CAPP) systems. First, large numbers of small batches put a great burden on process planning departments where skilled workforce is scarce. As different process planners make different process plans for the same parts, many companies have different process plans for the same part, resulting in inconsistencies and extra paper work. CAPP systems can help in overcoming these inconsistencies. The use of CAPP systems has the following potential advantages [Chang 90]:
- it reduces the demand on the skilled planner. - it reduces the process planning time. - it reduces both process planning and manufacturing cost. - it creates consistent plans. - it produces accurate plans. - it increases productivity.In chapter 2, section 2.2, CAPP systems are elaborated in more detail.
2. CAPP
Computer Aided Process Planning which forms the link between CAD and CAM is treated in this section. In section 2.2.1 a brief historical overview of CAPP is provided. In section 2.2.2 the functionality offered by today's CAPP systems is discussed. In section 2.2.3 the PART and PART-S systems, which are of particular relevance to this thesis, are discussed. Finally, the shortcomings of today's CAPP systems are detailed in section 2.2.4.
2.1 A brief historical overview of CAPP
CAPP has been a research issue since the 1960's. In the early seventies, the first industrial applications came into existence. They were directed only to the storage and retrieval of process plans for conventional machining. Surveys on CAPP systems can be found in [Ham 88], [Alting 89] and [ElMaraghy 93a]. Generally, two different types of CAPP systems are distinguished: variant and generative.
Variant CAPP
The variant approach to CAPP was the first approach used to computerize process planning. Variant CAPP is based on the concept that similar parts have similar process plans. The computer is used as a tool to assist in identifying similar process plans, as well as in retrieving and editing the plans to suit the requirements for specific parts. Variant CAPP is related to part classification and Group Technology coding. In these approaches, parts are classified and coded based upon several characteristics or attributes. A Group Technology code can be used for the retrieval of process plans for similar parts.
Generative CAPP
Generative CAPP has come into development in the late seventies. It aims at the automatic generation of process plans, starting from scratch for every new workpiece description. Often, the workpiece description is a CAD solid model, as this is an unambiguous product model. A manufacturing database, decision making logic and algorithms are the main ingredients of a generative CAPP system. In the early eighties, knowledge based CAPP made its introduction using AI techniques. A hybrid generative/variant CAPP system has been described by Detand in [Detand 93].
2.2 Functionality offered by today's CAPP systems
Today's more advanced CAPP systems take a CAD based product model as input. At best, this is a 3D solid model on which the CAPP system can perform automatic feature recognition. However, some CAPP systems exist that take wire frame models as an input and on which the process planner has to identify the manufacturing features manually. This is the case in the CAPP system described in [Detand 93]. As CAD models often do not contain tolerance and material information, some CAPP systems allow for adding this information to the product model manually in order to allow automatic reasoning. Most generative CAPP systems allow for human interaction. A lot of CAPP systems can be classified as semi-variant or semi-generative.
2.3 The PART and PART-S CAPP systems
The PART and PART-S process planning systems are both generative CAPP systems developed at the Laboratory of Production and Design Engineering. PART is an acronym for Planning of Activities Resources and Technology. The difference between the two is that PART is the older system, focused on prismatic parts and now commercially available while PART-S is the younger system, inspired on its predecessor, focused on sheet metal parts and still under development. The PART and PART-S systems are the result of a long line of research of the Laboratory of Production and Design Engineering in the field of CAPP. After the CUBIC [Stoltenkamp 79], ROUND [Houten 84] and XPLANE [Erve 88] CAPP systems, the PART and PART-S systems emerged. The PART system has been described extensively in [Houten 91]. PART-S is mainly described in [Vin 94a] and [Vries 95].
PART and PART-S share the same philosophy and roughly offer the same functionality (apart from specific product and process related functionality). First of all, there is the CAD interface in which a solid model representation from a CAD system like Pro-EngineerTM or CatiaTM can be converted into the internal representation of the modeller used in PART. If tolerances have not been added to the original model, it is possible to edit tolerances in the tolerance editor. Then automatic feature recognition can start. The sequence of feature recognition and other activities can be made application dependent. The following activities can be performed: set up selection, machine tool selection, design of jigs and fixtures, the determination of machining methods, cutting tool selection, machining operation sequencing, NC output generation and capacity planning. Figure 2.3 shows the architecture of the PART system, while figure 2.4 shows the very similar architecture of the PART-S system.
PART
The application area of PART is in the machining of 2.5D prismatic components focusing on processes like milling, drilling, finishing, boring, reaming etc.. The process planning functions that PART can perform are placed in functional modules (Figure 2.3). A PART module is a set of related phases. Phases are independent programs that need no input from other phases or operator interaction during its execution. The sequence in which the phases are executed, is prescribed by a scenario which is executed by a supervisor. Some of the different modules of the PART system have been studied in various PhD projects.
For instance, the jigs and fixtures module was the field of study in [Boerma 90]. The main philosophy behind this module is that in order to achieve the specified tolerances, tight tolerances should be machined in one set up as much as possible. However, it is difficult to compare different types of tolerance constraints. Especially, tolerances that cause rotational deviations should be machined in one set up as these are hard to compensate in subsequent set ups. To deal with this problem, the tolerances are converted into a tolerance factor by means of which different tolerance types can be compared in order to perform set up selection. The supervisor and the architecture of PART have been elaborated in [Jonkers 92]. Tool management, tool selection and cutting conditions have been studied in [Boogert 94]. The link of PART with production planning has been described in [Lenderink 93, 94]. Presently, the PART system is commercially available as ICEM-PART(TM).
PART-S
The application area of PART-S is small batch part manufacturing of sheet metal components, that are nested in sheets with a thickness between 0.5 and 5 mm. The main processes included in PART-S are laser cutting, nibbling, (special tool) punching, laser welding and air bending.
Like PART, PART-S has its process planning functions organized in functional modules (Figure 2.4). The modules are again subdivided in a group of related phases. The sequence in which the phases are executed is determined by a scenario. In [Vin 94a] the focus is towards bending sequence determination and the calculation of the process parameters in air bending operations. In a way, [Vin 94a] can be regarded as the sheet metal counterpart of [Boerma 90]. The main difference between sheet bending and the machining of prismatic components is that each bend does not only change the shape of the part locally, but globally as well. This has a profound impact on the way reasoning about tolerances is performed in the handling, positioning and collision checking module of PART-S. De Vin proposed to use a so-called tolerance tree for this. In [Vries 95] the focus is mainly towards process planning and its interaction with other manufacturing functions. In [Vries 94b] capacity planning and nesting in PART-S are considered. In [Vin 93a,b, 94b] and [Liebers 93] specific aspects of PART-S have been addressed.
2.4 Shortcomings of today's CAPP systems
Most present day CAPP systems cannot handle a feature based solid CAD model as input. That is, if they seem to be able to handle such models, the feature based model first has to be converted into its corresponding (B-rep) solid model representation from which the CAPP system can infer the manufacturing features by means of feature recognition. PART and PART-S also take a B-rep CAD model as input and perform feature recognition on it. Manual feature identification as for instance in the case of [Detand 93], is labour intensive and should be avoided. In the case of feature based design followed by feature recognition or feature identification, the feature information is first thrown away and recovered later. This inefficient information transfer could be improved, at least if the design features and the process planning features correspond or if they can be converted into one another. A feature conversion, however, can be obstructed by different feature representations in CAD and CAPP systems. In fact, in some feature recognition based CAPP systems, the features are described within the feature recognition algorithms. At least, in PART and PART-S this is indeed the case. The definition of new features in many CAPP systems such as PART and PART-S involves the programming of new feature recognition algorithms. Although feature recognition languages can be fairly high level programming languages, the definition of new features in a CAPP system can involve a lot of work.
One of the shortcomings of today's commercial CAPP systems is that they do not provide the CAD system, or the designer using the CAD system, with feedback information on cost, manufacturability etc.. Many researchers have proposed a lot of different ways in which CAD and CAPP systems could become more cooperative. As most of these proposals involve the use of features, they will be discussed in the next chapter and in Appendix A (A.1, A.2, A.3 and A.4.5).
Another shortcoming of present commercial CAPP systems is that they do not communicate with capacity planning functions. In his thesis, Lenderink deals with this problem extensively, focusing primarily on the PART system and its link with capacity planning [Lenderink 94]. Lenderink proposes to complete the detailed process plan only just before the manufacturing of the part can start. Before completing the process plan, the first part of the process plan is derived from information becoming available from feature recognition and set-up selection. Using alternative set ups, the jobs are assigned to the resources (loading), based on the actual availability and the actual workload of all the machines in the workshop. Subsequently, the detailed process plan is completed. Detand also addresses this problem [Detand 93]. Detand promotes non-linear process plans; a process plan that comprises different manufacturing alternatives and is represented by an AND/OR structure.
