not only represents feature parameters, tolerance, material, etc。 as attributes of class or instance, but also represents geometrical objects by importing UG/UDO and UG/UDF referenced solid geometry as an instance attribute or child。 And this realizes the connectivity and integration between the symbol of a feature and its geometrical object。 It can also get an attribute value from a function, rule, expression and database。

4。2 Operation feature

Accordingly, stamping operation features are categorized into initial and subsequent, such as drawing, bending, flanging, trimming, hemming, re-striking, and piercing and so on。 The relevant appropriate operations are assigned to form stamping features of products based on feature- operation criterion, parameters of the stamping feature and their correlativity。 This assignment is a  decision-making  activity  using  a  set  of  rules  with  decision-making  tree  and    model-based

reasoning methods。 For example, drawing→ trimming is reasoning from the main draw  feature,

flanging is reasoning from the flange feature, and piercing is reasoning from the hole。 Fig。 5 show the typical illustration of flanging operation features reasoning from its stamping feature, and the sequence rules of these operation features are attached to the operation features。 Customarily, relationships between features, especially the hierarchy, should be an important factor, while the operation feature is the reasoning。 In Fig。4, the features flange3, flange1 and bead1 should be in the form of a set together to deduce relevant appropriate operations。 In this way of reasoning, the operation features can get fundamental knowledge for subsequent planning。 In this paper, the operations features reasoned from stamping features are represented as object-oriented entities and UG/UDO, and the relationships are expressed as network based on the stamping feature model of the product (shown in Fig。 6)。 The operation features not only refer themselves to stamping features, but also relate to other operations, such as relevant forming dies and machinery with the relationships among them。 The relationships between operations consist of operations order constraint and operations combination constraint。 Another type of relation between processes and machines/dies is preference (preferred-to)。 For example, flanging is always preferred to flanging die and relevant press machine, while trimming is always preferred to trimming die and press machine。 ALL these constraints and relationships among them are reprsented  as  attributes,  rules  and  methods  in  the  operations。  It  is  through  the  citation of

stamping features that the description of the geometrical objects are realized in the operation features; while the relevant dies and machinery can be represented in operation by options and the constraints expressed as relevant rules, functions or methods。

4。3 Process information model (PIM) for path

Planning Now the PIM of process planning can be defined as the foundation to process planning。 And the PCS of IMM is created to control and monitor the stamping feature and its operation features by the control variable or state variable。 The process planning of large complicated stampings is a dynamic process。 The PIM must involve all data, such as process planning data, analysis data, die design data, and circumstance, etc。 The basic knowledge expression of the model should have adaptability to the changes of geometry, attribute, features, constraints and the way of thinking。 Using the knowledge multi-expression format of KBE technology, the integrated information model of process planning for  large complicated stampings is built based on the feature model。 The process information model is shown in Fig。 7。 The object-oriented method and feature technology are adopted mainly to form the model。 There are three basic classes of features: stamping design features, operation features, and sequence features, in which process knowledge, e。g。 database, parameter, rules, and experience, acts as rules and attributes of the objects。 Using feature technology and the geometry extraction method, the stamping design features, such as the main forming feature (e。g。 drawing, bend), flange, hole, emboss, notch, and so on, that are defined first as UG/UDO, can be extracted from a 3D solid model。 Relevant appropriate operations can then be assigned from stamping design features of a product using a set of rules with a decision-making tree and model-based reasoning methods of feature-operation criteria。 Hierarchy and framework of the product model and semantic net of feature-operation-tools are used to establish the relationship information for PIM。 With PIM, where the knowledge is encapsulated in objects or decision-making knowledge procedures, the process planning can be completed through corresponding sets of knowledge-reasoning。 This model is a dynamic expanded information model, in which the information can be added and updated along with the process of planning。 Information management and control is a part of PCS in IMM of process planning, which can monitor the change or modification of process plan- ning, and timely update the information to insure the process information model and process planning synchro-