數(shù)控機(jī)床上下料機(jī)械手的設(shè)計(jì),數(shù)控機(jī)床上下料機(jī)械手的設(shè)計(jì),數(shù)控機(jī)床,上下,機(jī)械手,設(shè)計(jì) 31 Development of STEP-NC Compliant Machine Tool Data Model W. Z. Yang 1 , X. Xu 2 and S. Q. Xie 3 Department of Mechanical Engineering, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand 1 wyan041ec.auckland.ac.nz 2 x.xuauckland.ac.nz 3 s.xieauckland.ac.nz Abstract. To implement STEP-NC based manufacturing, a STEP-NC compliant machine tool data model (STEP-NCMtDm) is developed by using EXPRESS modelling lanugage. It models sufficient machine tool data to transfer STEP-NC machining task-level data to machine tool dependent method-level data. The objective and two main technological aspects of developing STEP-NCMtDM are discussed. The model architecture is analyzed. Machine tool data in STEP-NCMtDm are categorized into two groups: “static data” and “dynamic data”, to support further system development. A Web-enabled STEP-NCMtDm (a corresponding XML schema) is developed to support Web-based application. STEP-NCMtDm has been implemented to model a milling machine tool and represent its data in a STEP Part 21 file and an XML file to support STEP-NC based manufacturing. Keywords: STEP-NC, Machine Tools, Data Model, XML Schema. 1. Introduction STEP-NC based manufacturing is a newly developed digital manufacturing approach to realize an I 4 (informative, intelligent, integrated and interoperated) manufacturing scenario. STEP-NC has several significant advantages when implemented in the whole CNC manufacturing processes: 1) providing a complete and structured data model to represent various product and technical data at different stages of product development processes, 2) supporting standardized data exchange and sharing within integrated manufacturing systems 1; 3) bi-directional transfer of high level manufacturing information between CAM and CNC systems, which is impossible with G/M code; and 4) using object-oriented concept “workingstep” to encapsulate machining feature information together with machining operation parameters, which makes manufacturing data reusable. The most important feature of STEP-NC concept is that of machine tool independence. STEP-NC data model mainly portrays data at the Task-level or the “What-to-do” data. Although it is possible to define data at the Method-level or the “How-to-do” data, such as machine tool trajectory, the main aim of STEP-NC is to allow these decisions to be made by a STEP-NC enabled controller. STEP-NC programs may be written once and used on different machine tools provided that the machine tool has the required process capabilities 2. Therefore, the implementation of STEP-NC is effectively a process of adapting its data model for different CNC systems. That is, the central issue to implement STEP-NC is therefore the transition from the Task-level data to the Method-level data, or from the “What-to-do” data to the “How-to-do” data. To do this, it is necessary to have in place a complete and readily accessible database of manufacturing resources, describing data such as product data, cutting tool data. Although some machining process data such as machining parameters, machining strategy, etc. have been defined in STEP-NC Part 10, 11 and 12 5, 6, 7, these data objects are not adequate for modelling machine tool data in support of STEP-NC enabled machining, especially in a dynamical shop floor environment. Thus, to implement STEP-NC based manufacturing, and to accomplish process planning and scheduling tasks, it is essential to build up a STEP-NC compliant machine tool data model that is able to capture all the required machine tool data for supporting STEP-NC enabled machining. In this paper, a STEP-NC compliant Machine tool Data model (STEP-NCMtDm) is described. When combined with the cutting tool data models defined in STEP-NC, STEP-NCMtDm can form a comprehensive Manufacturing Resource Data Model (MRDM) and ultimately provide sufficient information for STEP-NC based manufacturing. 2. Literature Review There are some efforts in the area of modelling machine tool data. The project Modelling of Manufacturing Resources Information was carried out at the National Institute of Standard and Technology (NIST) in the USA 8. It specifies two aspects of manufacturing resources: 1) machine tool (milling machines and vertical/horizontal turning machines); and 2) tool assembly (cutting tools, inserts, tool holders, etc.) This model has been considered as a basis to develop the proposed ASME B5.59-2 standard for representing machine tools. The NIST Manufacturing Resources Model and ASME B5.59-2 are developed to principally satisfy the requirements of manufacturing resource venders but not ideal to support process planning. To overcome this, Kulvatunyou et al developed a UML-based manufacturing resource model based on the NIST model 9. The new model interacts with the Resource Specific Process Planning (RSPP) module to perform process planning tasks for collaborative manufacturing, but it still lacks the 36 W. Z. Yang, X. Xu and S. Q. Xie capability of integrating CNC machining to form a close loop manufacturing chain. Ming et al. introduced a PDES/STEP based production resource model, including data for machine, cutting tool, fixture, gauge tool and fixture 10. The proposed model considered the external resources in support of the collaborative manufacturing environment. With the other two PDES/STEP-based models (part information model and process planning model), the entire information model can support CAPP applications. Lopec-Ortegsa and Ramirez proposed a STEP-based EXPRESS model to present flexible manufacturing resources (i.e. machining processing resources, handling resources, transportation resources and storage resources) in the shop-floor 11. This data model was implemented in a STEP-based manufacturing information system. Several standards have been developed for machine tool data modelling. ASME B5 committee has been working on two draft standards to represent machine tool performance data (ASME B5.59-1) and properties specification (ASME B5.59-2) respectively. These standards mainly aim to support machine tool vendors. Within ISO, STEP Part 240 12 stipulates a data model for process planning and eight application objects of Units of Functionality (UoF) manufacturing_machine_tool_ resources that are of direct relevance to machine tools 13. In addition, ISO TC39/SC2 is developing standards for enhancing interchangeability between machine tool components and testing machine tools (i.e. test method, test code, test component, etc.). None of the above models is fit for supporting STEP- NC based manufacturing. Some models (e.g. those developed by NIST, ASME and ISO) are for machine tool vendors and users but not for manufacturing applications such as process planning, shop floor scheduling, etc. Others (e.g. Mings and Lopec-Ortegass models, and STEP Part 240) are not completely compatible with the STEP-NC standards. The proposed STEP-NC machine model is to support STEP-NC based manufacturing. 3. Objective The purpose of developing STEP-NCMtDm is of two- folds. First, it is to meet the data requirements for implementing STEP-NC data model in process planning and scheduling. More specifically, it helps the user determine the “How-to-do” information for CNC machining based on the “What-to-do” information contained in a STEP-NC program. STEP-NC can be considered as a process plan model. It is not supportive of other activities, such as cost estimation, machine tool selection, etc. Thus, the second goal of STEP-NCMtDm is to support these functionalities of process planning in a distributed manufacturing environment. 4. Technological aspects of STEP-NCMtDm 4.1 Machine tool static data and dynamic data Machine tool data are classified into two types: static and dynamic. If the data stay unchanged during the lifecycle of a machine tool, they are classified as static machine tool data, e.g. machine loading capacity and machine tool dimensions. These data are mainly machine tools specifications, a gross indication of the machining capability of a machine tool. On the other hand, if the data change during different stages of machine tool usage, or take on different values for different applications, they are classified as dynamic machine tool data. Dynamic machine tool data are the key information for realizing flexible process planning and manufacturing. There may be two causes to the changes of a machine tools dynamic data. The first is to do with the machine tool wear, which leads to an accuracy drift or changes of mechanical and electrical properties. The second is to do with the changes or re-configuration of a machine tool setup. The static machine tool data, once modelled, can stay unchanged. For the two types of dynamic machine tool data, different data processing and updating procedures can be followed. The first type of dynamic data need to be updated on a more regular basis due to the factors such as machine tool wear. The second type of dynamic data is updated on an irregular basis. 4.2 Modelling language and Web-based data model The EXPRESS modelling language 14 is used to model STEP-NCMtDm. It defines an entity-attributes type data representation method and can fully support modelling constraints and functions. STEP standard has defined several implementation methods (through Java, C+, etc.). STEP-NC data models are also developed by using EXPRESS. Thus, use of EXPRESS can ensure the compliance of STEP-NCMtDm with STEP-NC, and STEP-NCMtDm can service the STEP-NC data models to form a comprehensive process planning data model in support of STEP-NC based manufacturing. STEP Part 21 15 files are neutral files and can be utilized to transfer data between different CAD/CAPP/CAM systems. However, it is not suited for Web-based data publications and transformations. To overcome this problem, a Web-enable data representation method is required. In STEP Part 28 (second edition) 16, a mapping mechanism between EXPRESS and XML schema is defined. Using this mechanism, the STEP- NCMtDm in EXPRESS format can be converted to an XML schema. The STEP Part 21 file with machine tool data can also be presented in the XML file format for Web-based data exchange and sharing. Development of STEP-NC Compliant Machine Tool Data Model 37 5. STEP-NCMtDm 5.1 Architecture of STEP-NCMtDm STEP-NCMtDm is constructed to model the following five sets of data (Figure 1.1): 1) Machine tool general data; 2) Machine tool component data; 3) Machining cost data; 4) Machine tool performance data; and 5) Machining capability data. Machine tool general data set represents the general information about a machine tool. The root entity is the abstract ENTITY machine_tool. The general information modelled about a machine tool includes its id, its location, and the links to other machine tool technique data. Three sub-entities are modelled to represent milling, turning, and mill-turn machine tools respectively. Machine tool component data set represents the information about each individual component of a machine tool. It is further divided into machine tool axis data (ENTITY axis), spindle data (ENTITY spindle), workpiece handling device data (ENTITY workpiece_handling_device), cutting tool handling device data (ENTITY cutting_tool_handling_device) and auxiliary device data (ENTITY aux_device). Machine tool capability data set contains the information which can be used to determine machining capability of a machine tool. In process planning, these data are the key factors to determine whether a machine tool is suitable for a particular machining job. The machining capability, represented by ENTITY machining_capability includes machine tools operation type capability, table loading capability, machinable workpiece size, machining accuracy capability, axis positioning accuracy capability, feeding capability and maximum cutting depth data. Machine tool performance data set is used along with some other data, in selecting a proper machine tool for machining. Currently, only machine tools accuracy data is modelled, which involves machine tool resolution (ENTITY resolution) and axis repeatability (ENTITY repeatablility_accuracy). Machine tool cost data set is modelled by ENTITY machine_tool_cost_information for estimating machining cost in process planning. It keeps the data such as the average cost per setup (attribute setup_cost) and machining hourly rate (attribute machining_cost_per_hour). The process planning system can use these data to calculate the corresponding manufacturing cost. 5.2 Static data and dynamic data As discussed in section 1.3.1, machine tool data can be divided into two categories: static and dynamic. Different data acquisition and updating methods are implemented for different categories. The dynamic machine tool data can be further divided into two groups according to their updating frequency, regular or irregular. Classification of static and dynamic data is essential for later model implementation. Tables 1-3 show the three types of data for ENTITIES machine_tool, travelling_axis and machine_tool_capability respectively. In general, data classification is based on the following guidelines: The machine tools general information and the data related to machine tool or its components capability are considered as static data. The machine tool data related to the machine tools performance, such as accuracy data, belongs to the first type of machine tool dynamic data. Frequency of updating this type of data depends on the usage of the machine tool. The machine tool data related to cutting tool and workpiece handling devices are of the second type Machine Tool Performance (ABS)milling_type_machin e (ABS)turning_machine milling_machine drilling vertical_turning_machine horizontal_turning_machine mill_turn_center its_id its_location number_of_axes axes_list S1:? (ABS)Axis (ABS)spindle (ABS)cuttiing_tool_handling_device (ABS)workpiece_handling_device label STRING INTEGER(ABS)aux_device_information machine_tool_capability its_spindle S1:?1 tool_handling_information S1:? workpiece_handling_information S1:? its_aux_devices S1:? its_capability 1 1 1 (ABS)machine_performance its_machine_tool machine_tool_cost_information its_machine_tool Machining Capability Machine Tool Components Machine Tool General information Machining Cost (ABS)ma c hine_ t ool Figure 1.1. Architecture of STEP-NCMtDm 38 W. Z. Yang, X. Xu and S. Q. Xie of machine tool dynamic data that needs to be updated on a regular basis. This is due to the fact that different cutting tools handling devices or workpiece handling devices may be utilized for machining a different part. The data are often situational subject to the changes at shop floor. Table 1.1 Static data and dynamic data in ENTITY machine_tool Dynamic data Static data Type 1 (regularly updated) Type 2 (irregularly updated) its_id its_location tool_handling _information number_of_axis axes_list workpiece_handling _information its_spindle its_aux_devices its_capability its_capability Table 1.2 Static data and dynamic data in ENTITY travelling_axis Dynamic data Static data Type 1 (regularly updated) Type 2 (irregularly updated) its_name its_machine _tool its_traveling _range its_feeding _range rapid_movement _speed its_accuracy_ information Table 1.3 Static data and dynamic data in Entity machine_tool_capability Dynamic data Static data Type 1 (regular updated) Type 2 (irregular updated) its_machine _tool machinable_workpiece _information operation_type _information machining_accur- acy_information max_cutting_depth table_loading _capacity positioning_accu- racy_information feeding_capability 5.3 Measurement unit The types of units supported by STEP-NCMtDm are SI units as well as derived or conversion based units defined in ISO 10303 Part 41(ISO 10303-41:1994(E). If no units are given then, the default values are assumed as defined in Clause 4.2.1 of ISO 14649 Part 10: length_measure: millimetre mm rot_speed_measure: revolutions per second r/s plane_angle_measure: degrees pressure_measure: Pascal pa speed_measure: meters per second m/s time_measure: seconds s The default unit for mass_measure (referenced from ISO 10303-41 17) is defined as: mass_measure: kilogram kg. The power measurement and its measurement unit are defined in ENTITY power_measure_with_unit in STEP-NCMtDm. ENTITIES measure_with_unit and si_unit, and TYPE parameter_value, referenced from STEP Part 41, are utilized to construct ENTITY power_measure_with_unit. 6. Web-based STEP-NCMtDm The EXPRESS-oriented STEP-NCMtDm is not web- enabled. To adapt the distributed manufacturing environment, an XML-oriented STEP-NCMtDm is required to model machine tool data. STEP Part 28 (2nd edition) defines a standardized method to convert EXPRESS data model into a set of XML schema, which can be used to represent machine tool data in a XML document. SETP Part 28 introduces two types of XML schema binding default XML schema binding (clause 7) and configured XML schema binding (clause 8). This paper utilizes the mapping mechanism corresponding to the default XML schema binding to map STEP-NCMtDm into the corresponding XML schema. These are the standardized mapping mechanisms defined for the EXPRESS elements, such as EXPRESS data type (clause 7.2), defined data type (clause 7.3), entity data type (clause 7.4), entity attributes except DERIVEd attributes and INVERSE attributes (clause 7.6), supertype/subtype relations (clause 7.5), etc. Figure 1.2 shows a portion of the XML schema (i.e. ENTITY machine_tool) converted from the STEP- NCMtDm EXPRESS data model, and the relationship between machine_tool and its subtype ENTITIES milling_type_machine, turning_machine, and turn_mill_center. The header part of the XML schema defines the name space of the STEP-NCMtDm EXPRESS schema and STEP-NC standards. The data part of schema is used to define the main body of the EXPRESS schema of STEP-NCMtDm. 7. Implementation STEP-NCMtDm has been implemented to model an EMCO Concept Milling 105 milling machine tool 18. Two types of data file are generated: a STEP Part 21 file (Figure 1.3) and an XML document (Figure 1.4). These files can support adequate machine tool data to meet the data requirements of STEP-NC Part 11 (milling application). Development of STEP-NC Compliant Machine Tool Data Model 39 For example, for the Entity milling_technology in STEP- NC Part 11, which defines the technological parameters of milling operations, the STEP-NCMtDm based Part 21 file/XML file represents data for the following attributes of this entity: 1) Attributes cutting_speed and spindle: Related to an instance of rotation_speed_range defined in Line #8 (Fig. 1.3). The actual spindle rotation Figure 1.3. Potion of the Part 21 File for Concept Mill 105 Figure 1.4. Potion of the XML file for Concept Mill 105 Figure 1.2. XML schema representation of STEP-NCMtDm Header Part Data Part 40 W. Z. Yang, X. Xu and S. Q. Xie speed must be within this range. The linear cutting speed range can also be set. 2) Attribute feedrate_per_tooth: Related to Line #5, which defines an instance of feeding_range for the cutter. When multiplying the tooth number, a feeding range is calculated to constrain the feedrate value of a cutting tool. 3) Attribute synchronize_spindle_with_feed: Defined as the ninth parameter in Line #7, which defines whether cutting speed and feed of the tool are synchronized. The corresponding STEP-NCMtDm based XML file supports the same machine tool data for Web-based STEP-NC manufacturing applications. 8. Conclusions STEP-NC based manufacturing is about utilizing the STEP and STEP-NC data models to support an informative, intelligent, integrated and interoperable CNC manufacturing scenario. The key for STEP-NC based manufacturing is to build machine tool data model to realize the transition of STEP-NC tasks level data to machine tool dependent method level data. STEP- NCMtDm models various machine tool data to meet the requirements of STEP-NC compliant