In order to prepare manufacturing companies to face increasingly frequent and unpredictable market changes with confidence, there is a recognized need for CNC machine tools to be further advanced so that they become more integrated with design models and adaptable to uncertain machining conditions. For a CNC system to be able to access any design information, this design information has to be at the task-level, that is what-to-do. For a CNC system to produce the final part, it has to turn the task-level information into method-level information which effectively is the machine control data. These topics are discussed at the beginning of this chapter. The rest of the chapter discusses a CNC native database used for converting the task-level data to method-level data, the methodology of converting the task-level data to methodlevel data, and implementation of the methodology to a conventional CNC machine that employs G-codes. Again both STEP-NC (ISO 14649-1, 2003) and function blocks (IEC 61499, 2005) are used.
Task-Level Data Vs. Method-Level Data
An important feature of the STEP-NC concept is that of “machine tool independency”; this makes STEP-NC codes interoperable across various CNC systems. This is because a STEP-NC data model mainly captures the task-level or the what-to-do information. Although it is possible to define data at the method-level or the how-to-do level, 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. This way, STEP-NC part programs may be written once but can be used on different machine tools providing that the machine tool has the required process capabilities. Figure 1 shows these two categories of data defined in the STEP-NC data model. The first two columns depict the manufacturing task information. The process-level data describe abstract manufacturing tasks at the macro-level. The geometry-level data are represented in terms of manufacturing_features. The how-to-do data are also divided into two types. Machine tool core data spell out the manufacturing requirements. Machine tool auxiliary data are method-level data and they are in fact defined in the STEP-NC data model as optional data. Both categories of how-to-do data take different forms when different machine tools are used.
What-to-do and how-to-do data in STEP-NC ©2008, Taylor and Francis Journals, http://www.informaworld.com, used with permission from the authors.
The central issue is therefore the ‘transition’ from the task-level data to the method-level, or from the what-to-do data to the how-to-do data. Since STEP-NC is utilized as a CNC machining data model, implementation of STEP-NC is effectively a process of adapting its data model for different CNC systems. This is illustrated in Figure 2.
Implementation of STEP-NC manufacturing ©2008, Taylor and Francis Journals, http://www.informaworld.com, used with permission from the authors.
Such a system may have three stages. First of all, a native version of STEP-NC program is generated based on the information in a generic STEP-NC program. Then, low-level, local NC commands can be generated based on the native STEP-NC information. This provides a direct interface with the targeted CNC machine, hence a “CAM-CNC transition”. This transition is intended to be hidden away from the user and ideally synchronised with the subsequent execution process. Finally, a STEP-NC enabled controller executes the STEP-NC program through the above native CNC commands.Top
Generate A Native Step-Nc Program
The key in this phase is to “map” a generic STEP-NC program to a native one. In other words, the main task is to convert what-to-do information in a generic STEP-NC program, into the how-to-do information for a specific CNC machine. Different CNC machines have differing machining capabilities and native parameters. Until a STEP-NC program takes into account all the information specific to a CNC system, it will not be possible to execute the Workplan(s) specified in the STEP-NC program. The mapping process is also a “checking” process to evaluate the manufacturability of the job on a particular machine tool. In the distributed manufacturing environment, this “plug-and-play” feature gives a process plan the mobility and portability that are desired.
Figure 3 shows the logical structure of the system (Wang, Xu & Tedford, 2006). The three main functional units are the Native CNC database, Adaptor and Human-Machine (HM) interface.
Logical structure of the system