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Making CNC machine tools more open, interoperable andintelligenta review of the technologiesX.W. Xua,*, S.T. NewmanbaDepartment of Mechanical Engineering, School of Engineering, The University of Auckland,Private Bag 92019, Auckland, New ZealandbDepartment of Mechanical Engineering, University of Bath, Bath, BA2 7AY, UKReceived 30 August 2004; accepted 7 June 2005Available online 10 October 2005AbstractThe aim of the next generation of computer numerically controlled (CNC) machines is to be portable, interoperable and adaptable. Over theyears, G-codes (ISO 6983) have been extensively used by the CNC machine tools for part programming and are now considered as a bottleneck fordeveloping next generation of CNC machines. A new standard known as STEP-NC is being developed as the data model for a new breed of CNCmachine tools. The data model represents a common standard specifically aimed at the intelligent CNC manufacturing workstation, making thegoal of a standardised CNC controller and NC code generation facility a reality. It is believed that CNC machines implementing STEP-NC will bethe basis for a more open and adaptable architecture. This paper outlines a futuristic view of STEP-NC to support distributed interoperableintelligent manufacturing through global networking with autonomous manufacturing workstations with STEP compliant data interpretation,intelligent part program generation, diagnostics and maintenance, monitoring and job production scheduling.# 2005 Elsevier B.V. All rights reserved.Keywords: CNC; Interoperability; STEP; STEP-NC1. IntroductionFrom the start of craft production in the 1800s to thepioneering mass production of the early 1900s there have been anumber of revolutionary changes to manufacturing systemsconfigurations. The most recognised traditional configuration ofmanufacturing systems was the dedicated transfer (machine)line, which enabled mass production at high efficiency and lowcost. With the need of the 1970s and 1980s to produce a widerrange of parts, flexible manufacturing was developed to meetthese needs for the production of smaller batches of differentparts. These systems used groups of computer numericallycontrolled(CNC)machinesthatcouldbereprogrammedtomakedifferent parts combined with automated transport systems andstorage.TheseCNCmachinesbecamethecentralelementsinthesystems such as flexible transfer lines, flexible manufacturingsystems (FMS) and flexible manufacturing cells (FMC).However, the amount of flexibility existing in thesesystems was still believed to be limited. In order to preparemanufacturing companies to face increasingly frequent andunpredictable market changes with confidence, interoperableand more open manufacturing systems are needed. In theprocess of designing and operating interoperable and openmanufacturing systems there is a need to distinguish fromamong system-level issues, component-level (i.e. machineand control) issues, and ramp-up time reduction issues 1,2.Most of the research effort has been spared on the issues at thesystem level, some at the component level and little on theramp-up time reduction issues. At the component level,research work has primarily centred around the control issuesconcerning machine tools, with the aim to provide enablingCNC technologies for modular and open-architecture control3,4.CNC machine tools are the main components in anymanufacturing system. There are demands and new opportu-nities to empower the current CNC machines with the much-needed features such as interoperability, adaptability, agilityand reconfigurability. To this end, there are two major issuesthat need to be addressed namely product data compatibility/locate/compindComputers in Industry 57 (2006) 141152* Corresponding author. Tel.: +64 9 373 7599x84527; fax: +64 9 373 7479.E-mail address: x.xuauckland.ac.nz (X.W. Xu).0166-3615/$ see front matter # 2005 Elsevier B.V. All rights reserved.doi:10.1016/pind.2005.06.002interoperability and adaptable CNC machines. Up till now littleresearch has been carried out in this field, but due to thedevelopments of the new CNC data model known as STEP-NC,there has been a surge of research activities in trying to addressthe above-mentioned issues. This paper reports on theseresearch activities and tries to address the issues of interoper-ability and adaptability for CNC machine tools.2. Impediments of current CNC technologiesTodays CNC machine designs are well developed withcapabilities such as multi-axis control, error compensation andmulti-process manufacture (e.g. combined mill/turn/laser andgrinding machines). In the mean time, these capabilities havemade the programming task increasingly more difficult andmachine tools themselves less adaptable. Some effort has beenmade to alleviate this problem, in particularly the trend towardsopen architecture control, based on OSACA 5 and openmodular architecture controller (OMAC) 6, where third partysoftwarecanbeusedatthecontrollerworkingwithinastandardwindows operating system. One further recognisable industrialdevelopment is the application of software controllers, wherePLC logic is captured in software rather than in hardware.Although these developments have improved software toolsand the architecture of CNC systems, vendorsand users are stillseeking a common language for CAD, CAPP, CAM, and CNC,which integrates and translates the knowledge of each stagewith no information loss. Though there are many CAM toolssupporting NC manufacture, the problem of adaptability andinteroperability from system to system was and is still seen asone of the key issues in limiting the wider use of these tools.2.1. Product data compatibility and interoperabilityCNC machine tools complete the product design andmanufacturing lifecycle, and more often than not they have tocommunicate with upstream sub-systems, such as CAD, CAPPand CAM. In the case when neutral data exchange protocols,such as SET, VDA, and initial graphics exchange specification(IGES) are used, information exchange can happen betweenheterogeneous CAD and/or CAM systems. This is howeveronly partially successful since these protocols are mainlydesigned to exchange geometrical information and not totallysuitable to all the needs of the CAD/CAPP/CAM industry.Thus, the international communitydeveloped the ISO10303 7set of standards, well known as STEP.By implementing STEP AP-203 8 and STEP AP-214 9withinCADsystems,thedataexchangebarrierisremoved.Yet,dataexchangeproblemsbetweenCAD/CAMandCNCsystemsremain unsolved. CAD systems are designed to describe thegeometry of a part precisely, whereas CAM systems focus onusing computer systems to generate plans and control themanufacturingoperationsaccordingtothegeometricalinformation present in a CAD model and the existing resourcesonthe shop-floor.Thefinal result fromaCAM system isa setofCNC programs that can be executed on a CNC machine. STEPAP-203 and STEPAP-214 only unify the input data for a CAMsystem. On the output side of a CAM system, a 50-year-oldinternational standard ISO 6983 (known as G-Code orRS274D) 10 still dominates the control systems of mostCNC machines. Outdated yet still widely used, ISO 6983 onlysupports one-way information flow from design to manufactur-ing. The CAD data are not utilised at a machine tool. Instead,they are processed by a post-processor only to obtain a set oflow-level, incomplete data that makes modification, verifica-tions and simulation difficult. The changes made at the shop-floor cannot be directly fed back to the designer. Hence,invaluable experiences on the shop-floor cannot be preservedand re-utilised.2.2. Inflexible CNC control regimeThe ISO 6983 standard focuses on programming the path ofthecutter centrelocation(CL)withrespect tothemachineaxes,rather than the machining tasks with respect to the part. Thus,ISO 6983 defines the syntax of program statements, but in mostcases leaves the semantics ambiguous, together with low-levellimited control over program execution. These programs, whenprocessed in a CAM system by a machine-specific post-processor, become machine-dependent. In order to enhance thecapability of a CNC machine, CNC controller vendors havealso developed their own tailored control command sets to addmore features to their CNC controllers to extend ISO 6983.These command sets once again vary from vendor to vendorresulting in further incompatible data among the machine tools.The current inflexible CNC control regime means that theoutput from a CAM system has no adaptability, which in turndenies the CNC machine tools of having any interoperability.The main reason is that a G-code based part program onlycontains low-level information that can be described as how-to-do information. The CNC machine tools, no matter howcapable they are, can do nothing but faithfully follow the G-code program.Itisimpossibletoperformintelligentcontrolnormachining optimization.3. The STEP-NC standardTodayanewstandardnamelyISO146491116recognised informally as STEP-NC is being developed byvendors, users and academic institutes world wide to provide adata modelforanewbreedofintelligent CNCs. Thedata modelrepresents a common standard specifically aimed at NCprogramming, making the goal of a standardised CNCcontroller and NC code generation facility a reality. CurrentlytwoversionsofSTEP-NCarebeingdevelopedbyISO.Thefirstis the Application Reference Model (ARM) (i.e. ISO 14649)and the other Application Interpreted Model (AIM) of ISO14649 (i.e. ISO 10303 AP-238 17). For more information onthe use and differences between them readers are referred to18,19.Contrary to the current NC programming standard (ISO6983), ISO 14649 is not a method for part programming anddoes not normally describe the tool movements for a CNCmachine. Instead, it provides an object oriented data model forX.W. Xu, S.T. Newman/Computers in Industry 57 (2006) 141152142CNCs with a detailed and structured data interface thatincorporates feature-based programming where a range ofinformation is represented such as the features to bemachined, tool types used, the operations to perform, andthe sequence of operations to follow. Though it is possible toclosely define the machine tool trajectory using STEP-NC, theaim of the standard is to allow these decisions to be made at alatter stage by a new breed of intelligent controllerSTEP-NC controller. It is the aim that STEP-NC part programs maybe written once and used on many different types of machinetool controller providing the machine has the required processcapabilities. In doing this, both CNC machine tools and theircontrol programs are made adaptable and interoperable. Fig. 1illustrates that both geometric and machining information cannow be bi-directionally transferred between a CAD/CAMsystem and a STEP-NC controller 20. One critical issue isthat the tool path movement information is optional andideally should be generated at the machine by the STEP-NCcontroller.Geometric information is defined by machining features(similar to AP-224 22) with machining operations termedWorkingsteps performed on one or more features. TheseWorkingsteps provide the basis of a Workplan to manu-facture the component. Fig. 2 illustrates an actual extract ofsuchdataforapartwithaWorkplanconsistingofWorkingstepsfor slotting, drilling and pocketing. One important point to noteis that this code is the STEP-NC transfer (physical) file, whichis imported/exported into and out of a STEP-NC intelligentcontroller. This file would be interpreted by the controller,enabling CNC operators to interact at a Workingstep (i.e.machining operation) level via an intelligent manual datainterface (MDI) or CAD/CAM system at the controller. Someof the benefits with using STEP-NC are as follows 23.? STEP-NC provides a complete and structured data model,linked with geometrical and technological information, sothat no information is lost between the different stages of theproduct development process.? Its data elements are adequate enough to describe taskoriented NC data.? The data model is extendable to further technologies andscalable(withconformanceclasses)tomatchtheabilitiesofaspecific CAM, SFP or NC.? Machining time for small to medium sized job lots can bereduced because intelligent optimisation can be built into theSTEP-NC controllers.? Post-processor mechanism will be eliminated, as the inter-face does not require machine-specific information.? Machine tools are safer and more adaptable because STEP-NC is independent from machine tool vendors.? Modification at the shop-floor can be saved and fed back tothe design department hence bi-directional information flowfrom CAD/CAM to CNC machines can be achieved.? XML files can be used as an information carrier hence enableWeb-based distributed manufacturing.Adetailed discussion onvalue propositionforSTEP-NCcanbe found in a report produced by the OMAC STEP-NCWorking Group 24 and other publications 20,23,25.4. STEP-NC international communityIn the second half of the 1990s, an effort from theinternational community backed by ISO started the majorchange in the concept of NC programming, through aninternational intelligent manufacturing systems (IMS) pro-gramme 26. The programme was co-ordinated across fourX.W. Xu, S.T. Newman/Computers in Industry 57 (2006) 141152143Fig. 1. Bi-directional information flow with STEP-NC 21.worldwide regions each with individual projects namelyEurope, Korea, Switzerland and the USA. The major co-ordinators of the programme are Siemens (EU), CADCAMa-tion (Switzerland), STEP Tools (USA) and ERC-ACI (Korea).STEP-NC Europe is responsible for milling, turning andinspection of the ISO 14649 standard. It has 15 partners, led bySiemens, with users such as Daimler Chrysler, Volvo, and thesupport of research institutes such as WZL RWTH-Aachen andISW Stuttgart University. The Swiss are leading the develop-ment of the standard for wire-cut and die-sink EDM incollaboration with vendors such as Agie, Starrag and CAMmanufacturer CADCAMation. The work in Korea has beencarried outby both PohangUniversityofScience &Technology (PosTECH) and the Seoul National Universityin the areas of milling and turning architectures for ISO 14649compliant controllers.Otherresearch teamsworkingin the areainclude those in the UK and New Zealand. In the UnitedKingdom, an Agent-Based, STEP-compliant CAM (AB-CAM)system has been developed in Wolfson School of Mechanicaland Manufacturing Engineering, Loughborough University27,28.InNewZealand,theManufacturingSystemsLaboratory at the University of Auckland has been using theAIM of STEP-NC 17 to develop a STEP-compliant CAPPsystem for collaborative manufacturing 29,30.The STEP-NC programme in the USA called SuperModelled by STEP Tools Inc. and sponsored by National Institute ofStandards and Technology (NIST) has made major advancesto fully automate the CAD to CNC manufacturing processthroughtheuseofSTEPorratherAP-238.This projectinvolvedastronggroupofindustrialpartnersincludingBoeing,LockheadMartin, General Electric and General Motors, together withrecognised CAM vendors such as Gibbs Associates andMasterCAM.5. STEP-NC for more open and interoperablemachine toolsThere are four types of research work related to STEP-NC:(1) conventional CNC control using STEP-NC; (2) new STEP-NC enabled control; (3) STEP-NC enabled intelligent control;X.W. Xu, S.T. Newman/Computers in Industry 57 (2006) 141152144Fig. 2. Example STEP-NC physical file 20.and (4) collaborativeSTEP-NC enabled machining. The degreeof adaptability increases from Type 1 to Type 4. It is to be notedthat STEP-NC together with STEP is now forming a commondata model for representing complete product information. Itsfar-reaching effect lies in a total integration of CAD, CAPP,CAM and CNC with desired interoperability and adaptabilityacross the complete design to manufacturing chain. Due to thelimited scope of this paper, only the research work directlyrelated to STEP-NC enabled CAM/CNC is discussed.5.1. Conventional CNC control using STEP-NCThis type of research marked the beginning of STEP-NCrelated research endeavour. The main purpose is to answer twoquestions: Does a STEP-NC file contain enough and justenough information for CNC machining? and if it does, Canit be used on a traditional CNC machine tool without makingchanges to the system hardware?. The main research is to dowith the development of translators which can read in aSTEPAP-203 or AP-224 file and convert it into G-code formatthat the targeted CNC machine tool can understand. Thetranslator is somewhat similar to the post-processor used inmany CAD/CAM or CAM systems. The only difference is thatthe CAD/CAM, CAM and CNC systems are now madeinteroperable in a sense that the STEP compliant informationcan be used across the board. Also, the design information thatcan be embedded in a STEP-NC file is made available to theCNC systems. This scenario represents conventional solid-based manufacturing as enabled by STEP AP-203.Thework carried out in the first two stages of the three-stageSuperModel Project falls into this type of work. In stage one, arange of software tools (i.e. ST-Plan, ST-Machine, and STIX31) were developed involving GibbsCAM and various piecesof third-party software. The GibbsCAM STEP translator canreadinthedemonstrationpartinSTEPAP-203format.Thepartis then programmed using GibbsCAMs graphical interface,and visually verified using its cut part rendering capability 32.In the second stage, the AP-238 file was read using aGibbsCAM STEP-NC Adaptor plug-in, developed by STEPTools Inc. An MDSI Open CNC controller (software-basedCNC) 33 retrofitted to a Bridgeport vertical machining centrewas used as the platform for the GibbsCAM and STEP-NCsoftware. Using the tooling and operation parameters specifiedin the AP-238 file, the STEP-NC Adaptor created GibbsCAMtooling,processandgeometryelementsandexecutedGibbsCAM functions to generate toolpaths correspondingto the AP-238 machining features. Once again, the cut partrendering was used for visual verification prior to post-processing the data to generate conventional G-code output.This work has demonstrated the ability of STEP-NC tocompletely automate CAM processing and toolpath genera-tion. It has also significantly reduced the lead-time in the CAD/CAM to CNC programming time by up to 85% 32.More recently, at the Jet Propulsion Lab (JPL) in Pasadena,California, in January 2003, STEP Tools Inc. demonstratedthe conversion of AP-203 design data into AP-238 (i.e. theAIM version of STEP-NC), feature by feature, with the use ofST-Plan AP-238 data. AP-238 data was then transferred toGibbsCAM with the assistance of ST-Machine, and then to afive-axis Fadal machining centre. In June 2003 at NIST, asimilar set-up saw MasterCAM interface with another five-axismachine tool.5.2. New STEP-NC enabled controlWorking closely with some of the popular CNC controllersor Open Modular Architecture Controller 6, several researchteams around the world have been able to process STEP-NCinformation internally is a CNC controller. This is madepossible by developing for, and integrating a STEP-NCInterpreter into, these controllers that can faithfully performsthe machining tasks as specified in ISO 14649.The third stage work of the US SuperModel Project sawGibbsCAM integrated with an OMAC machine tool. An AP-238 data file provided all the manufacturing information toallow GibbsCAM to generate the toolpath data. The toolpathdatawasthensenttoahorizontalmachiningcentreinso-calledstroke-level inter-process communication rather than con-ventional G-codes, demonstrating a higher level of CAM/CNCintegration than is normally realised through ISO 6983.Most of the work carried out in EU falls into this category ofresearch. The main focus has been on the development of theSTEP-NCenabled CNC control usingSiemens 840D controller34. This enables the STEP-NC physical files to be integrateddirectly with the controller, with visualisation of the machiningfeatures and associated Workingsteps in a STEP-NC compliantversionoftheirShopMillCAMsystem.Programmingdevelopments in parallel with this work have been undertakenat WZL, University of Aachen, Germany, with the WZLShopfloor Programming System incorporating WZL Mill, aSTEP-NC compliant programming system and WZL-WOP(workshop-oriented programming). Commercial applicationsin Europe with CATIA and OpenMind systems have beenpresented by Volvo and Daimler Chrysler 34,15 illustratingthe capability of incorporating the standard within the CAD/CAM products and exporting the STEP-NC output to theSiemens 840D controller.InadditiontoSTEP-NCmillingdevelopmentsthetechnology has also been extended to CNC turning. Theprototype STEPTurn software module has been developed byISW Stuttgart, working with a Siemens 840 control on aBoehringer NG200 lathe 34, STEPTurn software can importCAD geometry and machining features, define machiningstrategies and technologies and generate STEP-NC output. TheSiemens controller receives this output and converts it into theSiemens ShopTurn system via a STEP-NC import facility.5.3. STEP-NC enabled intelligent controlThe dream of performing intelligent control on a CNCmachine has never been truly realized. The main reason is thatthe information (G-code) available to a CNC machine is toolow-level information, with which only minimum amount ofoptimization work can be carried out in real time or near realX.W. Xu, S.T. Newman/Computers in Industry 57 (2006) 141152145time. With STEP-NC, both design and process planninginformation is available to a CNC machine. It is possible for theCNC machines, or their controllers, to perform high-level,intelligent activities, such as automatic part setup; automaticandoptimaltoolpathgeneration;accuratemachiningstatusandresult feedback; complete collision avoidance check (takinginto account of fixture and in-process geometry); optimalWorkingstep sequence; adaptive control and on-machineinspection.The researchers at the NRL-SNT (National ResearchLaboratory for STEP-NC Technology) in PosTECH, KoreahavedevelopedaFeature-BasedSTEP-NCautonomouscontrolsystem based on an Open Architectural Virtual ManufacturingSystem3537.TheinformationinanISO14649partprogramis converted through an Interpreter into the internaldata format,i.e. process sequence in form of Process Sequence Graph(PSG).The EXPRESScompiler in the Interpreterconvertsthephysical file, in form of task description into a PSG, basedon the information such as geometry, technology and tooldescription. PSG represents a non-linear sequence of Work-ingstepsdescribedintermsofmachining_featureandmachining_operationusingtheANDOR relationship(Table 1 and Fig. 3). As presented in the PSG, the part canbe machined in a number of ways, making CNC executionflexible, optimal, intelligent and autonomous. The non-linearprocess sequence schema enables a STEP-compliant CNCautonomous system. In preparation for executing a STEP-NCprogram, a Tool Path Generator (PosTPG), Tool Path Simulator(PosTPS) and a soft-CNC called NCK/PLC have beendeveloped 35. NCK/PLC can convert the STEP-NC datamodel into machine tool motion, and is capable of NURBSinterpolation, look-ahead control, position/velocity interpola-tion and PID (Proportional, Integral, Derivative) control. Itinterfaces with machine tool hardware (drivers and motors) viaan I/O board.A STEP-compliant CNC machine tool that demonstrated aG-code free machining scenario has been developed at theManufacturing Systems Laboratory, University of Auckland38. This research work consists of two parts: retrofitting anexisting CNC machine and the development of a STEPcNC(STEP-compliant NC) Converter. The CompuCams motioncontrol system 39 is used to replace the existing CNCcontroller,which isprogrammable using itsownmotioncontrollanguage6 K Motion Control language and capable ofinterfacing with other CAPP/CAM programs through lan-guages such as Visual Basic, Visual C+ and Delphi. TheSTEPcNC Converter can understand and process STEP-NCcodes,andinterfacewiththeCNCcontrollerthroughaHumanMachine Interface. It makes use of STEP-NC information suchas Workplan, Workingstep, machining strategy, machiningfeatures and cutting tools that is present in a STEP-NC AIMfile.5.4. Collaborative STEP-NC enabled machiningIt can be said that the ultimategoal for the STEP-NC enabledmachiningistosupportWeb-based,distributedandcollaborativemanufacturing (Fig. 4), a scenario of design anywhere/buildanywhere.ThisispossibleasaSTEP-NCprogramcanseparatethegenericmanufacturinginformation(what-to-do),fromthemanufacturing information (how-to-do) that is native to aspecific machine tool. Therefore, a generic STEP-NC programcanbemademachine-independentandhasanadvantageovertheconventional, G-code based NC program which is alwaysgenerated for a particular CNC machine. For this type of STEP-NC program to be implemented on a native CNC system, thenativemanufacturingknowledgehastobeincorporated.Tofulfilthisfunction,anativeSTEP-NCmappingsystemcalledNativeSTEP-NC Adaptor has been developed 40. The adaptor isbuiltwiththree parts: anative CNCsystem knowledgedatabase,a Translator and a HumanComputer Interface. Thenative CNCsystem knowledge database has a proprietary data structure sothat the work in developing the Translator is made simpler andcoherent programming of NC components across the enterpriseis enabled.Recently,therehasbeen atrendofusingXML(orratherISO10303 Part 28) instead of EXPRESS language (or ISO 10303Part 21 41) to represent the STEP-NC information. Thereason for this is obvious. The XML processing ability canX.W. Xu, S.T. Newman/Computers in Industry 57 (2006) 141152146Table 1Workingstep list 37WorkingstepIDFeatureOperation1Planar_facePlane_rough_milling2Closed_pocketBottom_and_side_rough_milling3Round_holeDrilling4Round_holeDrilling5SlotBottom_and_side_rough_milling6SlotBottom_and_side_rough_milling7Round_holeDrilling8SlotBottom_and_side_rough_millingFig. 3. Process sequence graph 37.Fig. 4. Distributed, STEP complaint NC machining 20.easily support the e-Manufacturing scenario. CNC machinetools can share information with other departments in andoutside the company over the Internet/Intranet.ERC-ACI (Engineering Research Centre for AdvanceControl and Instrumentation) in Seoul National University4244 has been working toward developing an XML-enabledSTEP-NC data model for milling. It can search for, extract andstore, the toolpaths generated in XML format. The millingmachine used to test the system contains four modules (Fig. 5):XML Data Input module, Interpreter, Tool Path Generator andMotion Control Board. The XML Data Input module andInterpreter generate STEP-NC programs from CAD files,whereastheothertwogenerateandexecutenativeCNCprocessplans.A framework of a STEP-compliant CAPP system has beendeveloped at the University of Auckland 29,30. The systemadopts three-tiered, Web-based network architecture (Fig. 6).The client tier consists of a set of applications and a WebX.W. Xu, S.T. Newman/Computers in Industry 57 (2006) 141152147Fig. 5. STEP-NC milling machine 42.Fig. 6. A STEP-compliant collaborative manufacturing model.browser, enabling interactions between users and the system.The process plans that are used as an input to a CNC system aredescribed in accordance with the STEP-NC AIM standard.Instead of low level information as stipulated by ISO 6983,higher level information such as machining features, Work-ingsteps and Workplans is used to constitute a process plan. Adatabase structure has been proposed for both generic andnativemanufacturinginformationandXMLisusedtorepresentthe STEP-NC information in these databases.6. Portable STEP-NC toolpathOn3rdFebruary2005,theOMACSTEP-NCWorkingGrouphosted an STEP-NC Forum in Orlando, FL, USA. The mainpurposeofthedemonstrationistwo-fold:(a)todemonstratehowSTEP-NC information can support portable machining on five-axis machining centres; and (b) to see if STEP-NC toolpathdescription capabilities can be used to streamline the data flowbetween existing CAD/CAM systems and machining centres.The AIM (AP-238) version of STEP-NC was adopted, and itsCC1 (conformance class 1) Machine Independent toolpaths17 was used for the demonstration. The component tested wasthe5-AxisNAS979circle/diamond/squarepartwithaninvertedNAS 979 cone test in the centre (Fig. 7) 45.The business case and the main industry participant areBoeings manufacturing plants. Like others, most five-axismachining centres at Boeing receive machine control data (orMCD) in G-code format that defines each axis movementrequiredinordertomanufactureapart.Thisdirectprogrammingmodel means that the orientation axes are traversed assynchronized axes, and are tied to a specific tool length. Theproblem with these MCD programs is that they are neitherportable nor adaptable. Lack of portability presents a problemsince unique axes position data must be generated for eachmachine control combination (part, tool, and machine config-uration) on which the part is to be run. MCD programs are notadaptable as no information is providedto the machine tohelp itadapt to real-time changes in machining dynamics (feed andspeeds) or machine tool alignment (tool and wear offsets).By comparison, tool centre programming (TCP) definesprogram geometry as cutter movement data, instead of axismovementdata.TCPissimilartorobotic6Dposerepresentation.Motionisdefinedasa3Dtool-tipposition(X,Y,Z)anda3Dtoolaxis orientation(I,J,K).ForeachTCP(X,Y,Z,I, J,K),the CNCcontrols the two rotation axes so that the tool is positioned andoriented as specified. In addition, the CNC controller performstool offset compensation along the tool axis according to theposition of the tool tip in the proper position and orientation.STEP-NC allows tool centre programming to defineprogram geometry as cutter movement data, instead of axismovement data. STEP-NC also provides rich, high levelinformation about the part features, materials, cutters, anddimensional tolerances. In the aerospace industry, tighter andtighterparttolerancesaretheexpectednormsothattheneedforSTEP-NC is pronounced. TCP can provide some directaccuracy improvements since each CNC will determine itstool tip position, as opposed to a CAM system generating statictoolpaths as a series of axes positions. Since machinegeometries can vary slightly even between identical machines,expected accuracy improvement should be significant.AttheSTEP-NCForuminOrlando,fourCAD/CAMsystems(i.e.Unigraphics,Catia,GibbsCAMandMasterCAM)wereusedto generate CL part programs. These CL data represent angularcutter motions in a CNC configuration-independent I, J, K way,with the assumption that the underlying machine tool controllerwill translate the I, J, K into machine specific five-axes angularconfiguration. CL-AP-238 converters have been developed totranslatetheCLfileintoAP-238Part21filebasedontheAP-238CC1toolpathstechnology.ThisSTEP-NCfileencodesmachineWorkingsteps as TCP toolpaths based on the AP-238 Expressschema, which is then suitable for the transfer between differingmachining centres. Different machine-specific converters havebeen developed to translate the STEP-NC file into a controller-specific TCP programs (Fig. 8). These converters will beeventually embedded in the controllers. It is to be noted that theSTEP-NC (AP-238) fileis now neutral to all five-axismachines,be it a five-axis gantry CNC, C on A machining centre or ConBmachiningcentre.Itisportablebecauseitdefinesprogramgeometry as cutter movement data, instead of axes movementdata.Itisadaptablebecauseitcanaccountforanyconfigurationschanges on a machine tool.7. Challenges and opportunitiesThough some early research work has shown that STEP-NCcanbeanenablingtoolfordevelopingmoreopen,interoperableand intelligent CNC machine tools, to gain acceptability by theNC community and particularly the CNC programmers andoperators, a number of challenges still lie ahead. Thesechallenges also present ample opportunities for various partiessuch as NC machine tool manufacturers, CNC controllermanufacturers and commercial CAD/CAPP/CAM vendors.7.1. STEP-NC information modelsThe use of STEP-NC brings the benefit of better integrationfor information models from design to manufacture, effectivelyeliminating semantic errors and bringing an end to theX.W. Xu, S.T. Newman/Computers in Industry 57 (2006) 141152148Fig. 7. Five-axis NAS 979 circle/diamond/square part.translation between proprietary and non-proprietary formats.However, the fact that both STEP-NC ARM (ISO 14649) andAIM (AP-238) co-exist and have each been implemented bydifferent groups, presents a less than satisfactory environmentfor users. It is of particular importance that one understands thedifference between these two versions of STEP-NC prior toimplementation. The main difference between these twomodels is the degree towhich they use the STEP representationmethods and technical architecture. Both versions can beviewed as different implementation methods of the STEP-NCstandard. The ISO 14649 standard is more likely to be used inan environment in which CAM systems have exact informationfrom the shop-floor, whereas STEP AP-238, as a part of theSTEP standard, is more suitable for a complete design andmanufacturing integration. The ISO 14649 standard has fewmechanism to incorporate other types of STEP data, hencemaking bi-directional data flow between design and manu-facturing more difficult. Unlike ISO 14649, STEP AP-238encompasses all the information from STEP AP-203 and AP-224 plus an interpreted model mapped from ISO 14649. Hence,bi-directional data exchange is enabled. A major problem withSTEP AP-238 though, is that the STEP Integrated Resourcesused in AP-238 are not adapted to application areas; hence thedata in its files are fragmented and distributed. It only providesan information view of the data, whereas the ARM provides afunctional view of the data.7.2. Feature recognition for STEP-NCFor STEP-NC to take off, machining feature recognition isan important prerequisite, as it is a linkage between STEP-enabled CAD and CAPP. Most of the current featurerecognition systems are prototypes and incapable of dealingwith a sufficient broad space of feature spectrum. Also, veryfew systems can perform feature recognition in a STEP in,STEP out manner, i.e. recognizing STEP AP-224 machiningfeatures based on the STEP AP-203 or AP-214 data.The literature to date has shown some effort spared in thisarea. Based on the technology developed at Honeywell FederalManufacturing & Technologies (FM&T), ST-Plan can createSTEP AP-224 machining features from STEP (AP-203 or AP-214) data. Parameters such as tolerances, features, processesand tool requirementscan be manipulated. ST-Planclaims tobethefirst-to-the-marketsoftwarepackagededicated toSTEP-NCand e-Manufacturing 31. The system has two major modules:feature-based machining (FBMach) and feature-based toleran-cing (FBTol). FBMach is used to recognise manufacturingfeatures and allow Workingsteps to be defined for thosefeatures, whereas FBTol is used to define tolerances. FBMachcontains a library of machining features and feature recognitionalgorithms. The system creates both surface and volume-basedmachining features. A surface-based machining feature isbased on sets of faces on the solid model-the skin thatrepresents the shape of a feature. A volumetric machiningfeature is represented by delta volumes, which are solidbodies showing the shape and amount of material to beremoved. Toolpaths may also be determined by delta volumesin applications for generating CNC routines.The SFP system developed at NRL-SNT can also generateSTEP AP-224 features based on AP-203 data 3537. Thesefeatures are then input to the process planning stage, duringwhich the native machine tool information and machiningparameters are added to the program. Workingsteps of eachmanufacturing feature are defined and saved into an ISO 14649partprogram.TheSTEPturnsystemdevelopedatISWalsohasafeature recognition function that precedes Workingstep sequen-cing in order to generate a STEP-NC (ISO 14649) physical file.7.3. Intelligent CNC controllersSTEP-NC delivers a complete package of information, be itdesign or manufacturing information, to the CNC machine. Onthe one hand, the CNC controller gets much richer informationmaking it possible to perform a true adaptable, optimal andX.W. Xu, S.T. Newman/Computers in Industry 57 (2006) 141152149Fig. 8. Smart CNC data flow through using STEP-NC.intelligent control. On the other hand, the NC controllermanufacturers,oncepersuaded,havetore-designtheircontrollerstructuresandstrategiestotakethisadvantage.Thisbeingdone,amajor shift in culture and user belief is also needed to trust thenew breed of intelligent controllers to translate feature-basedconversational programming to the correct toolpath at themachine. This is recognized as a paradigm shift where theequivalent of todays post processor functions at the off-lineCAD/CAM system will take place at the machine (or within thenew controller) leading to the end of G-code era. A closercollaboration between the CNC control vendors and CAxvendors may have to happen to enable this dramatic change.7.4. Manufacturing knowledgeThe adoption of the STEP-NC paradigm brings forth theopportunity to consider not just the ways of representinginformation but also the type of information to be represented.To be more precisely, it is the knowledge required for processplanning and machining discrete parts that needs to beaccumulated and validated. The knowledge-based systemsthus developed need to be robust. At present the developmentsinSTEP-NChaveconsideredrelativelysimplecomponents,i.e.2D-components with autonomous or compound features.Parts containing more complex geometry with intricateinteractions require additional knowledge-based intelligentsolutions. A sound balance between standardizing suchsolutions as much as possible and still leaving enough roomfor customization is hard to keep.7.5. Challenges related to the supporting technologiesFor a STEP-NCenabledmachine tool tobefullyautonomous as well as interoperable, a suite of supportingtechnologies needs to be further developed. At the CAD/CAPPend, it is the oldtopic of automatic feature recognition basedon a STEPAP-203 model. At the machine side, more adaptableand faster control is needed. Intelligent embedded machatronicsystems and OMAC seem to offer a viable solution, but moreresearch has to be carried out in the areas of modular softwareand hardware design, intelligent control algorithms anddistributed control technology. The knowledge-based systemsas mentioned above need to be mobile and easy to share. XMLis being recognized as a promising means of modellingknowledge and distributing it across the Internet. CurrentInternet technologies are not yet fit to serve such portablesystems, not to mention the venerability of the Web space.8. ConclusionsModern CNC machine tools, though capable in function-alities, lack adaptability, portability and intelligence. This isdue to the fact that a 50-year-old language is still employed bythese machine tools. NC programs following this format areonly meant for execution on a specific machine tool. Theycannot be reinterpreted by a CAM system or a SFP system for adifferent machine tool. Automatic generation of a 100%optimised NC program is not possible as design informationand know-how about the machine tools and materials isrepresented in different formats and on different databases.STEP-NC can provide a uniform NC program format forCAM, SFP and NC, avoiding post-processing and entail a trulyexchangeable format. The operator can now be supported atCAM, SFP and NC level by complete information containingunderstandable geometry (features), task oriented operations,strategies and tool definitions. Availability of design data at themachining stagealso enables a reliablecollisioncheck, accuratesimulation and feedback from the machining stage to the designstage. Part programs following the STEP-NC standard areinteroperable in a sense that they can be adapted to any CNCmachine tools that have the ability to execute the machiningtasks. CNC machines implementing STEP-NC can have a moreopen and adaptable architecture, making it easier to integratewith other manufacturing facilities, e.g. workpiece handlingdevice. STEP-NC also supports distributed manufacturingscenariothrough,forexample,Ethernetconnectionstoaccomplish data collection, diagnostics and maintenance,monitoring and production scheduling on the same platform.The demonstration exhibited at the OMAC STEP-NC Forum issignificant. It showed that different CAD/CAM systems cangenerate the same, machine-neutral STEP-NC information. TheSTEP-NC file has been adapted to different five-axis CNCmachines.Thetestpart(NAS979)isatruefive-axiscomponent.It is to be pointed out that only AP-238 CC1 machineindependent toolpath data is used. Information such asmachiningfeaturesanddesigndataisnotconsidered.Therefore,there is limited adaptability the CNC machines can exercise inthis case.Therearestillissuestobeaddressedandchallengestobemet.ThesechallengescomefromthedriveforauniformedSTEP-NCinformation model, development of STEP-NC enabled intelli-gent controllers, capture of necessary manufacturing knowledgeto support decision-making at the machine tool level as well asother under-developed pertaining technologies. The challengesco-exist with the opportunities that if seized in time can yield amultitude of benefits that STEP-NC promises.References1 M.G. Mehrabi, A.G. Ulsoy, Y. Koren, Reconfigurable manufacturingsystems: key to future manufacturing, Journal of Intelligent Manufactur-ing 11 (3) (2000) 403419.2 M.G. Mehrabi, A.G. Ulsoy, Y. Koren, P. Heytler, Trends and perspectivesin flexible and reconfigurable manufacturing systems, Journal of Intelli-gent Manufacturing 13 (2) (2002) 135146.3 J. Zhang, F.T.S. Chan, P. Li, H.C.W. Lau, R.W.L. Ip, P. Samaranayak,Investigation of the reconfigurable control system for an agile manufac-turing cell, International Journal of Production Research 40 (15 SPEC)(2002) 37093723.4 E. Carpanzano, D. Dallefrate, F. Jatta, A modular framework for thedevelopment of self-reconfiguring manufacturing control systems, in: in:2002 IEEE/RSJ International Conference on Intelligent Robots andSystems, 30 September4 October 2002, Institute of Electrical andElectronics Engineers Inc., Lausanne, Switzerland, 2002.5 P. Lutz,W. Sperling,OSACAthevendor neutralcontrol architecture,in:D. Fichtner (Ed.), Facilitating Deployment of Information and Commu-nications Technologies for Competitive Manufacturing, Proceedings ofX.W. Xu, S.T. Newman/Computers in Industry 57 (2006) 141152150the European Conference on Integration in Manufacturing liM97, Selbst-verlag der TU Dresden, Dresden, 1997.6 Open Modular Architecture Controls: OMAC-HMI, OSACA, JOP-Stan-dard CNC Data Type Analysis, /wgs/MachTool/HMI-API/standards_compare.pdf, accessed on: 30/07/2004.7 ISO 10303-1: 1994, Industrial Automation Systems and IntegrationProduct Data Representation and Exchange, Part 1. Overview and funda-mental principles.8 ISO 10303-203: 1994, Industrial Automation Systems and IntegrationProduct Data Representation and Exchange, Part 203. Application pro-tocol: configuration controlled 3D designs of mechanical parts andassemblies.9 ISO 10303-214: 1994, Industrial Automation Systems and IntegrationProduct Data Representation and Exchange, Part 214. Application pro-tocol: core data for automotive mechanical design processes.10 ISO6983-1: 1982,NumericalControlof MachinesProgramFormat andDefinition of Address Words, Part 1. Data format for positioning, linemotion and contouring control systems.11 ISO14649-1:2003,DataModelforComputerizedNumericalControllers,Part 1. Overview and fundamental principles.12 ISO 14649-10: 2003, Data Model for Computerized Numerical Control-lers, Part 10. General process data.13 ISO 14649-11: 2003, Data Model for Computerized Numerical Control-lers, Part 11. Process data for milling.14 ISO 14649-111: 2001, Data Model for Computerized Numerical Con-trollers, Part 111. Tools for milling.15 ISO/DIS 14649-12: 2003, Data Model for Computerized NumericalControllers, Part 12. Process data for turning.16 ISO/DIS 14649-121: 2003. Data Model for Computerized NumericalControllers, Part 12. Tools for turning.17 ISO/DIS 10303-238: 2003, Industrial Automation Systems and Integra-tionProduct Data Representation and Exchange, Part 238. Applicationprotocols: application interpreted model for computerized numericalcontrollers.18 J. Wolf, Requirements in NC machining and use cases for STEP-NC,Analysis of ISO 14649 (ARM) and AP 238 (AIM). White Paper, ISO T24STEP-Manufacturing Meeting, San Diego, USA, March 2003.19 A.B. Feeney, T. Kramer, F. Proctor, M. Hardwick, D. Loffredo, STEP-NCimplementationARM or AIM? White Paper, ISO T2 STEP-Manufac-turing Meeting, San Diego, USA, March 2003.20 S.T. Newman, Integrated CAD/CAM/CNC manufacture for the 21stcentury, in: Keynote Speech, The 14th International Conference onFlexible Automation and Intelligent Manufacturing (FAIM2004), 1214 July 2004, Ryerson University, Toronto, Canada, 2004.21 R.D. Allen, S.T. Newman, J.A. Harding, RSU Rosso Jr., The design of aSTEP-NC compliant agent based CAD/CAM system, in: Proceedings ofthe 13th International Conference on Flexible Automation and IntelligentManufacturing (FAIM2003), Tampa, FL, USA, 2003), pp. 530540.22 ISO 13030-224: 2001, Industrial Automation Systems and IntegrationProduct Data Representation and Exchange, Part 224. Application pro-tocol: mechanical product definition for process plans using machiningfeatures.23 X.W. Xu, Q. He, Striving for a total integration of CAD, CAPP, CAM andCNC, Robotics and Computer Integrated Manufacturing 20 (2004) 101109.24 OMAC STEP-NC Working Group, The value proposition for STEP-NC,OMAC Users Group, Draft Version 4, 2002.25 X.W.Xu,H.Wang,J.Mao,S.T.Newman,T.R.Kramer,
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