FUTURE ISSUES FOR CAD/CAM AND
INTELLIGENT CNC MANUFACTURE
Roberto S.U. Rosso Jr*, R.D. Allen, and Stephen T. Newman
Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University,
LE11 3TU, Leicestershire, UK
*To whom correspondence should be addressed. E-mail address: R.S.U.Rosso-Jr@lboro.ac.uk.
ABSTRACT
The search for automatic manufacture of components has been and continues to be a
major goal of researchers since CNC machines appeared in the 1980’s. The ability to generate an
NC tool path is now commonplace from CAD/CAM systems, but the technology used to
program NC machines is still based on 1950’s standards. Today under the IMS project named
STEP-NC in Europe and Asia, and Super Model in the USA, industrialists and academics are
collaborating to deliver a new data model for CNC machines entitled the ISO14649 standard.
The authors believe that this standard will provide software vendors with the basis to meet this
automatic CNC manufacturing goal. This paper provides a futuristic view of how this standard
could be used in manufacturing and highlights the various issues for CAD/CAM vendors;
machine control vendors and manufacturing users.
KEYWORDS: STEP, CNC, CAD/CAM
1.
INTRODUCTION
The current standard to program NC machine tools has had no significant change since the
early 1950’s when the first NC (numerical control) machine was developed at M.I.T.
(Massachusetts Institute of Technology), U.S.A. These early NC machines and today’s NC
machines continue to use the same standard for programming namely G & M codes based on the
ISO 6893 standard [1].
Since the 1970’s significant developments have been made towards more automatic and
reliable computer numerically controlled machines with new processes such as punching &
nibbling, laser cutting, and water jet cutting which are now common place. The advent of the
Computer Numerical Control (CNC) brought a massive improvement in the capabilities of these
machines. Currently CNC machines provide the ability of multi-axis, multi-tool, and multiprocesses manufacture. These capabilities have made the programming task more and more
difficult and off-line software tools for CAD/CAM a necessity for efficient code generation.
Though these developments have revolutionised CNC processes and capabilities, the
programming language has basically stayed the same with G/M code programming which was
developed in the 1950’s and later became the ISO 6983 standard that is based on the tool path
and machine status description.
Since the beginning of CAD and CAM software, the problem of a model’s portability from
system to system was one of the key issues to spread the use of these tools. Many solutions were
proposed in the direction of a standard way of data exchange such as SET, VDA, and IGES,
which were partially successful [2] but were not totally suitable to all the needs of the
CAD/CAPP/CAM industry. Thus, the international community have developed the ISO10303
[3] set of standards, well known as STEP, which has its foundations in many of the earlier
aforementioned standards [2].
In parallel with these developments the use of solid modeller’s has brought the possibility
of a more realistic and reliable CAD model, however the pure geometric solid modellers (GSM)
were not developed enough to take the designer’s intent to the manufacture engineers. Feature
technology has been researched since 1976 [4] and was expected to fill this gap between the
design and manufacturing environment. However, neither the design intent nor the production
planning could be transferred completely to the CAM systems. Most of the information created
by the designer was lost, or needed to be re-entered, during the process-planning phase, which
included the creation of NC programs by CAM systems. Some researchers have tried to solve
this problem using feature recognition [5] [6] [7]. The key problem is in the creation and
exchange of models where different feature recognisers’ give different results for the same
problem. By using a design by features approach, within a GSM based CAD system, where the
modelling history is generally stored via feature design interactions. Though once the data is
exchanged from CAD to CAD or CAD to CAM the feature information is lost and then most of
the work must be done again with the additional problem of not understanding or assuming the
designers intent. This problem has been addressed in the standard ISO 10303 with the
Application Protocol 224 (AP224) [8], which provides a set of standardised machining features
for use in process planning.
2.
STEP COMPLIANT NC PROGRAMMING
The software evolution for NC programming has seen a number of generations, from the
beginning of hardwired machines, with manual block to block programming, to APT
(Automated Programming Tool), ADAPT[9], AUTOMAP, COMPACT II, and UNIAPT[10] and
the extensions of APT such as EXAPT[11], EXAPT II, and EXAPT III [9] to the modern
graphic interactive Computer Aided Manufacture (CAM) systems. Today the software and
hardware available at machine tools makes it possible to simulate graphically the tool motion,
material removed, and use adaptive control for on-line improvement. The current trends are
towards open architectures such as OSACA [12] and OMAC [13] where third party software can
be used at the controller working within a standard PC operating system.
One further industrial development is the application of software controllers, where PLC
logic is captured in software rather than in hardware. Such systems for example the MDSI CNC
architecture [14], provides many opportunities to implement open control capabilities, but they
are mainly used in retrofitting applications for older CNC & NC machines. Although these
developments have improved software tools and the architecture of CNC machine tools, vendors
and users are still seeking a common language for CAD, CAPP, CAM, and CNC, which
integrates and translates the knowledge of each stage. It is with this aim that the STEPCompliant NC (STEP-C-NC) programming is being developed to provide consistent standards
for automatic and quality oriented CNC component manufacture. To this end in the second half
of the 1990’s an effort from the international community backed by the International
Organization for Standardization started a major change in the concept of NC programming. A
new data interface called ISO 14649 [15] started to be developed under the ISO technical
Committee TC184 in the Sub-Committees SC1 and SC4 using the ISO 10303 known as STEP as
a basis. Contrary to the current NC programming standard ISO 6983, known by G/M codes, the
ISO 14649 is not a method for programming and does not describe the tool movements for a
CNC machine. Instead, ISO 14649 provides a object oriented data model for CNC’s with a
detailed and structured data interface that incorporates feature based programming where there is
a range of information such as the feature to be machined, type of tools used, the operations to
perform, and the work plan [16].
Header (supply general information)
FILE_DESCRIPTION;
FILE_NAME;
FILE_SCHEMA;
DATA (Information about manufacturing tasks & geometry)
Part Identification
Workpiece
Material
Property Parameter
Identification of the Part
Part Material Identification
Material Property Parameter
Project Start Point of executions
Workplan & Executables:
NC Functions
Program Structure
Workingsteps
(Workplans)
Technology description :
Descriptions of Workingsteps used in the Workplan
Association with Workingsteps &: Regions, Surfaces and Features
Technological Information
Tool Description
Geometry & Topology description :
Positions
Axis Placements
Directions …..
Cutting width,
speed,
Finish Allowance,
Spindle,
Feed,
Tool used
Dimensions,
Tool holder…..
Type,
ISO 10303 Part 41
Reference TO
Part 42
Part 511
Part 514
Figure 1- The general structure of a STEP compliant NC code (Based on [17]).
For each operation performed on one or more features, a statement called a workingstep is
defined. These workingsteps provide the basis of the workplan to manufacture the component.
Figure 1 shows the general structure of the ISO 14649 data. Figure 2 illustrates the actual extract
of such data for a part with a workplan consisting of workingsteps, for facing, drilling, and
pocketing.
Figure 2 - Part of a STEP-C-NC code (Adapted from [18])
3.
IMPLEMENTATION FRAMEWORKS TOWARDS STEP
COMPLIANT CAD/CAM
The development of ISO 14649 provides a number of options for interpretation and
implementation of this standard within CAD/CAM systems. The implementations in this paper
are defined by the authors as ISO 14649 compliant, and referred to as STEP compliant NC
(STEP-C-NC). Three frameworks are defined for STEP-C-NC as outlined below:
i)
ii)
iii)
A CAD/CAM system which imports/generates a STEP-C-NC output;
A CAD/CAM Systems with STEP-C-NC data support structures;
A CAD/CAM environment with kernel STEP-compliant data structure.
3.1 A CAD/CAM system which imports/generates STEP-C-NC output
The first form of STEP-C-NC framework provides a CAD/CAM System with the ability to
generate ISO 14649 output. This framework can be further classified into two variants; the first
has the sole ability to export STEP-C-NC code and the second with import and export
capabilities.
3.1.1 CAD/CAM system exports STEP-C-NC data
The first variant of this CAD/CAM framework is used in its normal operational form using
its own native feature representation and manufacturing strategies for the design and
manufacture of components. The generation of the ISO14649 output is created by mapping the
native CAD/CAM information structures onto the STEP compliant data through a post processor
specifically for ISO 14649. A major issue for this basic form of STEP compliance is that the
CAD/CAM information stored on manufacturing technology (eg. materials, tooling, clamping
and machining strategy data) has to be converted into ISO 14649 format via the post processor. It
should be noted that many of the current CAD/CAM systems will probably not have the
flexibility to incorporate this STEP output, as the output is so different to the normal G/M code
output plus further data is required on the geometry of the component not just machining data.
This framework is illustrated in figure 3.
3.1.2 CAD/CAM System imports and exports STEP-C-NC code
The second of framework is able to both import and generate STEP-C-NC output. The
imported STEP compliant data is translated into the native geometric and manufacturing data
structures of the CAD/CAM system using a reverse post-processor. The user would then use the
system in its normal operating manner, and have the ability to generate STEP-C-NC output as in
section 3.1.1.
Figure 3 – Framework that exports
STEP-C-NC data
Figure 4 – Variant framework
imports/exports STEP-C-NC data
3.2 A CAD/CAM Systems with STEP-C-NC data support structures
This framework is classified by the CAD/CAM system having a STEP-C-NC data support
structure that maybe held internally or externally to the CAD/CAM system. Two variants of this
framework are outlined below.
3.2.1 A CAD/CAM system integrated with an external STEP-compliant data support
system
In this case, the CAD/CAM system is integrated with an external software system to
provide the STEP compliant data management support. The framework illustrated in figure 5,
shows the CAD/CAM system functioning independently to a STEP-compliant data management
system. This is used not only for importing information and generating STEP-compliant code,
but also in interpreting the native CAD/CAM geometric and manufacturing routines. In addition
it has STEP-compliant workplans and greater control to configure the output compared to using
the solutions represented in section 3.1
3.2.2 A CAD/CAM environment with both native and STEP-compliant internal data
structures
This case represents a CAD/CAM system, which has a dual internal representation of
geometric and manufacturing data, both in the native format of the CAD/CAM system and in the
ISO 14649 format. Thus in the operational use of the system both sets of data are updated in real
time rather than generating the STEP output through a post processor or having the STEP
information structures in an external software environment. Although this framework has
essentially the same principle as in 3.2.1, it represents a significant improvement since no postprocessing is required. This framework is illustrated in figure 6.
Figure 5 – An external STEP-C-NC
interfaced CAD/CAM environment
Figure 6– An internally shared STEP-C-NC
CAD/CAM environment
3.3 A CAD/CAM environment with kernel STEP-compliant data structure
This highest level of compliance is based on a CAD/CAM system, which uses ISO 14649
and ISO 10303-224 for both geometric and manufacturing data models. The authors believe this
structure, will form the basis of new developments for CAD/CAM vendors and will also provide
the direct and simplest generation of ISO 14649 NC code. One overhead of this framework will
be the need to post process the STEP data into ISO 6983 G/M code output for conventional CNC
controllers.
Figure 7 - Framework to for an internal
STEP-compliant data structure
4.
ISSUES FOR THE IMPLEMENTATION OF STEP COMPLIANT CAD/CAM
As the development of the ISO 14649 data model gains significant impetus on the way to
becoming an International Standard, the issues of how to create CAD/CAM systems that are ISO
14649 compliant and their levels of compliance will depend on the industrial uses and software
vendors acceptance of the standards. These developments raise a number of issues some of
which are well recognised previously from feature based CAD/CAM plus new issues related to
the STEP standards. These issues are discussed below.
4.1 Features
It is recognised that the use of features has brought many advantages to the design and
manufacturing environment. It is particularly valuable in the way it communicates in
manufacturing terms such as holes, bosses, and pockets instead of the Boolean operations using
cylinders, cubes, and other geometric entities. Features add meaning to the geometric model and
this is a major contribution of this technology.
The view of CAD/CAM developers consider design, process planning and manufacturing
as very related fields which can be considered complimentary in their use of knowledge.
However, it seems that that the use of features in CAD/CAPP and CAM shows a remarkable lack
of consensus.
Companies prefer to define theirs own set of features, sometimes slightly different from
each other. The consequence is a lack of consensus between implementations and the consequent
loss or misunderstanding of the designer’s intent. In the case of a design and manufacturing
environment this problem adds frustration to the user, moreover the geometric and
manufacturing models suffer from a lack consensus throughout the scientific and engineering
community. To help solve this problem AP224 [8] was developed in the scope of the ISO 10303
standard. AP 224 is generally recognised as a large set of machining features that can be used to
exchange information needed to create machined mechanical parts. The ISO 14649 standard [17]
uses the ISO 10303-224 as a basis to define its features. During the development of ISO 14649
some features were re-defined resulting in the need to map from AP224 to ISO 14649 during
import export routines. It is the authors believe that the use of features defined in ISO 10303-224
should be used in order to avoid any multiple definitions.
The literature has shown the use of both, design by features and features recognition
approaches [5][6][7][19] for the generation of features within a CAD/CAM model. A third
approach is the hybrid solution [20][21], which combines the feature recognition to understand
imported geometric models and the design by feature capabilities to provide the means to create
models or to change imported models. A fourth approach can be envisaged, which uses a
translator to exchange feature information among two or more software systems. The use of
translators to transfer geometric information based on neutral standards such as ISO AP203 [22]
and AP214 [23] or proprietary standards such as ACISâ and Parasolidâ models plus feature
translators provides a way to import a component model to a CAD/CAM system. Since both the
geometry and the features would be available to create a model with the same type of
information available in the original system. There are a number of proposed solutions to use
geometry plus features either in research institutions [24] [25] or at a commercial level [26]. The
method proposed by Allen et all [24] uses the AP 224 features together with the geometric model
imported using AP203/214 as a way to have STEP compliant data to perform process planning
and to generate a STEP compliant NC program.
4.2 Machining Strategies
The ISO 14649 standard has a defined set of machining strategies that can be used in a
program. Some of them have definitions quite near to the well-recognised G-code canned cycles
in ISO 6983. Although the task is performed in a similar manner, the parameters sometimes are
different. At the highest levels of STEP-compliance it is desirable that the CAD/CAM system
should be able to capture the machining operations/strategies according too ISO 14649. Without
this level of knowledge this system will merely be translating G and M codes cycles into a basic
STEP-C-NC program resulting in much of the operation intelligence being lost.
This is not a problem at present since there are only few prototype CNC controllers that are
able to intelligently process a STEP-C-NC program. It should be recognised that this would be a
future drawback for the vendors, who don’t follow the standard.
4.3 Generic versus Machine Specific NC Programs
The new data interface provides a way to create very portable programs, which means
write once use many times. The idea is feasible in theory, since the development a controller
which, has the intelligence to adapt the program to the specific environment as proposed by
Yamazaki [27] and ISO14649 compliant as proposed by Suh [28]. Within Europe, Siemens had
demonstrated the first prototype of an industrial model of a STEP compliant CNC controller for
milling using their own vendor proprietary architecture [29]. Some issues could be raised about
the limits of the portability since there are physical restrictions to machining a part. A program
written to perform a task that demands a five-axis machine tool would not work on a three-axis
machine.
4.4
Process Planning
There were many issues in the past about the way to create, store, and transfer information
about the process of machining components. The ISO 14649 [15][17][18] has a systematic and
detailed structure to solve at least the issue of storage and transfer of this information. One of its
most powerful points as mentioned in section 2 is the richness of information available in a
program written according to the new standard. In reality, the information in a STEP-C-NC
program would be recognised as a detailed process plan rather than a NC program. Details such
as the type of operation, strategy, and tool used to machine a given feature in a workingstep and
the sequence of operations given by the workplan statement. Furthermore, one of the aims of the
standard is to provide capability to be used as the basis for bi- and multi-directional data
exchange between other information systems [17]. Once a program is modified and tested at the
shop floor it can be uploaded back to the process plan database maintaining the data integrity.
4.5
CNC Machining knowledge
For the current generation of ISO 6983 based CNC it is generally a problem to know the
format of the program and what each canned cycle means in terms of tool behaviour. This is
more complex for the new generation of STEP-Compliant than the current CAM and CNC
controllers since it is not a matter of what is the format of the program but how the controller
would react to each instruction. It should be observed that if you have intelligent CNC
controllers they could perform the same statement differently since the knowledge bases are
different. One new issue, which arises with the standard, is the problem that different pairs of
machine-tool/controllers would provide different tool paths. This means the knowledge
embedded in each controller needs to be available in the CAD/CAM system. Therefore
knowledge bases are expected to be very large in order to provide support for many controllers.
4.6
Validation of a STEP-compliant program
Machine capability data is crucial information during the simulation of an NC program.
Although ISO 14649 lets the user create programs without the need to describe the tool path,
during the simulation the user would expect to see the actual tool’s trajectory for a given
machine. Therefore, the CAD/CAM system must have access to the knowledge embedded in the
controller being simulated. Presently the developed ISO 14649 solutions use their native
machining strategies, mostly based on G code canned cycles, to simulate the tool path. The
authors believe this is not the best way to perform the task since different intelligent controllers
would act in different ways. They also believe in the need of an agreement between controllers
and CAM systems vendors to solve this problem.
5.
CONCLUSIONS
The recent development of the ISO 14649 standard provides a major opportunity to take
full advantage of the programs and machining capabilities of CNC controllers. This paper
outlines thee major CAD/CAM frameworks to support the implementation of the standard with
various levels of STEP compliant architecture. Issues related to the implementation of these
frameworks and their use with STEP compliant NC controllers provide a major change in the
current day use of CAD, CAPP, CAM and CNC systems. This change will bring new challenges
to industrial users and software vendors to identify the new boundaries and define intelligent
CNC manufacture in the 21st century.
6.
ACKNOWLEDGMENTS
The authors wish to thank the Engineering and Physical Science Research Council
(EPSRC), Loughborough University, UDESC-State University of Santa Catarina, and Conselho
Nacional de Ciência e Tecnologia (CNPq-Brazil) in support of the studentships to undertake this
research.
7. BIBLIOGRAPHY
[1]
[2]
International Organization for Standartization – ISO 6983/1 – Numerical control of
machines – Program format and definition of address words – Part 1,: Data format for
positioning, line and contouring control systems. First edition, 15/09/1982.
Sharon J. Kemmerer (Editor), STEP The Grand Experience. Manufacturing Engineering
Laboratory. National Inst. of Standards and Technology. Gaithersburg (MD), July 1999.
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
International Organization for Standartization-ISO10303- Industrial automation systems
and integration – Product Data Representation and Exchange - Part 1: Overview and
fundamental principles. 1994.
A. R. Grayer, A Computer Link Between Design and Manufacture. Ph.D. Dissertation.
University of Cambridge. September 1976.
Thomas R. Kramer, A parser that converts a boundary representation into features
representation. Intl Journal of CIM. Vol. 2. No. 3. (1989) 154-163.
Z. Gu, Y.F. Zhang, A. Y. C. Nee, Generic form feature recognition and operation selection
using connectionist modeling. Journal of Intelligent Manufacturing. Vol. 6, (1995), 263273.
V. Allada and S. Amand, Feature-based Modelling Approaches for Integrated
Manufacturing: State-of-the-art Survey and Future Directions. International Journal of
Computer Integrated Manufacturing. Vol. 8, No. 6, (1995), 411-440.
International Organization for Standartization - ISO 10303 - Industrial Automation
Systems and Integration -Product Data Representation and Exchange - Part 224:
Application Protocol: Mechanical Product Definition for Process Planning Using
Machining Features. December 2000.
Mikell P. Groover, Automation, production systems, and computer-integrated
manufacturing. 2nd Ed., London, Prentice Hall International (2001).
Warren S. Seames, Computer Numerical Control: concepts and programming. 3rd Ed.,
Albany, Delmar Publishers, (1995).
R. I. M. Young, Machine Tool Programming – Techniques and Trends. In: Handbook of
Design, Manufacturing and Automation. Richard C. Dorf and Andrew Kusiak (Editors).
New York, John Wiley, (1994).
Peter Lutz and Wolfgang Sperling, OSACA - The vendor neutral Control Architecture. In:
”Facilitating Deployment of Information and Communications Technologies for
Competitive Manufacturing“, Proc. of the European Conference on Integration in
Manufacturing IiM’97, Dresden, Ed. by D. Fichtner, et al., Selbstverlag der TU Dresden,
1997, ISBN 3-86005-192-X
Open Modular Architecture Controls Users Group. Business Justification of Open
Architecture Control. White Paper V1.0. 08/04/1999. Word Wide Web URL:
http://www.arcweb.com/omac/ guidelines/BusJustV1.pdf. Available in 07/03/2001.
Manufacturing Data Systems Inc., OpenCNC Architecture. PDF document.
http://www.mdsi2.com/images/DataS_Arch.pdf. Available in 17/10/2000.
International Organization for Standartization – ISO/DIS 14649-1 Industrial automation
systems and integration – Physical device control – Data model for computerized
numerical controllers – Part 1: Overview and fundamental principles. 28/09/2000.
M. Weck and Jochen Wolf. STEP-NC – The STEP compliant NC Programming Interface:
Evaluation and Improvement of the modern Interface. In: IMS Forum. Ascona /
Switzerland, October/2001.
International Organization for Standartization – ISO/DIS 14649-10 Industrial automation
systems and integration – Physical device control – Data model for computerized
numerical controllers – Part 10: General process data. 09/2000.
International Organization for Standartization – ISO/DIS 14649-11 Industrial automation
systems and integration – Physical device control – Data model for computerized
numerical controllers – Part 11: Process data for milling. 09/2000.
K. Case and M. S. Hounsell, Feature modelling: a validation methodology and its
evaluation. J. of Materials Processing Technology. Vol. 107 (2000), 15-23.
[20] J. HAN, and A. A. G. Requicha, Integration of Feature based Design and Feature
Recognition. Computer Aided Design. Vol. 29 No.5, (1997) 393-403.
[21] T. Laako and M. Mäntylä, Feature Modelling by Incremental Feature Recognition.
Computer Aided Design. Vol. 25 No. 8 (1993) 479-492.
[22] International Organization for Standartization - ISO 10303- Industrial Automation Systems
and Integration - Product Data Representation and Exchange-Part 203: Application
Protocol: Configuration Controlled Design First Edition; Technical Corrigendum 1: 1995.
[23] International Organization for Standartization - ISO 10303-214 – Industrial Automation
System and Integration – Product Data Representation and Exchange - Part 214:
Application protocol: Core data for automotive mechanical design processes. March 2001.
[24] R. D. Allen, R. S. U. Rosso Jr. and S. T. Newman. AB-CAM: an agent-based methodology
for the manufacture of STEP compliant feature based components, In: Metal Cutting and
High Speed Machining. Edited by D. Dudzinski et al., Kluwer Academic/Plenum (2002),
351-362. ISBN 0-306-46725-9
[25] Mangesh P. Bhandarkar and Rakesh Nagi, STEP-based feature extraction from STEP
geometry for Agile Manufacturing.Computers in Industry.Vol. 41(1)January 2000,3-24.
[26] Anonymous , Features and History. CAD User. Vol. 15(01), 2002, 20-22.
[27] Kazuo Yamazaki, Yoshimaro Hanaki, Masahiro Mori, Kazusaka Tezuka. Autonomously
proficient CNC controller for high-performance machine tools based on an open
architecture concept. Annals of the CIRP. (1997)Vol. 46/1. 275-278.
[28] Suk-Hwan Suh, Jung-Hoon Cho, Hee-Dong Hong. On the architecture of intelligent STEPcompliant CNC. Int. Journal of CIM. (2002).Vol.15. no. 2. 168-177.
[29] Michael Weyrich, Bernard Rommel, Siegmar Haasis, Peter Muller, First Prototype of a NC
Controller based on STEP-NC. In: Sciences Days 2000. (2000), Available at the URL:
http://public.prostep.de/