03-08-2012, 10:29 AM
Striving for a totalintegration of CAD, CAPP, CAM and CNC
Striving for a totalintegration of CAD, CAPP, CAM and CNC.pdf (Size: 298.28 KB / Downloads: 105)
Abstract
Over the last 50 years, conventional programming of the numerical controlled (NC) machine tools has been based on a
data model stipulated by ISO 6983, or otherwise known as G-codes. It was designed to sequentially pass instructions to the controls
of machine tools with little, if any, intelligence. It has been realized that this standard has led to serious impediments for the
fulfilment of total CAD/CAM integration. The forthcoming ISO 14649 defines a new interface for exchange of information between
CAD/CAM systems and NC machine tools. ISO 14649 is strictly harmonized with ISO 10303 (STEP), hence the name STEP-NC
meaning an extended STEP for numericalcontrol . There has been substantialamount of research being carried out concerning
STEP-NC, in particular EU and USA; its benefits and potentials are revealing and appealing. The manufacturing industry may have
to embrace itself with a revolutionary change even more dramatic than that of introduction of numerical control about half a
century ago.
r 2003 Elsevier Ltd. All rights reserved.
Keywords: G-code; Numericalcontrol ; STEP,STEP-NC,CAD/CAM
1. Introduction
Modern manufacturing enterprises are built from
facilities spread around the globe, which often contain
equipment from different manufacturers. Immense
volume of product information must be transferred
between the various facilities at different locations.
Today’s digital communication technology has solved
the problem of reliably transferring information across
the global networks. For mechanical parts, the description
of product data has been standardized by ISO
10303 (also known as STEP) in form of exchangeable
files, application programming interfaces and database
implementation. This leads to the possibility of using
standard data throughout the entire product process
chain in the manufacturing environment. The impediments
to realizing this principle are believed to be the
data format used at the machine level. Most computer
numericalco ntrol(CNC ) machines are programmed
using the ISO 6983 ‘‘G-code’’ language, or otherwise
known as RS274D. CNC programs are typically
generated by computer-aided manufacturing (CAM)
systems that use information from a computer-aided
design (CAD) system. The problems occurred to the
CNC machines are further magnified due to the
indispensable role these CNC machines are playing in
the manufacturing world.
ISO 14649 is a new standard for data transferring
between CAD/CAM systems and CNC machines. It
remedies the shortcomings of ISO 6983 by specifying the
machining processes rather than machine toolmotion
by means of ‘‘Workingstep’’, as the basic entity.
Workingsteps are effectively machining tasks that
correspond to high-level machining features and associated
process parameters. The future CNC controllers
are responsible for translating Workingsteps, instead of
G, M codes, to axis motion and tooloperation s. A
major benefit of using ISO 14649 is its totalcon formity
to ISO 10303. In fact, the standard that ISO 14649
defines is called STEP-NC, namely STEP extended
to NC.
This paper overviews the STEP-NC standard and the
G-code that is to be replaced by STEP-NC. The current
research activities are summarized; the benefits, potentials,
challenges and opportunities concerning STEP-NC
are also discussed.
*Corresponding author. Tel.: +64-9-373-7599; fax: +64-9-373-
7479.
E-mail address: x.xu[at]auckland.ac.nz (X.W. Xu).
0736-5845/$ - see front matter r 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.rcim.2003.08.003
2. Integrating CAD with CAM
The basic role of CAD is to precisely define the
geometry of a design, as it is criticalto all the subsequent
activities in the product cycle. Similarly, CAM is the
technology concerned with the use of computer systems
to plan, manage and control manufacturing operations
through either direct or indirect computer interface with
the plant’s production resources so that a design can be
materialized. Because a 3D model contains enough
information for NC cutter-path programming, the
linking between CAD and CAMis made easier. Socalled
turnkey CAD/CAM systems were developed based on
this concept and became popular in late 1980s. One of
the earliest developed technologies in the areas of CAM
is Numeric Control(NC), which is the technique of
using programmed instructions to controla machine
that mills, cuts, punches, grinds, bends or turns raw
stock into a finished part.
In an integrated CAD/CAM environment, there
are two types of interfaces, the interface for neutral
(mostly geometric) data exchange between CAD and
CAM or two different CAD or CAM systems, and
the interface for communications between a CAM
system and a CNC machine tool. The former is now
available and generally considered sufficient. The later,
which is effectively the machine control data (NC part
program), presents a weak link in a much-needed
integrated CAD/CAM environment. Admittedly, it is
rooted from a long established international standard—
ISO 6983.
3. ISO 6983 and the problems
According to ISO 6983, the CNC coding is based on
the following stipulations [1]:
Preparatory functions: From G0 to G99;
Miscellaneous commands: M (also called Machine
function);
Axis motion commands: X, Y, Z, A , B, C;
Feed and speed commands: F (Feed Rate), S (Spindle
Speed);
Identification commands: N (Block number);
T (Cutting Tool).
Because of the efficiency in processing, precision in
machining and easiness in operating, CNC machines
have been widely used in manufacturing industries all
over the world. Nevertheless, a number of problems
are found with ISO 6983, which are summarized
underneath.
(a) The language focuses on programming the path of
the cutter centre location (CL) with respect to the
machine axes, rather than the machining tasks with
respect to the part.
(b) The standard defines the syntax of program
statements, but in most cases leaves the semantics
ambiguous.
© Vendors usually supplement the language with
extensions that are not covered in the limited scope
of ISO 6983, hence the CNC programs are not
exchangeable.
(d) It only supports one-way information flow from
design to manufacturing, the changes in the shopfloor
cannot be directly fed back to the designer.
Hence, invaluable experiences on the shop-floor can
hardly be preserved.
(e) There is limited control of program execution and it
is difficult to change the program in the workshop.
(f) The CAD data are not used directly on the
machine, instead, they have to be processed by a
machine-specific post-processor, only to obtain a set
of low-level, incomplete data that makes verification
and simulation difficult, if not impossible.
(g) ISO 6983 does not support the spline data, which
makes it incapable of controlling five or more axis
milling.
4. Data exchange in CAD/CAM
Many data exchange formats have been developed in
the past. Their primary purpose is to exchange
geometric data. The most widely accepted formats have
been the Drawing Transfer File (DXF), the Initial
Graphics Exchange Standard (IGES) and the Product
Description Exchange for Standard (PDES).
Published in 1994, STEP/ISO 10303 defines the
industrialautoma tion systems and integration for
product data representation and exchange. Unlike its
other predecessors, STEP provides a neutralmechani sm
by specifying a form capable of describing the entire
product data throughout the life cycle of a product,
independent of any particular system. Part of STEP
(AP11) renders a data description language ‘‘EXPRESS’’,
which is used to describe some of the
application protocols (APs). EXPRESS is general and
similar to an object-oriented programming language. It
provides a set of basic types that are predefined and
available to use in defining higher-level types.
A graphical form of the language, called EXPRESSG,
was also defined in the standard. The big difference
between STEP and other data format is that STEP is
designed so that virtually all the information pertaining
to a product, not just geometric information, can be
communicated among different users.
There are three main reasons for implementing STEP:
* Data Exchange—Exchange product data with consumers
and suppliers to improve contracting;
102 X.W. Xu, Q. He / Robotics and Computer-Integrated Manufacturing 20 (2004) 101–109
* Data Sharing—Store product data in a standard
database for use by externaland internal supply
chains; and
* Internet Collaboration—Product data can be easily
accessed via Internet.