Making CNC Machine Tools More Open, Iterroperable and Intelligent

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Making CNC machine tools more open, interoperable andintelligent—a review of the technologies

X.W. Xu a,*, S.T. Newman b

a Department of Mechanical Engineering, School of Engineering, The University of Auckland,

Private Bag 92019, Auckland, New Zealand b Department of Mechanical Engineering, University of Bath, Bath, BA2 7AY, UK

Received 30 August 2004; accepted 7 June 2005

Available online 10 October 2005

Abstract

The aim of the next generation of computer numerically controlled (CNC) machines is to be portable, interoperable and adaptable. Over the

years, G-codes (ISO 6983) have been extensively used by the CNC machine tools for part programming and are now considered as a bottleneck for

developing next generation of CNC machines. A new standard known as STEP-NC is being developed as the data model for a new breed of CNC

machine tools. The data model represents a common standard specifically aimed at the intelligent CNC manufacturing workstation, making the

goal of a standardised CNC controller and NC code generation facility a reality. It is believed that CNC machines implementing STEP-NC will be

the basis for a more open and adaptable architecture. This paper outlines a futuristic view of STEP-NC to support distributed interoperable

intelligent 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-NC

1. Introduction

From the start of craft production in the 1800s to the

pioneering mass production of the early 1900s there have been a

number of revolutionary changes to manufacturing system’s

configurations. The most recognised traditional configuration of

manufacturing systems was the dedicated transfer (machine)

line, which enabled mass production at high efficiency and low

cost. With the need of the 1970s and 1980s to produce a wider

range of parts, ‘‘flexible’’ manufacturing was developed to meet

these needs for the production of smaller batches of different

parts. These systems used groups of computer numericallycontrolled (CNC) machinesthat couldbe reprogrammed to make

different parts combined with automated transport systems and

storage. These CNC machinesbecame thecentral elements in the

systems such as flexible transfer lines, flexible manufacturing

systems (FMS) and flexible manufacturing cells (FMC).

However, the amount of flexibility existing in these

systems was still believed to be limited. In order to prepare

manufacturing companies to face increasingly frequent and

unpredictable market changes with confidence, interoperable

and more open manufacturing systems are needed. In the

process of designing and operating interoperable and open

manufacturing systems there is a need to distinguish from

among system-level issues, component-level (i.e. machine

and control) issues, and ramp-up time reduction issues [1,2].

Most of the research effort has been spared on the issues at the

system level, some at the component level and little on the

ramp-up time reduction issues. At the component level,research work has primarily centred around the control issues

concerning machine tools, with the aim to provide enabling

CNC technologies for modular and open-architecture control

[3,4].

CNC machine tools are the main components in any

manufacturing system. There are demands and new opportu-

nities to empower the current CNC machines with the much-

needed features such as interoperability, adaptability, agility

and reconfigurability. To this end, there are two major issues

that need to be addressed namely product data compatibility/

www.elsevier.com/locate/compindComputers in Industry 57 (2006) 141–152

* Corresponding author. Tel.: +64 9 373 7599x84527; fax: +64 9 373 7479.

E-mail address: [email protected] (X.W. Xu).

0166-3615/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.compind.2005.06.002


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interoperability and adaptable CNC machines. Up till now little

research has been carried out in this field, but due to the

developments of the new CNC data model known as STEP-NC,

there has been a surge of research activities in trying to address

the above-mentioned issues. This paper reports on these

research activities and tries to address the issues of interoper-

ability and adaptability for CNC machine tools.

2. Impediments of current CNC technologies

Today’s CNC machine designs are well developed with

capabilities such as multi-axis control, error compensation and

multi-process manufacture (e.g. combined mill/turn/laser and

grinding machines). In the mean time, these capabilities have

made the programming task increasingly more difficult and

machine tools themselves less adaptable. Some effort has been

made to alleviate this problem, in particularly the trend towards

open architecture control, based on OSACA [5] and open

modular architecture controller (OMAC) [6], where third party

software can be used at the controller working within a standardwindows operating system. One further recognisable industrial

development is the application of software controllers, where

PLC logic is captured in software rather than in hardware.

Although these developments have improved software tools

and the architecture of CNC systems, vendors and users are still

seeking a common language for CAD, CAPP, CAM, and CNC,

which integrates and translates the knowledge of each stage

with no information loss. Though there are many CAM tools

supporting NC manufacture, the problem of adaptability and

interoperability from system to system was and is still seen as

one of the key issues in limiting the wider use of these tools.

2.1. Product data compatibility and interoperability

CNC machine tools complete the product design and

manufacturing lifecycle, and more often than not they have to

communicate with upstream sub-systems, such as CAD, CAPP

and 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 between

heterogeneous CAD and/or CAM systems. This is however

only partially successful since these protocols are mainly

designed to exchange geometrical information and not totally

suitable to all the needs of the CAD/CAPP/CAM industry.

Thus, the international community developed the ISO10303 [7]set of standards, well known as STEP.

By implementing STEP AP-203 [8] and STEP AP-214 [9]

within CAD systems, the data exchange barrier is removed. Yet,

data exchange problems between CAD/CAM and CNC systems

remain unsolved. CAD systems are designed to describe the

geometry of a part precisely, whereas CAM systems focus on

using computer systems to generate plans and control the

manufacturing operations according to the geometrical

information present in a CAD model and the existing resources

on the shop-floor. The final result from a CAM system is a set of

CNC programs that can be executed on a CNC machine. STEP

AP-203 and STEP AP-214 only unify the input data for a CAM

system. On the output side of a CAM system, a 50-year-old

international standard ISO 6983 (known as G-Code or

RS274D) [10] still dominates the control systems of most

CNC machines. Outdated yet still widely used, ISO 6983 only

supports 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 of

low-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 preserved

and re-utilised.

2.2. Inflexible CNC control regime

The ISO 6983 standard 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. Thus,

ISO 6983 defines the syntax of program statements, but in most

cases leaves the semantics ambiguous, together with low-levellimited control over program execution. These programs, when

processed in a CAM system by a machine-specific post-

processor, become machine-dependent. In order to enhance the

capability of a CNC machine, CNC controller vendors have

also developed their own tailored control command sets to add

more features to their CNC controllers to extend ISO 6983.

These command sets once again vary from vendor to vendor

resulting in further incompatible data among the machine tools.

The current inflexible CNC control regime means that the

output from a CAM system has no adaptability, which in turn

denies 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 how

capable they are, can do nothing but ‘‘faithfully’’ follow the G-

code program. It is impossible to perform intelligent control nor

machining optimization.

3. The STEP-NC standard

Today a new standard namely ISO 14649 [11–16]

recognised informally as STEP-NC is being developed by

vendors, users and academic institutes world wide to provide a

data model for a new breed of intelligent CNCs. The data model

represents a common standard specifically aimed at NCprogramming, making the goal of a standardised CNC

controller and NC code generation facility a reality. Currently

two versions of STEP-NC are being developed by ISO. The first

is the Application Reference Model (ARM) (i.e. ISO 14649)

and the other Application Interpreted Model (AIM) of ISO

14649 (i.e. ISO 10303 AP-238 [17]). For more information on

the use and differences between them readers are referred to

[18,19].

Contrary to the current NC programming standard (ISO

6983), ISO 14649 is not a method for part programming and

does not normally describe the tool movements for a CNC

machine. Instead, it provides an object oriented data model for

X.W. Xu, S.T. Newman / Computers in Industry 57 (2006) 141–152142


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CNCs with a detailed and structured data interface that

incorporates feature-based programming where a range of

information is represented such as the features to be

machined, tool types used, the operations to perform, and

the sequence of operations to follow. Though it is possible to

closely define the machine tool trajectory using STEP-NC, the

aim of the standard is to allow these decisions to be made at a

latter stage by a new breed of intelligent controller—STEP-

NC controller. It is the aim that STEP-NC part programs may

be written once and used on many different types of machine

tool controller providing the machine has the required process

capabilities. In doing this, both CNC machine tools and their

control programs are made adaptable and interoperable. Fig. 1

illustrates that both geometric and machining information can

now be bi-directionally transferred between a CAD/CAM

system and a STEP-NC controller [20]. One critical issue is

that the tool path movement information is optional and

ideally should be generated at the machine by the STEP-NC

controller.

Geometric information is defined by machining features(similar to AP-224 [22]) with machining operations termed

‘‘Workingsteps’’ performed on one or more features. These

Workingsteps provide the basis of a ‘‘Workplan’’ to manu-

facture the component. Fig. 2 illustrates an actual extract of

such data for a part with a Workplan consisting of Workingsteps

for slotting, drilling and pocketing. One important point to note

is that this code is the STEP-NC transfer (physical) file, which

is imported/exported into and out of a STEP-NC intelligent

controller. 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 data

interface (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, so

that no information is lost between the different stages of the

product development process.

Its data elements are adequate enough to describe task

oriented NC data.

The data model is extendable to further technologies and

scalable (with conformance classes) to match the abilities of a

specific CAM, SFP or NC.

Machining time for small to medium sized job lots can be

reduced because intelligent optimisation can be built into the

STEP-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 to

the design department hence bi-directional information flow

from CAD/CAM to CNC machines can be achieved.

XML files can be used as an information carrier hence enableWeb-based distributed manufacturing.

A detailed discussion on value proposition for STEP-NC can

be found in a report produced by the OMAC STEP-NC

Working Group [24] and other publications [20,23,25].

4. STEP-NC international community

In the second half of the 1990s, an effort from the

international community backed by ISO started the major

change in the concept of NC programming, through an

international intelligent manufacturing systems (IMS) pro-gramme [26]. The programme was co-ordinated across four

X.W. Xu, S.T. Newman / Computers in Industry 57 (2006) 141–152 143

Fig. 1. Bi-directional information flow with STEP-NC [21].

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worldwide regions each with individual projects namely

Europe, 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 and

inspection of the ISO 14649 standard. It has 15 partners, led by

Siemens, with users such as Daimler Chrysler, Volvo, and the

support 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 in

collaboration with vendors such as Agie, Starrag and CAM

manufacturer CADCAMation. The work in Korea has been

carried out by both Pohang University of Science &

Technology (PosTECH) and the Seoul National University

in the areas of milling and turning architectures for ISO 14649

compliant controllers. Other research teams working in the area

include those in the UK and New Zealand. In the United

Kingdom, an Agent-Based, STEP-compliant CAM (AB-CAM)

system has been developed in Wolfson School of Mechanical

and Manufacturing Engineering, Loughborough University

[27,28]. In New Zealand, the Manufacturing Systems

Laboratory at the University of Auckland has been using the

AIM of STEP-NC [17] to develop a STEP-compliant CAPP

system for collaborative manufacturing [29,30].

The STEP-NC programme in the USA called SuperModel

led by STEP Tools Inc. and sponsored by National Institute of

Standards and Technology (NIST) has made major advances

to fully automate the CAD to CNC manufacturing processthrough the use of STEP or rather AP-238. This project involved

a strong group of industrial partners including Boeing, Lockhead

Martin, General Electric and General Motors, together with

recognised CAM vendors such as Gibbs Associates and

MasterCAM.

5. STEP-NC for more open and interoperable

machine tools

There 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;


Fig. 2. Example STEP-NC physical file [20].


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and (4) collaborative STEP-NC enabled machining. The degree

of adaptability increases from Type 1 to Type 4. It is to be noted

that STEP-NC together with STEP is now forming a common

data model for representing complete product information. Its

far-reaching effect lies in a total integration of CAD, CAPP,

CAM and CNC with desired interoperability and adaptability

across the complete design to manufacturing chain. Due to the

limited scope of this paper, only the research work directly

related to STEP-NC enabled CAM/CNC is discussed.

5.1. Conventional CNC control using STEP-NC

This type of research marked the beginning of STEP-NC

related research endeavour. The main purpose is to answer two

questions: ‘‘Does a STEP-NC file contain enough and just

enough information for CNC machining?’’ and if it does, ‘‘Can

it be used on a traditional CNC machine tool without making

changes to the system hardware?’’. The main research is to do

with the development of ‘‘translators’’ which can read in a

STEP AP-203 or AP-224 file and convert it into G-code formatthat the targeted CNC machine tool can understand. The

translator is somewhat similar to the ‘‘post-processor’’ used in

many CAD/CAM or CAM systems. The only difference is that

the CAD/CAM, CAM and CNC systems are now made

interoperable in a sense that the STEP compliant information

can be used across the board. Also, the design information that

can be embedded in a STEP-NC file is made available to the

CNC systems. This scenario represents conventional solid-

based manufacturing as enabled by STEP AP-203.

The work carried out in the first two stages of the three-stage

SuperModel Project falls into this type of work. In stage one, a

range of software tools (i.e. ST-Plan, ST-Machine, and STIX[31]) were developed involving GibbsCAM and various pieces

of third-party software. The GibbsCAM STEP translator can

read in the demonstration part in STEPAP-203 format. The part

is then programmed using GibbsCAM’s graphical interface,

and visually verified using its cut part rendering capability [32].

In the second stage, the AP-238 file was read using a

GibbsCAM STEP-NC Adaptor plug-in, developed by STEP

Tools Inc. An MDSI Open CNC controller (software-based

CNC) [33] retrofitted to a Bridgeport vertical machining centre

was used as the platform for the GibbsCAM and STEP-NC

software. Using the tooling and operation parameters specified

in the AP-238 file, the STEP-NC Adaptor created GibbsCAM

tooling, process and geometry elements and executedGibbsCAM functions to generate tool–paths corresponding

to the AP-238 machining features. Once again, the cut part

rendering 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 to

completely automate CAM processing and tool–path 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. demonstrated

the conversion of AP-203 design data into AP-238 (i.e. the

AIM version of STEP-NC), feature by feature, with the use of

ST-Plan AP-238 data. AP-238 data was then transferred to

GibbsCAM with the assistance of ST-Machine, and then to a

five-axis Fadal machining centre. In June 2003 at NIST, a

similar set-up saw MasterCAM interface with another five-axis

machine tool.

5.2. New STEP-NC enabled control

Working closely with some of the popular CNC controllers

or Open Modular Architecture Controller [6], several research

teams around the world have been able to process STEP-NC

information internally is a CNC controller. This is made

possible by developing for, and integrating a STEP-NC

Interpreter into, these controllers that can faithfully performs

the machining tasks as specified in ISO 14649.

The third stage work of the US SuperModel Project saw

GibbsCAM integrated with an OMAC machine tool. An AP-

238 data file provided all the manufacturing information to

allow GibbsCAM to generate the tool–path data. The tool–path

data was then sent to a horizontal machining centre in so-called’’stroke-level inter-process communication’ rather than con-

ventional G-codes, demonstrating a higher level of CAM/CNC

integration than is normally realised through ISO 6983.

Most of the work carried out in EU falls into this category of

research. The main focus has been on the development of the

STEP-NC enabled CNC control using Siemens 840D controller

[34]. This enables the STEP-NC physical files to be integrated

directly with the controller, with visualisation of the machining

features and associated Workingsteps in a STEP-NC compliant

version of their ShopMill CAM system. Programming

developments in parallel with this work have been undertaken

at WZL, University of Aachen, Germany, with the WZLShopfloor Programming System incorporating WZL Mill, a

STEP-NC compliant programming system and WZL-WOP

(workshop-oriented programming). Commercial applications

in Europe with CATIA and OpenMind systems have been

presented by Volvo and Daimler Chrysler [34,15] illustrating

the capability of incorporating the standard within the CAD/

CAM products and exporting the STEP-NC output to the

Siemens 840D controller.

In addition to STEP-NC milling developments the

technology has also been extended to CNC turning. The

prototype STEPTurn software module has been developed by

ISW Stuttgart, working with a Siemens 840 control on a

Boehringer NG200 lathe [34], STEPTurn software can importCAD geometry and machining features, define machining

strategies and technologies and generate STEP-NC output. The

Siemens controller receives this output and converts it into the

Siemens ShopTurn system via a STEP-NC import facility.

5.3. STEP-NC enabled intelligent control

The dream of performing intelligent control on a CNC

machine has never been truly realized. The main reason is that

the information (G-code) available to a CNC machine is too

low-level information, with which only minimum amount of

optimization work can be carried out in real time or near real



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time. With STEP-NC, both design and process planning

information is available to a CNC machine. It is possible for the

CNC machines, or their controllers, to perform high-level,

intelligent activities, such as automatic part setup; automatic

and optimal tool path generation; accurate machining status and

result feedback; complete collision avoidance check (taking

into account of fixture and in-process geometry); optimal

Workingstep sequence; adaptive control and on-machine

inspection.

The researchers at the NRL-SNT (National Research

Laboratory for STEP-NC Technology) in PosTECH, Korea

have developed a Feature-Based STEP-NC autonomous control

system based on an Open Architectural Virtual Manufacturing

System [35–37]. The information in an ISO 14649 part program

is converted through an Interpreter into the internal data format,

i.e. process sequence in form of ‘‘Process Sequence Graph

(PSG)’’. The EXPRESS compiler in the Interpreter converts the

physical file, in form of ‘‘task description’’ into a PSG, based

on the information such as geometry, technology and tool

description. PSG represents a non-linear sequence of Work-ingsteps described in terms of machining_feature and

machining_operation using the ‘‘AND–OR’’ relationship

(Table 1 and Fig. 3). As presented in the PSG, the part can

be machined in a number of ways, making CNC execution

flexible, optimal, intelligent and autonomous. The non-linear

process sequence schema enables a STEP-compliant CNC

autonomous system. In preparation for executing a STEP-NC

program, a Tool Path Generator (PosTPG), Tool Path Simulator

(PosTPS) and a soft-CNC called NCK/PLC have been

developed [35]. NCK/PLC can convert the STEP-NC data

model into machine tool motion, and is capable of NURBS

interpolation, look-ahead control, position/velocity interpola-tion and PID (Proportional, Integral, Derivative) control. It

interfaces with machine tool hardware (drivers and motors) via

an I/O board.

A STEP-compliant CNC machine tool that demonstrated a

G-code free machining scenario has been developed at the

Manufacturing Systems Laboratory, University of Auckland

[38]. This research work consists of two parts: retrofitting an

existing CNC machine and the development of a STEPcNC

(STEP-compliant NC) Converter. The CompuCam’s motion

control system [39] is used to replace the existing CNC

controller, which is programmable using its own motion control

language—6 K Motion Control language and capable of

interfacing with other CAPP/CAM programs through lan-

guages such as Visual Basic, Visual C++ and Delphi. The

STEPcNC Converter can understand and process STEP-NC

codes, and interface with the CNC controller through a Human–

Machine Interface. It makes use of STEP-NC information such

as Workplan, Workingstep, machining strategy, machining

features and cutting tools that is present in a STEP-NC AIM

file.

5.4. Collaborative STEP-NC enabled machining

It can be said that the ultimate goal for the STEP-NC enabled

machiningis to support Web-based,distributed and collaborative

manufacturing (Fig. 4), a scenario of ‘‘design anywhere/build

anywhere’’. This is possible as a STEP-NC program can separate

the ‘‘generic’’ manufacturinginformation (what-to-do), from the

manufacturing information (how-to-do) that is native to a

specific machine tool. Therefore, a generic STEP-NC program

canbe made machine-independent andhas an advantage over the

conventional, 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, the

native manufacturing knowledge has to be incorporated. To fulfil

this function, a native STEP-NC mapping system called ‘‘Native

STEP-NC Adaptor’’ has been developed [40]. The adaptor is

built with three parts: a native CNC system knowledge database,

a Translator and a Human–Computer Interface. The native CNC

system knowledge database has a proprietary data structure so

that the work in developing the Translator is made simpler and

coherent programming of NC components across the enterprise

is enabled.

Recently, there has been a trend of using XML (or rather ISO

10303 Part 28) instead of EXPRESS language (or ISO 10303

Part 21 [41]) to represent the STEP-NC information. The

reason for this is obvious. The XML processing ability can


Table 1

Workingstep list [37]

Workingstep

ID Feature Operation

1 Planar_face Plane_rough_milling

2 Closed_pocket Bottom_and_side_rough_milling

3 Round_hole Drilling


5 Slot Bottom_and_side_rough_milling




Fig. 3. Process sequence graph [37].

Fig. 4. Distributed, STEP complaint NC machining [20].


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easily support the e-Manufacturing scenario. CNC machine

tools can share information with other departments in and

outside the company over the Internet/Intranet.

ERC-ACI (Engineering Research Centre for Advance

Control and Instrumentation) in Seoul National University

[42–44] has been working toward developing an XML-enabled

STEP-NC data model for milling. It can search for, extract and

store, the tool–paths generated in XML format. The milling

machine used to test the system contains four modules (Fig. 5):

XML Data Input module, Interpreter, Tool Path Generator and

Motion Control Board. The XML Data Input module and

Interpreter generate STEP-NC programs from CAD files,

whereas the other two generate and execute native CNC process

plans.

A framework of a STEP-compliant CAPP system has been

developed at the University of Auckland [29,30]. The system

adopts three-tiered, Web-based network architecture (Fig. 6).

The client tier consists of a set of applications and a Web


Fig. 5. STEP-NC milling machine [42].

Fig. 6. A STEP-compliant collaborative manufacturing model.


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browser, enabling interactions between users and the system.

The process plans that are used as an input to a CNC system are

described 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. A

database structure has been proposed for both generic and

native manufacturing information and XML is used to represent

the STEP-NC information in these databases.

6. Portable STEP-NC tool–path

On 3rd February 2005, the OMAC STEP-NC Working Group

hosted an STEP-NC Forum in Orlando, FL, USA. The main

purpose of the demonstration is two-fold: (a) to demonstrate how

STEP-NC information can support portable machining on five-

axis machining centres; and (b) to see if STEP-NC tool–path

description capabilities can be used to streamline the data flow

between existing CAD/CAM systems and machining centres.

The AIM (AP-238) version of STEP-NC was adopted, and itsCC1 (conformance class 1) Machine Independent tool–paths

[17] was used for the demonstration. The component tested was

the 5-Axis NAS 979 circle/diamond/square part with an inverted

NAS 979 cone test in the centre (Fig. 7) [45].

The business case and the main industry participant are

Boeing’s manufacturing plants. Like others, most five-axis

machining centres at Boeing receive machine control data (or

MCD) in G-code format that defines each axis movement

required in order to manufacture a part. This direct programming

model means that the orientation axes are traversed as

synchronized axes, and are tied to a specific tool length. The

problem with these MCD programs is that they are neitherportable nor adaptable. Lack of portability presents a problem

since unique axes position data must be generated for each

machine control combination (part, tool, and machine config-

uration) on which the part is to be run. MCD programs are not

adaptable as no information is provided to the machine to help it

adapt to real-time changes in machining dynamics (feed and

speeds) or machine tool alignment (tool and wear offsets).

By comparison, tool centre programming (TCP) defines

program geometry as cutter movement data, instead of axis

movement data.TCP is similar to robotic 6D poserepresentation.

Motion is defined as a 3D tool-tip position ( X , Y , Z ) and a 3D tool

axis orientation ( I , J , K ).For eachTCP ( X , Y , Z , I , J , K ), the CNC

controls the two rotation axes so that the tool is positioned and

oriented as specified. In addition, the CNC controller performs

tool offset compensation along the tool axis according to the

position of the tool tip in the proper position and orientation.

STEP-NC allows tool centre programming to define

program geometry as cutter movement data, instead of axis

movement data. STEP-NC also provides rich, high level

information about the part features, materials, cutters, and

dimensional tolerances. In the aerospace industry, tighter and

tighter part tolerances are the expected norm so that the need for

STEP-NC is pronounced. TCP can provide some direct

accuracy improvements since each CNC will determine its

tool tip position, as opposed to a CAM system generating static

tool–paths as a series of axes positions. Since machine

geometries can vary slightly even between identical machines,

expected accuracy improvement should be significant.

At the STEP-NC Forumin Orlando, four CAD/CAM systems(i.e.Unigraphics, Catia, GibbsCAMand MasterCAM) wereused

to generate CL part programs. These CL data represent angular

cutter motions in a CNC configuration-independent I , J , K way,

with the assumption that the underlying machine tool controller

will translate the I , J , K into machine specific five-axes angular

configuration. CL-AP-238 converters have been developed to

translate the CLfile into AP-238 Part 21 file based onthe AP-238

CC1tool–pathstechnology. ThisSTEP-NCfile encodesmachine

Workingsteps as TCP tool–paths based on the AP-238 Express

schema, which is then suitable for the transfer between differing

machining centres. Different machine-specific converters have

been developed to translate the STEP-NC file into a controller-specific TCP programs (Fig. 8). These converters will be

eventually embedded in the controllers. It is to be noted that the

STEP-NC (AP-238) file is now neutral to all five-axis machines,

be it a five-axis gantry CNC, ‘‘C on A’’ machining centre or ‘‘C

on B’’ machining centre. It is portable because it defines program

geometry as cutter movement data, instead of axes movement

data. It is adaptable because it can account for any configurations

changes on a machine tool.

7. Challenges and opportunities

Though some early research work has shown that STEP-NC

can be an enabling tool for developing more open, interoperableand intelligent CNC machine tools, to gain acceptability by the

NC community and particularly the CNC programmers and

operators, a number of challenges still lie ahead. These

challenges also present ample opportunities for various parties

such as NC machine tool manufacturers, CNC controller

manufacturers and commercial CAD/CAPP/CAM vendors.

7.1. STEP-NC information models

The use of STEP-NC brings the benefit of better integration

for information models from design to manufacture, effectively

eliminating semantic errors and bringing an end to the


Fig. 7. Five-axis NAS 979 circle/diamond/square part.


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translation between proprietary and non-proprietary formats.

However, the fact that both STEP-NC ARM (ISO 14649) and

AIM (AP-238) co-exist and have each been implemented by

different groups, presents a less than satisfactory environment

for users. It is of particular importance that one understands the

difference between these two ‘‘versions’’ of STEP-NC prior to

implementation. The main difference between these two

models is the degree to which they use the STEP representation

methods and technical architecture. Both versions can be

viewed as different implementation methods of the STEP-NC

standard. The ISO 14649 standard is more likely to be used in

an environment in which CAM systems have exact informationfrom the shop-floor, whereas STEP AP-238, as a part of the

STEP standard, is more suitable for a complete design and

manufacturing integration. The ISO 14649 standard has few

mechanism to incorporate other types of STEP data, hence

making bi-directional data flow between design and manu-

facturing more difficult. Unlike ISO 14649, STEP AP-238

encompasses 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 with

STEP AP-238 though, is that the STEP Integrated Resources

used in AP-238 are not adapted to application areas; hence the

data in its files are fragmented and distributed. It only provides

an information view of the data, whereas the ARM provides afunctional view of the data.

7.2. Feature recognition for STEP-NC

For STEP-NC to take off, machining feature recognition is

an important prerequisite, as it is a linkage between STEP-

enabled CAD and CAPP. Most of the current feature

recognition systems are prototypes and incapable of dealing

with a sufficient broad space of feature spectrum. Also, very

few systems can perform feature recognition in a ‘‘STEP in,

STEP out’’ manner, i.e. recognizing STEP AP-224 machining

features based on the STEP AP-203 or AP-214 data.

The literature to date has shown some effort spared in this

area. Based on the technology developed at Honeywell Federal

Manufacturing & Technologies (FM&T), ST-Plan can create

STEP AP-224 machining features from STEP (AP-203 or AP-

214) data. Parameters such as tolerances, features, processes

and tool requirements can be manipulated. ST-Plan claims to be

the first-to-the-market software package dedicated to STEP-NC

and e-Manufacturing [31]. The system has two major modules:

feature-based machining (FBMach) and feature-based toleran-

cing (FBTol). FBMach is used to recognise manufacturing

features and allow Workingsteps to be defined for those

features, whereas FBTol is used to define tolerances. FBMachcontains a library of machining features and feature recognition

algorithms. The system creates both surface and volume-based

machining features. A surface-based machining feature is

based on sets of faces on the solid model-the ‘‘skin’’ that

represents the shape of a feature. A volumetric machining

feature is represented by ‘‘delta volumes’’, which are solid

bodies showing the shape and amount of material to be

removed. Tool–paths may also be determined by delta volumes

in applications for generating CNC routines.

The SFP system developed at NRL-SNT can also generate

STEP AP-224 features based on AP-203 data [35–37]. These

features are then input to the process planning stage, during

which the native machine tool information and machiningparameters are added to the program. Workingsteps of each

manufacturing feature are defined and saved into an ISO 14649

part program. The STEPturn system developed at ISWalso has a

feature recognition function that precedes Workingstep sequen-

cing in order to generate a STEP-NC (ISO 14649) physical file.

7.3. Intelligent CNC controllers

STEP-NC delivers a complete package of information, be it

design or manufacturing information, to the CNC machine. On

the one hand, the CNC controller gets much richer information

making it possible to perform a true adaptable, optimal and


Fig. 8. Smart CNC data flow through using STEP-NC.


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intelligent control. On the other hand, the NC controller

manufacturers, oncepersuaded, have to re-design theircontroller

structures and strategies to takethisadvantage.Thisbeing done, a

major shift in culture and user belief is also needed to trust the

new breed of intelligent controllers to translate feature-based

conversational programming to the correct tool–path at the

machine. This is recognized as a paradigm shift where the

equivalent of today’s post processor functions at the off-line

CAD/CAM system will take place at the machine (or within the

new controller) leading to the end of G-code era. A closer

collaboration between the CNC control vendors and CAx

vendors may have to happen to enable this dramatic change.

7.4. Manufacturing knowledge

The adoption of the STEP-NC paradigm brings forth the

opportunity to consider not just the ways of representing

information but also the type of information to be represented.

To be more precisely, it is the knowledge required for process

planning and machining discrete parts that needs to beaccumulated and validated. The knowledge-based systems

thus developed need to be robust. At present the developments

in STEP-NC have considered relatively simple components, i.e.

2½D-components with autonomous or compound features.

Parts containing more complex geometry with intricate

interactions require additional knowledge-based intelligent

solutions. A sound balance between standardizing such

solutions as much as possible and still leaving enough room

for customization is hard to keep.

7.5. Challenges related to the supporting technologies

For a STEP-NC enabled machine tool to be fully

autonomous as well as interoperable, a suite of supporting

technologies needs to be further developed. At the CAD/CAPP

end, it is the ‘‘old’’ topic of automatic feature recognition based

on a STEP AP-203 model. At the machine side, more adaptable

and faster control is needed. Intelligent embedded machatronic

systems and OMAC seem to offer a viable solution, but more

research has to be carried out in the areas of modular software

and hardware design, intelligent control algorithms and

distributed control technology. The knowledge-based systems

as mentioned above need to be mobile and easy to share. XML

is being recognized as a promising means of modelling

knowledge and distributing it across the Internet. CurrentInternet technologies are not yet fit to serve such portable

systems, not to mention the venerability of the Web space.

8. Conclusions

Modern CNC machine tools, though capable in function-

alities, lack adaptability, portability and intelligence. This is

due to the fact that a 50-year-old language is still employed by

these machine tools. NC programs following this format are

only meant for execution on a specific machine tool. They

cannot be reinterpreted by a CAM system or a SFP system for a

different machine tool. Automatic generation of a 100%

optimised NC program is not possible as design information

and know-how about the machine tools and materials is

represented in different formats and on different databases.

STEP-NC can provide a uniform NC program format for

CAM, SFP and NC, avoiding post-processing and entail a truly

exchangeable format. The operator can now be supported at

CAM, SFP and NC level by complete information containing

understandable geometry (features), task oriented operations,

strategies and tool definitions. Availability of design data at the

machining stage also enables a reliable collision check, accurate

simulation and feedback from the machining stage to the design

stage. Part programs following the STEP-NC standard are

interoperable in a sense that they can be adapted to any CNC

machine tools that have the ability to execute the machining

tasks. CNC machines implementing STEP-NC can have a more

open and adaptable architecture, making it easier to integrate

with other manufacturing facilities, e.g. workpiece handling

device. STEP-NC also supports distributed manufacturing

scenario through, for example, Ethernet connections to

accomplish data collection, diagnostics and maintenance,monitoring and production scheduling on the same platform.

The demonstration exhibited at the OMAC STEP-NC Forum is

significant. It showed that different CAD/CAM systems can

generate the same, machine-neutral STEP-NC information. The

STEP-NC file has been adapted to different five-axis CNC

machines. The test part (NAS 979) is a true five-axis component.

It is to be pointed out that only AP-238 CC1 machine

independent tool–path data is used. Information such as

machining features and design data is not considered. Therefore,

there is limited adaptability the CNC machines can exercise in

this case.

There are still issues to be addressed andchallenges to be met.These challenges come from the drive for a uniformed STEP-NC

information model, development of STEP-NC enabled intelli-

gent controllers, capture of necessary manufacturing knowledge

to support decision-making at the machine tool level as well as

other under-developed pertaining technologies. The challenges

co-exist with the opportunities that if seized in time can yield a

multitude of benefits that STEP-NC promises.

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fications, National Standards Association, Washington, DC, USA.

Xun W.Xu received a BSc and MSc from Shenyang

Jianzhu University and Dalian University of Tech-

nology, PR China in 1982 and 1988, respectively. In

1996, he received a PhD from the Department of

Mechanical Engineering, University of Manchester

Institute of Science and Technology (UMIST), UK.

He is now a senior lecturer at the Department of

Mechanical Engineering, the University of Auck-

land, New Zealand. Dr. Xu is a member of ASME

and IPENZ. In addition to his teaching and research

activities at the University of Auckland, Dr. Xu has been actively engaged in

various industrial consultancy work. He heads the Manufacturing Systems

Laboratory and the CAD/CAM Laboratory in the University of Auckland. His

main interests lie in the areas of CAD/CAPP/CAM, STEP, and STEP-NC.



12/12

Stephen T. Newman gained BSc honours degree in

production technologyand management in 1982 from

the University of Aston, Birmingham, on a sandwich

degree sponsored by Land Rover Ltd. Having worked

at Land Rover for 4 years he joined Loughborough

University as a research associate and gained his PhD

in 1990. In 1989, he was appointed, as a lecturer in

manufacturing engineering, promoted to senior lecturer in 1997 and Reader in

computer-aided manufacturing in 2000. In January 2006, he joined

the University of Bath as a professor in the area of Innovative Manufacturing.

He has 20 years of experience in European and National R & D programmes

being involved in Eureka Factory, EU Framework V and framework

VI programmes together with numerous national EPSRC research pro-

grammes.


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Making CNC Machine Tools More Open, Iterroperable and Intelligent