Russell Claus

Glenn Research Center, Cleveland, Ohio

Ilan Weitzer

Ford Motor Company, Dearborn, Michigan

CAD Services—An Industry Standard Interface

for Mechanical CAD Interoperability

NASA/TM—2002-211902

October 2002
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Russell Claus

Glenn Research Center, Cleveland, Ohio

Ilan Weitzer

Ford Motor Company, Dearborn, Michigan

CAD Services—An Industry Standard Interface

for Mechanical CAD Interoperability

NASA/TM—2002-211902

October 2002

National Aeronautics and

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NASA/TM—2002-211902 1 

CAD Services—An Industry Standard Interface for 

Mechanical CAD Interoperability 

Russell Claus 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Ilan Weitzer 

Ford Motor Company 

Dearborn, Michigan 48120 

1 ABSTRACT 

Most organizations seek to design and develop 

new products in increasingly shorter time periods. At 

the same time, increased performance demands require 

a team-based multidisciplinary design process that 

may span several organizations. One approach to meet 

these demands is to use “Geometry Centric” design. 

In this approach, design engineers team their efforts 

through one united representation of the design that is 

usually captured in a CAD system. Standards-based 

interfaces are critical to provide uniform, simple, 

distributed services that enable the “Geometry 

Centric” design approach. This paper describes an 

industry-wide effort, under the Object Management 

Group's (OMG) Manufacturing Domain Task Force, to 

define interfaces that enable the interoperability of 

CAD, Computer Aided Manufacturing (CAM) and 

Computer Aided Engineering (CAE) tools. This 

critical link to enable “Geometry Centric” design is 

called: Cad Services V1.0. This paper discusses the 

features of this standard and proposed application. 

2 BACKGROUND 

Design and engineering of complex systems is 

increasingly becoming distributed and collaborative. 

The complexity of modern products also dictates that a 

single designer or design team can no longer manage 

the complete product development effort. Driven by 

such issues, companies are increasingly staffing only 

their core competencies in-house and depending on 

other vendors and therefore other tools, to provide the 

complementary design knowledge and design effort 

needed for a complete product. Design and 

engineering tools are no longer merely exchanging 

geometric data, but interact with other tools to acquire 

knowledge including analysis, simulation, constraints, 

and rules. Furthermore, these knowledge exchanges 

increasingly cross organization boundaries, product 

line and tool set. The need for interoperability and 

rapid integration between systems, more than ever, 

reveals the need for an industry-wide solution for 

integration and computational frameworks. 

Despite these needs, a recent study, ref. 1, 

conducted for the U.S. auto-industry conservatively, 

estimates that $1B per year is wasted due to reentering 

or translating data between CAD systems and 

downstream applications (such as Computer Aided 

Engineering—CAE). 

There is recognition that well-defined and stable 

interfaces are essential for achieving rapid 

interoperability between tools. Integrating dissimilar 

systems from multiple disciplines, CAD, CAM, CAE, 

KBE and PDM to name a few, is now one of the 

primary challenges for integrating product 

development systems. Industry standards in the areas 

of distributed computing and domain-specific 

interfaces are, for now, the only viable solution to 

enable that kind of interoperability. 

3 OVERVIEW 

The CAD Services V1.0 interface standard 

described in this paper is an effort to provide uniform, 

simple, distributed services that enable a “Geometry 

Centric” design approach. This new standard would 

provide uniform interfaces to design geometry and 

topology features that can be easily and accurately 

shared among design team members. These high-level 

interfaces can respond to engineering queries without 

the need for data translation or low-level data 

structures. These interfaces can, therefore, shield the 

design engineer from the full complexity of a CAD 

system. 

Standards benefit both the end-user and vendors 

community. A primary goal for both user and vendor 

is to reduce software development, maintenance, and 

support costs. Vendors can benefit by addressing 

customer needs in a standards based environment. This 

should improve the usability of their product while not 

reducing their core competitive advantages. Users 

community will benefit from the improved 

functionality of their software tools and lower 

integration costs. A customer developing applications 

for multiple systems must write and maintain one-toone interfaces for each system. A software vendor may 

have to address software support and maintenance for 

multiple platforms, languages and requirements. A 

standards based interface will ease both of these 

situations (as seen in figure 1). 
NASA/TM—2002-211902 2

CAD Services interfaces map to business 

requirements in accordance with the Object 

Management Group’s vision of interoperability. This 

initiative has developed a standard CAD interface that 

complies with and uses standard CORBA Services and 

other adopted domain standards. 

4 OMG AND CORBA 

4.1 Object Management Group (OMG) 

OMG is a consortium of vendors, developers and 

end users with the goal of creating and promoting a 

standardized object-oriented architectural framework 

for distributed applications based on distributed object 

technologies. The OMG objective is to promote and 

support the development of standard interfaces to 

object-oriented software components that enable 

reusability, portability and interoperability in a 

heterogeneous environment. 

4.2 Object Management Architecture 

The Object Management Architecture (OMA) 

defines the conceptual infrastructure upon which 

object-oriented applications and objects are based. The 

Reference Model identifies and characterizes the 

components, interfaces, and protocols that compose 

the OMA. This includes the Object Request Broker

(ORB) component that enables clients and objects to 

communicate in a distributed environment, and four 

categories of object interfaces: 

♦ Object Services are interfaces for general services 

that are likely to be used in any program based on 

distributed objects. 

♦ Common Facilities are interfaces for horizontal 

end-user-oriented facilities applicable to most 

application domains. 

♦ Domain Interfaces are application domainspecific interfaces. 

♦ Application Interfaces are non-standardized 

application-specific interfaces. 

The Common Object Request Broker Architecture

(CORBA) defines the programming interface to the 

Object Management Architecture. Through a series of 

RFPs, OMG is populating the OMA with detailed 

specifications for each component and interface 

category in the Reference Model. 

The wide-scale industry adoption of OMG’s 

OMA provides application developers and users with 

the means to build interoperable software systems 

distributed across all major hardware, operating 

system, and programming language environments. 

One of these industry wide efforts is under the 

umbrella of the Manufacturing Domain Task Force 

(MfgDTF). 

4.3 Manufacturing Domain Task Force 

The MfgDTF’s mission is to foster the emergence 

of cost effective, timely, commercially available and 

interoperable manufacturing domain software 

components through CORBA technology. Its goals 

include: 

♦ Recommend technology for adoption that enables 

the interoperability and modularity of CORBA 

based manufacturing domain software 

components. 

♦ Encourage the development and use of CORBA 

based manufacturing domain software 

components, thereby growing the object 

technology market. 

♦ Leverage existing OMG specifications. 

Recommend liaison with other appropriate 

organizations in support of the preceding goals. 

Most of MfgDTF work is done in workgroups, 

each covering a specific manufacturing domain. The 

Product and Process Engineering workgroup (PPE) is 

one of the most active workgroups. This Work Group 

was chartered by the MfgDTF to develop standardized 

interfaces for software systems supporting design and 

analysis of products and the processes and facilities 

used to make them. The scope of this effort 

encompasses engineering throughout the product and 

process lifecycle. CAD Services is one of the active 

initiatives under the PPE workgroup. 

5 CAD SERVICES V 1.0 

CAD Services is a CORBA interface standard that 

was jointly developed by a team of CAD users and 

vendors. This Object-Oriented interface was designed 

to allow geometry and topology sharing between 

members of a design team. The adopted standard has 

the following features: 

1. Geometry and topology queries for both manifold 

and non-manifold geometries. 

2. Parametric regeneration of solid models. 

3. Tagging geometric entities with application 

specific information. 

4. Geometry creation. 

5. Integration with PDM Enablers V 2.0 standard. 

Figure 1. Systems link through industry 

standard services. 

I-DEAS 

UG 

Catia 

ProE 

Others 

Metaphase 

Enovia/ 

UG/IMAN

CAD Services

PDM Enablers

Enterprise Apps

CAD 

Apps. 

CAE 

Apps. 
NASA/TM—2002-211902 3

This Cad Services standard provides uniform 

access the native geometry kernels in Catia, 

Unigraphics, ProE and a number of other CAD 

systems. Commercial implementations will be 

available within the next year. At least one vendor 

plans to provide an open-source implementation, 

ref. 2. 

6 CAD SERVICES INTERFACE 

MODEL 

CAD Services interfaces are divided into eight 

different modules with a significant inter-dependency 

between modules (as illustrated in the figure 2). The 

first of these modules is the CadConnection module 

that provides standard interfaces to connect with the 

CAD Services server. CadConnection interfaces 

provide access to CadMain module interfaces. 

CadMain interfaces include Model interfaces that 

provide support for assemblies. The CadFoundation

module provides primary interfaces that are inherited 

by geometric entities. It also provides a general 

Attributable interface that can be used to “tag” 

geometric entities with application specific 

information through the use of DynAny local data 

structures. Basic geometric (tessellation) data 

structures and interfaces can be found in the 

CadGeometry module. The CadGeometryExtens 

module provides additional geometric entities that are 

subtypes of those in CadGeometry. Boundary 

representations (BREPs) can be found in the CadBrep

module. CadBrep contains interfaces for solid models 

that are represented through Bodies, Faces, Edges and 

others. These solid models expose parametric features 

that allow shape regeneration through interfaces in the 

CadFeature module. Finally, CadUtility provides a 

series of basic data structures used throughout the 

standard. One of these structures is a CadError

exception. This exception can be raised by almost all 

of the CAD Services interfaces. This general exception 

is needed due to the wide dissimilarity of CAD system 

implementations and the variability of native CAD 

API support. 

6.1 CadConnection Module 

The CadConnection module provides high-level 

connectivity to the Model interface in the CadMain

module. To reach and connect to the Model, a typical 

client will interact with both the CadServer and the 

CadSystem interfaces within the CadConnection 

Module. CadServer is a lightweight interface used 

primarily to connect with the CadSystem interface. It 

provides two read-only attributes that contain 

information on the native CAD system and properties 

needed to activate the underlying native CAD system. 

 The read-only attributes essentially provide a 

template that is “filled-in” by the client and passed into 

a “connect” request that (if correctly applied) returns a 

reference to the CadSystem interface. This connection 

CadBrep

<<CORBAModule>>

CadConnection

<<CORBAModule>>

CadM ain

<<CORBA M odule>>

CadU t ility

< < C O R B AM odule> >

CadG eom etry

<<CORBA M odule>>

C adF oundation

<<CORBAModule>>

CadFeature

<<CORBA M odule>>

CadG eom etryE xtens

<<CORBA M odule>>

Figure 2. CAD Services V 1.0 module structure. 

operation has various security provisions so that user 

authentication can be performed. The CadSystem 

interface allows clients to return a list of available 

Models, access to these Model interface, creation of 

Models and an optional access to a Graphical User 

Interface (GUI). 

The structure of this module closely follows the 

approach to be followed in the proposed new PDM 

Enablers V. 2.0 standard, ref. 5. This commonality 

allows vendors and implementers to use common 

authentication policies between the two systems. 

6.2 CadMain Module 

CadMain interfaces include the Model interface 

that provides the primary access to a wide variety of 

geometric entities. Specifically, the Model interface 

supports access to parts and assemblies through 

operations that identify, if this model contains other 

Models (ModelInstances). For example, the 

CadSystem::available_models() operation might return 

a sequence of strings that identifies an assembly file 

that contains various parts within that assembly. A 

CadSystem::open_model operation on this assembly 

file would also load the various parts contained within 

the Model and make them available to the client 

through a Model::model_children() operation. The 

Model interface also provides access to any geometric 

entities within the model (e.g. Surfaces, faces, etc.). It 

also provides an operation that creates an 

EntityFactory that can be used to construct geometric 

entities within a Model. 

6.3 CadFoundation Module 

CadFoundation defines general elements and 

behavior that are shared by all CAD geometry entities. 

These behaviors are inherited by all geometric entities 

in the Model through the Entity and the Attributable 

interfaces. The Attributable interface provides the 

ability to “tag” a geometric entity is a broad range of 
NASA/TM—2002-211902 4

possible data structures through the DnyAny data 

structure. The DnyAny data structure is very flexible 

and provides a mechanism for Engineering 

Applications (as well as others) to “tag” or label the 

geometric entity with application specific information 

(e.g. cost, maximum load, other data). The Entity 

interface provides a series of basic properties that 

define certain characteristics of the Entity (for 

example, dimensionality). The CadFoundation module 

also provides interfaces that support grouping of CAD 

entities through Layer (entities with shared color) or 

application specific groupings, EntityGroup. 

6.4 CadGeometry Module 

The CadGeometry module contains basic 

geometric data structures and interfaces that are used 

throughout the CAD Services interfaces. The two 

primary interfaces in this module are Surface and 

Curve. They inherit common functionality through the 

CadFoundation::Entity interface. A common use for 

either of these interfaces is to establish the exact threedimensional location of the surface or curve through a 

point query projection. These operations receive a 

sequence of three-dimensional point locations and 

return a sequence of points closest to the particular 

curve or surface. 

6.5 CadBrep Module 

The CadBrep module contains Boundary 

REPresentations (BREPs). BREPs are solid models 

such as Bodies, Faces, Edges and others. These solid 

models may expose parametric features that allow 

shape regeneration through interfaces in the 

CadFeature module. 

6.6 CadFeature Module 

The CadFeature Module provides interfaces 

(through inheritance with CadFoundation::Entity) that 

enable modification of native CAD entities. These 

interfaces enable suppression of various design 

features and a parameter set of expressions or values 

that define the geometry of the CAD entities. For 

example, a solid model of a box might have an 

associated parameter set that uniquely defines the 

width, length and height of the box. A client 

application might alter any of these parameters to 

regenerate the geometry, but would be unable to 

specify new parameters. 

6.7 CadUtility Module 

The CadUtilty module is a collection of generalpurpose data definitions used throughout the 

specification. This includes data structures for 

NURBS surfaces and curves, presentation information 

and basic geometry types (for example, points, rays, 

and vectors). No interface definitions are in this 

module. 

6.8 Performance 

The primary requirement for CAD Services 

interfaces is to ensure high-performance data access. 

Each CAD Services call may involve extensive 

network connections; therefore, the aggregation of 

data calls is essential. In other words, data access to 

geometry and topology information has to be 

developed using interfaces that provide substantial 

“chunks” of data as opposed to a series of calls that 

access data in fine granularity. 

A test of the performance differences that can 

occur with data aggregation was performed on two 

similar 300 Mhz NT computers using a commercial 

Object Request Broker (ORB). A chart comparing the 

transfer of a tessellated surface (message type 1) 

versus an equivalent series of point queries (message 

type 2) can be seen in figure 3. The timings display 

approximately an order-of-magnitude slower data 

access with the point queries. The tessellated surface 

information is transferred in only one distributed call. 

The point queries involved 8100 individual distributed 

calls. This chart clearly indicates the importance of 

aggregation to achieve high performance data access. 

CAD Services interfaces have been designed to 

aggregate distributed calls. Most geometric interfaces 

have been designed to allow a single call to return all 

relevant properties for that interface. 

6.9 Accuracy 

The accurate representation of model information 

to external applications was a key consideration in the 

CAD Services interface design. Native CAD kernels 

typically represent geometric entities through analytic 

curves (such as Non Uniform Rational B Splines, 

NURBS, ref. 6). This curve is frequently exported to 

external applications using a tessellation of the 

bounded surface. The process of creating the 

tessellated representation can result in an artificially 

roughen surface especially in regions of high 

curvature. 

Figure 3. Performance Chart 

0

10

20

30

Message Types

Time (secs)

Time (secs) 1.42 22.4

1 2 0

10

20

30

Message Types

Time (secs)

Time (secs) 1.42 22.4

1 2
NASA/TM—2002-211902 5

The CAD Services interface provides multiple 

options to reduce or eliminate this source of errors. 

First, a NURBS representation of the geometric entity 

can be obtained. Second, the tessellation mechanism 

provides controls to allow the creation of good 

curvature approximations. Finally, the geometric 

surface can be exactly described through a series of 

point queries that will return the exact threedimensional location of the surface. External 

applications are expected to use a combination of these 

options to obtain an accurate description of the 

geometric entity. 

6.10 Feature-Based Design 

A major use for CAD Services is to provide 

feature-based design linkages between engineering 

analysis and the CAD representation. This can be 

achieved by tagging the various CAD system entities 

with parametric information that provides some 

geometry regeneration capability. This parametric 

information can also be used to suppress certain 

features of the geometry. For example, a small detail 

that is not needed for flow analysis could be 

suppressed from the geometry. 

Body

<<Interface>>

Entity

(from CadFoundation)

<<Interface>>

BrepEntity

design_features()

<<Interface>>

Edge <<Interface>>

EdgeLoop

<<Interface>>

Face <<Interface>>

OrientedEdge

<<Interface>>

OrientedEdgeLoop <<Interface>>

OrientedFace

<<Interface>> OrientedShell<<Interface>>

Shell <<Interface>>

Vertex

<<Interface>>

VertexLoop

<<Interface>>

Figure 4. UML diagram of CadBrep Module. 

This capability is supported in CAD Services 

through a BrepEntity interface that is inherited from all 

Bounded Representations (BREPs) in the CadBrep 

Module as seen in figure 4. The BrepEntity interface is 

inherited by all CadBrep interfaces and supports access 

to design features through a design_features() operation. 

This operation provides access to a sequence of named 

parameters that are identified as read-only or 

modifiable. If these parameters can be modified, a client 

can change the parameters and regenerate the model 

through an operation at the Model interface. 

6.11 External Application References 

The CAD Services V 1.0 provides interfaces that 

permit the CAD system geometry to be tagged with 

external application information. For example, cost, 

inflow boundary conditions, and other parameters 

could be attached to relevant geometry entities. This 

connection to external application data should provide 

great flexibility to build integrated design systems with 

external applications providing important design 

feedback. 

This connection can be made through the 

Attributable interface in the CadFoundation Module. 

This interface allows a client (user) to associate any 

data structure with a geometric entity. 

6.12 Additional Requirements 

CAD Services V 1.0 supports querying capability 

to obtain high-level engineering data from the CAD 

system. Clients can query for mass properties 

including volume, center-of-gravity (CG), principal 

axes, and moments of inertia. 

An additional aspect of CAD Services V 1.0 is the 

optional support for a Graphic User Interface. Placing 

an additional interface into the CAD system graphics 

event loop can be a significant challenge. For this 

reason, the CAD Services User Interface support is 

optional. When supported, it provides the ability to 

select, highlight and tag (with text) geometric entities 

that are being displayed in the native CAD system. It 

also provides for user input in the form of a text string. 

 

7 CAD SERVICES APPLICATIONS 

To develop CAD Services interfaces a series of 

engineering Use Cases were examined. 

7.1 Computation Fluid Dynamics (CFD) 

NASA Glenn has developed a prototype CAD 

Services implementation applied to CFD grid 

generation. The approach has been tested for a 

Reusable Launch Vehicle geometry as shown in 

figure 5. 

Figure 5. Reusable Launch Vehicle geometry 

used for demonstration purposes. 

This vehicle was modeled using the ProEngineer 

CAD system. It employed parametric variables that 

could change the position of an inlet-spike. This inlet-
NASA/TM—2002-211902 6

spike can change the flow capture area to optimize 

inlet aerodynamic performance over the operating 

envelope. A close-up view of this inlet is shown in 

figure 6. 

CAD Services interfaces exposed parametric 

features that permitted automated grid generation for a 

variety of flow conditions. 

Figure 6. Close-up view of variable-geometry 

inlet used in the RLV concept. 

7.2 Suspension Analysis 

At Ford, a CAD Service prototype was developed 

for suspension analysis. This application requires 

interaction with a CAD tool to access car platform 

geometry and the suspension system attributes. 

A suspension analysis (figure 7) was developed 

that simplifies the interaction of a geometry-authoring 

tool and with analysis enhancements such as 

distribution and collaboration. This approach proved to 

be a good fit for other client applications and geometry 

tools acting as geometry servers. 

Figure 7. Illustration of a suspension analysis. 

8 CONCLUSIONS 

Design and engineering tools are no longer merely 

exchanging geometric data, but interact with other 

tools to acquire knowledge including analysis, 

simulation, constraints, and rules. Furthermore, these 

exchanges of knowledge increasingly cross 

organization boundaries, product line and tool set. The 

need for interoperability and rapid integration between 

systems, more than ever, reveals the need for domain 

specific services captured in well defined standard 

interfaces. 

The CAD Services standard defines a stable 

standard interfaces to CAD tools, which is essential for 

achieving rapid integration with design, engineering 

and analysis applications. Ultimately, it should provide 

a major step toward achieving a CAD "plug-and-play" 

environment that will enable users of design and 

engineering systems to seamlessly integrate, best-inclass, software across a wide variety of CAD/CAM 

and CAE applications. 

9 REFERENCES 

1. Tassey, G. 1999. “Interoperability Cost Analysis 

of the U.S. Automotive Supply Chain.” Research 

Triangle Institute Final Report. RTI Project 

7007–03. 

2. OpenCascade Web site: 

http://www.opencascade.org/3dwb/

cadservices/index.html

3. Slama, D., Garbis J., Russell P., 1990, Enterprise 

CORBA, Prentice Hall PTR, Upper Saddle River, 

NJ. 

4. Object Management Group: Manufacturing 

Domain Task Force: CAD Services 

V1.0 RFP, Document Number mfg/2000–06–07. 

Framingham, MA: Object Management Group, 

1996. 

5. Object Management Group: Manufacturing 

Domain Task Force: PDM Enablers V2.0 RFP, 

Document Number 2000–01–02. 

6. DeBoor, C., “A Practical Guide to Splines,” 1978, 

New York, Springer-Verlag. 
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12

CAD Services—An Industry Standard Interface for Mechanical CAD

Interoperability

Russell Claus and Ilan Weitzer

Software engineering; Object-oriented programming; Mechanical engineering; CAD;

CORBA; Data exchange

Unclassified -Unlimited

Subject Category: 07 Distribution: Nonstandard

Russell Claus, NASA Glenn Research Center; and Ilan Weitzer, Ford Motor Company, Dearborn, Michigan 48120.

Responsible person, Russell Claus, organization code 5880, 216–433–5869.

Most organizations seek to design and develop new products in increasingly shorter time periods. At the same time,

increased performance demands require a team-based multidisciplinary design process that may span several organizations.

One approach to meet these demands is to use “Geometry Centric” design. In this approach, design engineers team their

efforts through one united representation of the design that is usually captured in a CAD system. Standards-based interfaces are critical to provide uniform, simple, distributed services that enable the “Geometry Centric” design approach. This

paper describes an industry-wide effort, under the Object Management Group’s (OMG) Manufacturing Domain Task

Force, to define interfaces that enable the interoperability of CAD, Computer Aided Manufacturing (CAM), and Computer

Aided Engineering (CAE) tools. This critical link to enable “Geometry Centric” design is called: Cad Services V1.0. This

paper discusses the features of this standard and proposed application.