Modelling CAD Models

Method for the Model Driven Design of CAD Models for Deep Drawing Tools

Robert Scheffler

, Sergej Koch

, Gregor Wrobel

, Matthias Pleßow

Christian Buse

and Bernd-Arno Behrens

Society for the Advancement of Applied Computer Science (GFaI), Berlin, Germany

Institute of Forming Technology and Machines (IFUM), Leibniz Universität Hannover, Garbsen, Germany

Keywords: Model-based Systems Engineering, SysML, Graphical Domain-Specific Languages, Meta-modelling, Sheet

Metal Forming, Parametric Three-dimensional Computer-Aided (3D CAD) Models.

Abstract: Designing a fully parametric CAD model of a sheet forming tool in a 3D CAD system expends temporal

and financial effort and thus engineers shy away from it. The Institute of Forming Technology and

Machines (IFUM) and the Society for the Advancement of Applied Computer Science (GFaI) are currently

developing a new method for the model driven design of deep drawing tools. The core of this method is a

graphical modelling language for the domain of deep drawing tools. Meta models of these tools allow the

generation of models which in turn can be transformed to parametric CAD models and completed by

geometric modelling. The new method makes the modelling of parametric relations and dependencies easier

and less error-prone.

1 MOTIVATION

The use of 3D CAD modelling in design and

construction processes is state of the art in modern

engineering. Interactive CAD models are created,

completed or expanded by means of a CAD system.

Such a product model includes geometrical data,

technological and functional information as well as

information about design and manufacturing process

(Feldhusen and Grote, 2013). For many years design

processes of deep drawing tools have also been

carried out by means of 3D CAD systems. Current

CAD systems allow for a direct integration of

product logic and design knowledge in the CAD

model. Furthermore, it is possible to create new

model variation and modification by changing

parameters. However, the design of fully

parametrical CAD models involves a precise

planning and modelling of parameter relations in and

between the individual parts and assemblies. Such

CAD model design allows a simple parameter and,

consequently, model change. Nevertheless, most

design engineers prefer to avoid parameter-based

design due to high time and cost pressure. As a

consequence, product logic and designer knowledge

will subsequently be integrated in the CAD model in

the form of equations and rules. This type of model

extension is very error-prone and could lead to

model instability (Bergholz and Sachse, 2009).

In particular, method planning and tool design in

sheet metal working companies are significantly

affected due to the high diversity of variants

(Griesbach, 2005).

Nowadays, CAD systems offer a high degree of

automation and thus simplify designer tasks.

However, in the earlier developing phases of sheet

metal tools these CAD systems fail to present the

functional interactions at the required level of

abstraction (Marchenko et al., 2011). The missing

support of these developing phases could prevent the

development of new and innovative tool concepts

(Prieur, 2006). In this case, the designer needs an

integrated CAD tool, which allows computer-aided

modelling with major and minor functions, operating

principles as well as structure and parameter

coherence. Suitable methods and support tools for

the simplified modelling of parameter-based

coherences do not exist.

In order to facilitate the reproduction of product

logic and of designer expertise, a new method for

the model-driven design of deep drawing tools has

been developed at the IFUM and the GFaI. The main

component of this method is a new graphical

modelling language based on Systems Modelling

Schefﬂer, R., Koch, S., Wrobel, G., Pleßow, M., Buse, C. and Behrens, B-A.

Modelling CAD Models - Method for the Model Driven Design of CAD Models for Deep Drawing Tools.

DOI: 10.5220/0005799403770383

In Proceedings of the 4th International Conference on Model-Driven Engineering and Software Development (MODELSWARD 2016), pages 377-383

ISBN: 978-989-758-168-7

377

Language (SysML). This new modelling language

should expand the conventional CAD model to an

eXtended CAD (XCAD) model. By means of this

graphical language, parameter coherence could be

defined in earlier developing phases of deep drawing

tools. Thus, the conventional geometric modelling of

deep drawing tools could be replaced by applying

this new graphical language. This simplifies the

complexity of the parametric modelling of deep

drawing tools and thus the designer tasks.

2 STATE OF THE ART

2.1 Parametric 3D CAD Models

Market-leading 3D CAD systems, such as CATIA

V5, Solid Edge, Creo Parametric and Inventor

provide different approaches to creating geometric

3D CAD models of mechanical constructions. Here,

a differentiation is made between explicit and

parametric-associative modelling. In both modelling

methods, a 3D CAD model consists of a part or an

assembly of parts, and the visual 3D CAD model is

described by geometric parameters. In explicit

modelling there are no dependencies between

individual geometric parameters. This modelling

method is primarily used for customised individual

constructions with short development times. The

CAD system only saves part properties and topology

of the last modelling step (Schumacher, 2013).

During parametric-associative modelling, the

CAD system saves the genesis of the parts instead of

their geometries. These specific parts represent not

only object geometry, but also object attributes as

well as the creation history (Abulawi, 2012).

Parametric-associative modelling allows creating

parametric CAD models, which can be adapted to

planned modifications quickly and consistently.

Such CAD models consist of parameters, features

and their dependencies. In the context of CAD

systems a parameter is a variable by which the CAD

model can be controlled. Parameters can have

various data types and store integer values, float-

point numbers, truth values as well as strings (Vajna

et al., 2009). In parametric modelling a feature is

defined as an aggregation of geometry objects and

attributes (or parameters), which are used for the

common representation of functional elements. A

distinction is made between semantic and shape

features. A shape feature includes geometry

elements and their dimensioning parameters.

Additionally, a semantic feature stores technological

information, for example the hole tolerance H7.

Consequently, a feature contains the relevant

properties with their values as well as their relations

and constraints (Vajna et al., 2009).

The dependencies between parameters and CAD

elements such as features, parts or assemblies are

modelled in the form of constraints and relations.

These constraints and relations can be created as

functions and equations, but also as algebraic, logic

and semantic constraints (Marchenko et al., 2011).

2.2 Graphical Domain-Specific

Languages in Mechanical

Construction

Domain-specific languages (DSL) are formal

languages that are tailored for the use of a specific

domain. They are widely used in the form of textual

languages (e.g. Modelica) to model artefacts

(objects, facts, functions, behaviours) of the

individual domain. Graphical languages have been

established as well, the most widely used being

Unified Modelling Language (UML) and Systems

Modelling Language (SysML) as graphical domain-

specific languages for the domains Software

Engineering and Systems Engineering respectively.

While textual languages are usually described by

grammars, and it’s possible to specify graphical

languages by grammars (e.g. graph grammars) the

typical approach is to model the abstract syntax of a

language by defining a meta-model of the domain.

The meta-modelling of the languages UML and

SysML allows for specification and extension, so

that new DSL can be defined in addition to the

existing languages.

Graphical domain-specific languages (GDSL)

are already introduced into the domain of

mechanical construction: Hochgeladen and Vyas

explored the use of UML for the design of complex

assemblies and concluded that particularly the

knowledge embedded in rules and algorithms is

easier to grasp in the form of UML than in the

classic geometric construction in a CAD model

(Hochgeladen and Vyas, 2004). Au and Yuen

developed a graphical DSL for the modelling of

sculpt objects. Although these objects are defined by

their geometric representation the user works with

abstract features and their relations (Au and Yuen,

2000).

Other approaches focus on earlier stages in the

construction process, for example Andersson

developed a tool for the concept phase to create

geometric and non-geometric models (Andersson et

al., 1995). Wölkl and Shea examined the use of

SysML for the concept modelling in mechanical

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construction and showed the utility of several

SysML diagrams (Wölkl and Shea, 2009). Peak and

Zingel used SysML to generate a model usable for

simulations even in the early design phase (Peak et

al., 2007). Albers and Zingel applied SysML based

functional modelling techniques to the product

development process of mechatronic systems

(Albers and Zingel, 2013a).

2.3 SysML and Model-based Systems

Engineering

The mentioned research and industrial projects

highlight the potential of SysML for the domain of

parametric construction. Several surveys have also

examined the low acceptance of the Model-Based

Systems Engineering (MBSE) approach in

mechanical construction, notably (Bone and

Cloutier, 2009) and (Albers and Zingel, 2013b). The

main challenge identified was the steep learning

curve for SysML and particularly the application of

concepts like inheritance that aren’t present in

mechanical construction. SysML models differ

considerably from the mechanical models (CAD

models) a designer is used to. Other challenges are

the complexity and usage of specific diagrams. In

the survey conducted by Albers and Zingel only

48% of the participants claim to have knowledge of

Constraint (i.e. Parametric) diagrams and only 4%

find them to be “crucial”. A common improvement

recommendation by those surveyed was to increase

the usability of the existing modelling software tools

instead of the SysML language itself, which was

also one of the recommendations by Bone and

Cloutier.

3 METHOD

In order to simplify parametric 3D modelling of

deep drawing tools, a new method for the model-

driven design of deep drawing tools has been

developed at the IFUM and GFaI.

The major idea of the new method is to create

eXtended CAD (XCAD) models outside of CAD

software in a tool better suited for modelling

parametric relations. XCAD models don’t need to be

fully congruent to CAD models; they contain

information about the structure of the modelled

tools, but no geometric information. Since modelling

languages like SysML come with a graphical

notation and have seen industry acceptance they

should be considered as resources for this task.

The proposed workflow for a designer of deep

drawing tools is to first create the main structure of

the tool by building elements and connections

between them. This is based on a meta-model of

deep drawing tools that is loaded in the background

of the prototype software. The focus of the prototype

is on relations as the most important parts of the

CAD models to be created. The user is extensively

supported by layout algorithms and interaction

helpers. After an XCAD model is created it can be

exported to a CAD software system, where it is

completed to a full CAD model. Typically,

geometric parameters would be set in the CAD

software, which is uniquely suited for this task.

Figure 1: Development of the Method for Model-Driven Design of Deep Drawing Tools.

Modelling CAD Models - Method for the Model Driven Design of CAD Models for Deep Drawing Tools

379

The realised steps in developing the new method

are shown in Figure 1.

3.1 Analysis of CAD Models

Initially, deep drawing tools were selected for the

development of the new method and furthermore

their selection has been limited to three typical

geometries with varying complexity: round,

rectangular and trapezoidal geometries with flat

bottom area. In order to completely cover the

domain of deep drawing tools, single- and multiple-

acting deep drawing tools were modelled

parametrically in the 3D CAD system CATIA V5.

Figure 2 illustrates the 3D CAD model of a

single-acting deep drawing tool for a rectangular

cup. This tool consists of four assemblies, which

include several parts. Parametrical tool modelling

focuses on punch assembly since the punch is the

main forming component. Thus, the main

parameters in the form of length, width and height

are assigned to the forming punch. These three

parameters can be varied by the tool designer and

can consequently be called free parameters. These

are connected to the parameters of the other punch

assembly parts by corresponding relations. Thus,

this connection influences the geometry of the whole

assembly. The punch assembly is fixed on the upper

fixing plate and affects the column guide frame

assembly by the parameters and their relations. The

punch and the die are active tool elements, which

form the sheet metal part. Consequently, it is useful

to model the die assembly depending on the punch

assembly by corresponding relations. Figure 2 shows

that in this case the blank holder assembly is a

reversed copy of the die assembly. Therefore, the

blank holder assembly parameters depend on the die

assembly parameters.

Figure 2: CAD model of single-acting deep drawing tool.

In order to create a basis for formalising the

model data and the development of the meta-model,

development and construction documents in the

form of design drawings, requirement specifications

and part lists were created. In order to get an

overview of parameters and their dependencies in

the 3D CAD model, these were integrated in the

requirement specification documents in written

form.

3.2 Modelling in SysML

After designing the deep drawing tools as CAD

models, plain SysML was used to recreate these

models in order to get a better understanding of the

benefits and shortcomings of the model-based

approach. The diagrams used were Block Definition

Diagrams (BDD) for the hierarchical structure of the

CAD models and Parametric Diagrams (PD) for the

relations between model entities. Advanced SysML

concepts like profiles and stereotypes were also

explored and have proven helpful in adding

information without increasing diagram complexity.

The resulting diagrams are visual representations

of the parametric CAD models. The BDD hold

information about the structure of the models and a

rough outline of relations between them. Large

models are turned into very complex drawings that

are hard to read and understand clearly. A main

difference between the typical visualization of the

structure in a CAD software tool and a SysML

diagram is the representation of the composition

relation. CAD software often shows this as a tree

view, while SysML diagrams use relations

visualized by line connections. The abundance of

lines and the alikeness of different types of relations

make them hard to follow.

Studies on the readability of graphical languages

have found several important aesthetic criteria, most

notably the clear drawing of lines by avoiding

overlaps, crossings, bends and dense areas (Huang

and Eades, 2005; Ware et al., 2002). In a sufficiently

complex model essentially all of these criteria are

violated in BDD. While it is possible to separate

blocks into different diagrams this makes relations

between elements even more difficult to follow, as

they then cannot be contained in a single diagram.

While BDD can show that model elements are

related, the specific characteristics of these relations

are modelled in Parametric Diagrams. Surveys on

SysML use showed these to be among the most

puzzling diagram forms in SysML. For the domain

of parametric construction this is certainly true. To

model even the simplest constraint relations between

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model elements (e. g. mathematical formulas on

geometric properties), one has to use several

diagrams that need to be read in a precise order to

understand the model. Modelling the large number

of relations usually present in a fully parametric 3D

CAD model by hand is extremely time-consuming.

The main benefit of SysML for the domain of

parametric construction is the standardization and

the availability of tools and implementations. The

easy extensibility through profiles is another

advantage. As a graphical language SysML has to be

considered severely lacking for this domain. The

visual representations of the information in CAD

models are needlessly complicated (PD) or so

complex that they suffer from poor readability

(BDD).

3.3 Building XCAD Models

Creating an XCAD model is an important step of the

method proposed in this paper and a main focus of

the prototype software to be developed. It can be

achieved in three ways:

1. The user can build the XCAD model entirely in

the software (by instantiating the meta-model).

2. The user can import a CAD model to be

extended into an XCAD model.

3. The user can import a SysML model to be

extended into an XCAD model.

Imports of existing CAD models were implemented

for the CAD software CATIA V5 in an earlier

research project (Marchenko et al., 2011). SysML

models can be imported through XML Metadata

Interchange (XMI) files, which are transformed into

the intermediate format by Extensible Stylesheet

Language Transformations (XSLT).

The basis for the modelling of XCAD models is

a meta-model of deep drawing tools. Borrowing

from language features of UML, this meta-model

was also created in the software prototype, using the

same interactions described below.

Figure 3: Dual Tree View (simplified snippet).

The prototype software uses a graphical language

that was developed to help users identify the

interconnectedness of relations in parametric 3D

CAD models. A Dual Tree View (see Figure 3)

contains two tree views for the same model and

displays relations between them in the middle. This

visualization allows the user to intuitively grasp the

hierarchical structure of the model, a feature that is

very prominent in the design of CAD models, while

at the same time putting relations between elements

into focus. Connecting lines can be kept relatively

short and mostly overlap and crossing free. As a

contrast to typical SysML diagrams both colours and

icons are used intensively to differentiate between

types of model elements. Dual Tree Views make use

of interactive user behaviour to show long chains of

relation connections.

Editing XCAD models consists of two main

tasks: Firstly the creation of the hierarchical

structure and secondly the modelling of relations

between the elements of this structure. The first task

is relatively straight forward and can be

accomplished with the usual set of interactions:

Adding elements, editing their properties, moving

and deleting elements. The software prototype

presents a graphical way to model relations: The

user connects elements by drawing a line, thus

creating a relation shell, which can then be filled out

in a second step. If, for example, a user wants to

create a geometric constraint between two parts of

an assembly, he would connect the two parts and in

the relation editing view all the (CAD) properties

and parameters of the two parts would be presented

to him. These can then be connected to a relation

element of the type “formula constraint”. The

relation editing view uses a graphical language

called Parameter Map (see Figure 4) that is

influenced by mind map visualizations. Here the

relation element is the centre of the view and the

input and output elements are positioned at the top

or the bottom of the layout area. The hierarchical

information about model elements is preserved by

the dynamic generation of symbols for elements as

stacked rectangles.

Figure 4: Parameter Map for single relation.

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The XCAD models created with the software

prototype can be exported in an XML format. To

complete the proposed design method it is necessary

to transform these models into a format that is

reusable by other software tools, e.g. the XML

scheme formats of modern CAD systems.

While SysML diagrams aren’t used in the design

of the meta-model and the creation of models, the

resulting models are still MOF compliant. That

means the output of the software prototype can be

XSL-transformed into an XMI format of a SysML

model. This allows a variety of systems modelling

tools to reuse the created XCAD models. In

particular, simulation tools can be used in rapid

prototyping before even using CAD software in the

design process. The proposed method therefore fits

nicely into other approaches to model driven design

and can be integrated with existing MBSE tools.

4 CONCLUSIONS

The goal of the presented research project was to

create a holistic, graphical and model-based method

for the concept stage of deep drawing tool design.

SysML was recognized as a potent instrument

for MBSE with rising acceptance and existing

applications in different stages of the engineering

process. SysML diagrams, particularly parametric

diagrams, were identified as a weak point of the

language regarding usability.

Analysing CAD models of deep drawing tools

helped creating a meta-model for these tools and on

the basis of the meta-model a new graphical domain-

specific language (GDSL) was created. It features

diagram types that allow for a more intuitive usage

by engineers and thus for a faster design iteration.

On the other hand, the relation to and reliance on

SysML was kept intact to facilitate the integration

with existing MBSE software tools.

A software prototype implements the GDSL and

its diagrams as well as various import and export

operations to showcase the method.

ACKNOWLEDGEMENTS

The authors thank the German Research Foundation

(DFG) for the financial support of the research

project “Method for the Model-Driven Design of

Deep Drawing Tools” (project numbers BE

1691/164-1 and PL 706/1-1).

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