Machine Dynamics Research
2013, Vol. 37, No 2, 34–41
Approaches to Assemblies Modelling
in MCAD Systems
Mariusz Jabłoński, Andrzej Łukaszewicz
Bialystok University of Technology
a.lukaszewicz@pb.edu.pl
Abstract
In this paper some aspects of assemblies modelling in parametric Mechanical CAD systems
(MCAD) are described. Main approaches for assemblies design are discussed. Special
techniques and interesting tips and topics using for improve the desing process in assembly
modelling are presented.
Keywords: computer aided design, prodact data manegment, parametric CAD
1. Introduction
Nowadays, we are the witnesses of an enormous expansion of computer aided
technologies (CAx) [Dankwort et al., 2004; Łukaszewicz, 2008]. Replacement of
engineers' tools related with the process of design, analysis and products'
manufacturing has been forced by this expansion. Flexible automated production
which requires both, computer aided manufacturing processes and appropriate
organizational structure, is the main tendency in the industry. Computer aided
design (CAD) is an inseparable part of computer integrated manufacturing (CIM)
as well as product data management (PDM) and product lifecycle management
(PLM) systems during general workflow of information about product (Fig. 1).
Opportunities offered by 3D geometric modelling has caused that design based on
parametric modelling way is the standard [Yea et al., 2004; Łukaszewicz, 2009a].
The virtual 3D model, built in true scale in line with constructional plans is
subjected to full kinematic, static and functional analysis. It is possible to optimize
shape permanently and to make simulations of the final product behaviour which is
tested on measurement stands or during operation [Wełyczko, 2005]. Computer
aided design systems usually work on three types of files: parts, assemblies and 2D
documentation. Modelling of assemblies in parametric CAD systems in mechanical
branch (MCAD) is an influential phase in virtual design process and also in testing
of any devices built of two or more functionally separate elements named
assemblies components.
Approaches to Assemblies Modelling in MCAD Systems
35
Fig. 1. Workflow in CAx environment
2. Design for assemblies
Assembly is a necessary part of each larger project. Modelling of assemblies in
parametric CAD systems can cause certain difficulties. Project management, its
storage, access for contributors and nomenclature often make problems. Efficient
planning of the design process to make designer's work smooth and effective is
equally important task during the work with project [Łukaszewicz, 2009b; DS
SolidWorks, 2011].
2.1. Project planning
Before starting work we have to plan our action to optimize the process of
assembly design which is a component of the project. Therefore objective and
range must be exactly defined. It allows to see all the most important aspects of the
project just before the beginning of creating model.
If we work on large assembly it is useful to arrange the team, where every
person is responsible for completing specific task. We must remember to choose
the right person for the right design.
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Large projects are able to be divided into smaller areas. It creates a possibility to
simply introduce some changes to the project. It is easier to bring small pieces
together later for making an assembly.
2.2. Project management
Good solution is to use Product Data Management (PDM) systems during the work
with project. They allow to share files faster and effectively protect the project and
archive it.
Correct naming of parts and subassemblies need to be remembered. At the
beginning we need to define the method of components naming and use it for each
designed part. During naming of files it is good to save individual names to each of
them.
2.3. Stages of the project
During the work on a project it is good to know what level of the design stage we
are working on. The type of created models, the kinds of used constraints, and level
of created detail are all defined by what stage we are in (Fig. 2).
Fig. 2. Stages of the project [DS SolidWorks, 2011]
3. Assemblies in CAD systems
Model of assembly is a set of components saved as one file. Contemporary
parametric systems of body-surface modelling offer users the usage of separate
environments to define assemblies and also special tools and techniques intended
for assemblies designing. Separate parts and other assemblies which functionally
are components of larger assembly, can be the components of assembly. Every
Approaches to Assemblies Modelling in MCAD Systems
37
change in component's file is automatically rebuilt in assembly [DS SolidWorks,
2011].
3.1. Parts
Performance and efficiency of assembly largely depend on individual pieces.
Precise determination of design assumptions is one of key elements during
modelling of single components. Part designing must be initiated from specifying
and planning how to create the component. While constructing the modelling parts
plan it is good to pay attention to parts modelling rules [Łukaszewicz, 2009c].
3.2. Subassemblies
Subassembly is an assembly which is a component of another assembly. It can be
nested on many levels to reflect the hierarchy of the project. Subassemblies
designing is possible by using several methods [Lombard, 2011]:
• forming of the subassembly by selecting a group of components, which
are already element of assembly and moving them one level down in
assembly hierarchy,
• creating of assembly as a separate feature and inserting it into a higher
level assembly as a component,
• inserting of empty subassembly during editing of the highest level
assembly and adding components to it.
By default, the subassembly is rigid. Inside the main assembly, the subassembly
is treated as a single element and its components cannot move relative to each
other. There is a possibility to create flexible assemblies, what makes to move
individual components as part of the main assembly. When the assembly is already
opened, it has to solve all the constraints in the top-level assembly. The constraints
in subassemblies will not be disturbed unless the subassembly is set to be solved as
flexible. Consequently it is an advantage to minimize the number of constraints in
the top-level assembly [DS SolidWorks, 2011].
3.3. Constraints of components
The constraints are the basic element of assemblies designing. They make
geometrical correlations between components in the assembly. Each unrelated
component in the assembly has six degrees of freedom. There is a possibility of
moving and rotating it around the axis X, Y, Z. By putting constraint into two
elements, removing of one or several degrees of freedom is occurred. By adding
the constraint to components we can specify permitted movement directions of
component. The elements associated in the constraint can be: planar, cylindrical
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and conical faces, planes, axes, origins, edges of components, elements of sketches,
vertices or sketch points.
There are many possibilities of creating the constraints. For example, in
SolidWorks system they are divided into 3 groups such as: standard, advanced and
a mechanical constraints (mates).
In creating constraints, in design tree based CAD system, certain rules
improving and optimizing of assembly modelling process should be respected
(Lombard, 2011):
• all components should be associated with one or two immobilized
components or references. Designing long links of components extend
constraints process and they are prone to errors,
• creating loop of constraints causes conflicts when you add additional
constraints. It should be avoided,
• after adding of constraints we can drag components to test available
degrees of freedom,
• redundant constraints should be avoided and also instant correction of
constraints errors is recommended,
• if component with positioned constraints makes problems, better option is
to delete all constraints and set them once more than looking for problem,
• suppression and restoring of constraint can fix existing errors,
• it is important to define each part of assembly (exception is the part
which needs to be able to move).
4. Methods of creating assemblies
Assemblies modelling in parametric CAD systems often use Bottom-up Design and
Top-down Design strategies. There is also the method which is the combination of
these two methods. In order to select the optimal order of creating parts and
components as well the method of the design, before start designing, should be
answered:
• what task need to be met by model and for what purpose need to be
created?
• will technical documentation be created on its basis?
• will it be subjected to analyses of motion or strength?
• is it possible to split a large assembly in several subassemblies?
• does project include some parts presented in libraries of software?
4.1. Bottom-up approach
This strategy of creating assemblies is a traditional approach to assemblies
modelling process. Create of finished parts in separate files which will be included
Approaches to Assemblies Modelling in MCAD Systems
39
in assembly is the first step in designing method of Bottom-up. Then, components
and subassemblies created earlier are put into assembly file, where with
constraints, certain geometrical correlations are given. Parts must be edited
separately to make changes. Bottom-up approach is recommended in situation
when all parts from assembly are earlier modelled or available in the library.
4.2. Top-down approach
The name of the Top-down comes from the fact that the general assumptions of the
project (size of the feature, the standoff of remaining parts, location of components,
etc.) comes from the top (the assembly) and goes down (to parts)
(www.help.solidworks.com). This method is often used to plan of assembly and to
see relevant aspects of specific parts. It is particularly useful in conceptual stage of
the design process, when the project is subjected to many experiments and changes.
Much smaller number of repeated operations in changes is the basic advantage of
top-down method. Created components based on other parts are edited
automatically when only main parts are modified. In top-down design technique we
can use one or all methods listed below [DS SolidWorks, 2011]:
• features can be created by references to other parts in the assembly. In
bottom-up technique, the part is created in different window, where there
is no access to geometry of the assembly. Some CAD environments allow
to edit specific part when it already is into assembly. It gives us the
possibility to reference the geometry of other components,
• the possibility of designing parts through the creation of components in
context of the assembly. The component is joined to other component
existing in the assembly. Geometry of new component is based on
existing component. This method works mainly in parts design where
shape and size are dependent on other parts,
• the assembly can be designed with layout sketches specifying the location
of components, main dimensions, etc. Defining the project objectives and
designing of components with appropriate assumptions should start the
work. The list of known assembly parameters is made and then based on
them editable parts are designed.
4.3. Mixed design
The vast majority of the designed assemblies consist two modelling strategies,
bottom-up technique and top-down technique. Generally, most of the assemblies
are defined of already existing components. New part is designed in the case when
design assumptions are changed or when the project needs to have some design
changes. The new part is added to existing components connected with constraints
usually by creating it in the context of assembly. This component has a full
reference of geometry to the part. In case where change of shape or position cannot
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A. Łukaszewicz, M. Jabłoński
be ruled, leaving them not fully dimensioned is a good option. There is a possibility
then to change shape and size after giving it constraints with other components in
the assembly.
5. Functional design
The essence of functional design is a collection of materials and documentation,
which represent abandonment of geometric descriptions for acquisition and use of
knowledge (KBE). Functional design can be used in every step of project
designing. It allows to analyze the product in detail and solve problems occurring
during the design. By using functional design it is possible to automatically create a
mechanically efficient components [Gil et al., 2011; Skarka, 2009]. The use of
functional design during the design phase can provide many advantages:
• the high level of design quality and the accuracy of its implementation,
• accelerating the process of designing and making changes,
• avoiding many errors early in the design,
• no need to build physical prototypes to verify certain properties.
6. Conclusions
The growing demand for new products is a challenge for all producers who are
forced to find cheaper and better solutions which can be quickly placed on the
market. Contemporary parametric CAD systems significantly support this process,
therefore modelling with the use of CAD environment is such an important aspect
of this process. The use of certain techniques and principles of model design
consisting of single parts or assemblies of products to introduce on the market, can
vitally improve the process of virtual design.
Acknowledgements
This paper was performed within a framework of S/WM/1/2013 realised in Bialystok
University of Technology.
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