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Parametric and Feature-Based Modeling

Another feature of modern CAD systems is the ability to create parametric models. In a

parametric model, each entity, such as a boolean primitive, a line or arc in a wireframe, or a

filleting operation, has parameters associated with it. These parameters control the variousgeometric properties of the entity, such as the length, width and height of a rectangular prism, or

the radius of a fillet. They also control the locations of these entities within the model.

These parameters can be changed by the operator as necessary to create the desired part.

Parametric modelers that use a history-based method keep a record of how the model was built.

When the operator changes parameters in the model and regenerates the part, the programrepeats the operations from the history, using the new parameters, to create the new solid. There

are many uses for this type of modeling. Designers can test various sizes of parts to determinewhich is the ``best'' part for their use by simply adjusting the model parameters and regenerating

the part.

Some parametric modelers also allow constraint equations to be added to the models. These canbe used to construct relationships between parameters. If several parameters always require thesame value, or a certain parameter depends on the values of several others, this is the best way to

ensure that these relationships are always correct.

These modelers allow other methods of relating entities as well. Entities can be located, for

example, at the origin of curves, at the end of lines or arcs, at vertices, or at the midpoints of

lines and faces. They can also be located at a distance or at the end of a vector from these points.When the model is regenerated, these relationships are maintained. Some systems will also allow

geometric constraints between entities. These can require that entities be, for example, parallel,

tangent, or perpendicular.

Feature-based modelers allow operations such as creating holes, fillets, chamfers, bosses, and

pockets to be associated with specific edges and faces. When the edges or faces move because ofa regeneration, the feature operation moves along with it, keeping the original relationships. The

choices made developing these models are very important. If the features aren't referenced

correctly, they may not end up in the correct place if the model is regenerated. A feature that is

located at an X and Y offset from a corner of the face instead of at the center of the face will notremain at the center of the face when the model is regenerated unless constraints are added to the

model that will change the X and Y offsets to keep the feature at the center of the face. [Shah]

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Parametric Modelling BasicsNowadays, parametric 3D CAD solid and surface models are the principal means of

communicating design ideas and developing new products and systems. 3D parametric

modelling facilitates visual thinking and the design process, and represents a welcome additionto the traditional three R's of reading,writing and arithmetic. It stimulates students to use their

imagination and problem solving skills and helps them to become more technologically literate.

Worldwide, parametric modelling systems are part of a technology education reform movement

that seeks to improve critical thinking and multidimensional problem-solving skills, while alsoinspiring and preparing a growing number of students to become the engineers, designers and

technologists of tomorrow.

Parametric modelling enables learners to think and create in three dimensions with sophisticated

design software typically used by manufacturers. Integrating it into technological subjects willinspire more students to become the innovators of tomorrow by choosing careers in product

design, engineering and technology. The integration of parametric modelling into the

technological subjects will give these subjects a great future and make them even more relevantto the needs of society. It will modernise these subjects and bring excitement, interest, and

vibrancy to them and facilitate the realisation of their potential.

What is Parametric Modelling?

Parametric modellers are often referred to as Mechanical CAD (MCAD) modellers and can be

described as parametric, feature-based, solid and surface modelling design tools. Let us look

briefly at these terms.

Parametric

The term parametric essentially means that MCAD software uses parameters. The mostsignificant of these parameters are dimensions, and in MCAD software, dimensions drive the

geometry, as opposed to the geometry driving the dimensions, which is the case in 2D and

traditional 3D solid modellers. Therefore when you change a dimension value, this causes the

model size to change. In addition, the relations or constraints used to create the features of a partare also captured in the model.

Features

A feature is the basic unit of a parametric solid model. Just as an assembly is made up of

individual parts, a part file is made up of individual elements called features. Each feature hasintelligent properties that define it. When a feature is created, the geometric constraints and

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dimensions that apply to it are specified. The modeller stores these properties and uses them to

generate the feature. Examples of these basic building blocks called features are bosses, holes,

ribs, cuts, fillets, and chamfers. New features are dependent on existing features in such amanner that design changes are captured automatically. In essence, feature-based modelling

captures the designer's intent. If an element of the feature, or a related part of the model, changes,

the modelling software re-generates that feature in accordance with the defining propertiesassigned to it. For example, an edge that is defined to be tangent to an arc will move to preservethe tangency constraint if the size of the arc is changed. Features can be classified into two main

types, namely sketched features and applied or placed features.

Sketched Features

A sketched feature requires a 2-D sketch that is then transformed into a feature in one of fourmain ways. These part modelling methods are extrude, revolve, sweep and loft.

Applied features

Applied features are applied directly to the model and do not require a sketch. Fillets, chamfers,

draft and shell are examples of these features.

Solid Modelling

A solid model completely and unambiguously represents the geometry and topology of a part. In

addition to the information contained in surface models, solid models contain volume

information. This means that a solid model can provide such information as the mass propertiesof a part and interference checking between parts in an assembly.

Associativity

3D modelling software can automatically update related parts of the model when design changesare made and there is full bi-directional associativity between parts, assemblies and drawings.This means that your drawings are always correct as they are based on the parts and assembly

models and changes to a drawing transfer back to parts and assemblies.

Advantages of Parametric Modelling.

3D parametric solid modelling offers the following advantages over traditional 2D drawings:

In addition to standard orthographic views, 3D solid models also offer an unlimited range ofways to view the model, including rendered and animated views.

3D modelling software can automatically update related parts of the model when designchanges are made and there is full bi-directional associativity between parts, assemblies and

drawings.

3D systems provide easier design revisions. Changes can be made at the level of each individualsketch and feature. If a sketch is not the required size, it can be easily edited by selecting the

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relevant dimensions. Similarly, the definition of individual features can be edited by changing

their properties.

3D systems are more motivational, interesting and appealing for today's students who havenever used a typewriter, owned a vinyl record or a black and white television.

Parametric modellers have a rollback feature that shows the sequence in which the model wascreated. This is an invaluable tool for learning modelling strategies from existing models and is

also very useful for assessing student work

Not alone is the modelling sequence captured in a parametric system but modelling errors arehighlighted for the user. With 2D systems there is no error checking.

3D conveys a superior sense of what an artefact will look like. Form and shape and overall modelproportions are more easily understood and defined in 3D. In essence, 3D systems provide

better design visualization.

3D systems better capture design intent. This essentially relates to how the model shouldbehave when design changes are made.

3D systems provide automated drawing production. Within industry, 3D systems provide better integration with downstream applications and

reduced engineering cycle time. The accuracy and completeness of the design definition in the

CAD database make the models suitable for use in analysis and for transfer to rapid prototypingand manufacturing machinery.

The Parametric Solid Modelling Process

The starting point for a parametric solid model is a sketch that need only be the approximate sizeand shape of the part or feature being created, as dimensions can be added later to change thesize and shape of the geometry. While a parametric solid model is an intelligent representation of

a part, it is important to analyse and plan every part before modelling to determine the most

efficient sequence for creating the features. Poor modelling strategies will result in parts that takelonger to create and that are difficult to edit. Features should be created to allow for maximum

part flexibility and variation. Rather than perceiving the finished solid model as a large solidmass, it needs to be viewed as a composition of features that are likely to be modified.

Before starting to sketch, the model should be studied to identify the best profile to use forcreating the base feature. The best profile is that which best describes the overall shape of the

part, and will minimise the number of remaining features needed to complete the model. Each

new part contains three infinite reference planes, which represent the front, top and right planes

in space, each of which passes through the origin, which is the zero point in space.

The general procedure for parametric modelling is to decide on the best or most descriptiveprofile for the first sketch for the base (first) feature of the model. You then select the mostappropriate sketch plane on which to create this first sketch so that the final model will have

the correct orientation when viewed pictorially. The sketch geometry should be created by

capturing constraints as you sketch, and then dimensioned to fully define the geometry.

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Although sketches do not have to be fully defined to create features, normally it is better to do

so to avoid possible later model distortion. A fully defined sketch is blackand is the desired

state, whereas an underdefined sketch is blue. An overdefined sketch is red.

The 2D sketch is then turned into a 3D solid usually by an extrusion or a revolve process. Asnoted previously, sketches can also be turned into solid features through a sweep or loft process.

Extrusions pull the sketch normal to the sketch plane, while a revolved feature rotates the sketch

around an axis. Sweeping moves the sketch along a path made up of straight or curved geometry,while lofting uses multiple sketches to transition from one shape to another. Each sketch is

linked to its resulting feature. If you go back and change the sketch, the feature will update to

reflect the change. Normally each sketched feature will require its own sketch. Fillets and rounds

can be added to the model to round sharp corners that would be inappropriate to do in a sketch.From the finished solid model you can create a drawing file with standard dimensioned

orthographic and isometric views.