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Parametric Modelling Basics

Nowadays, parametric 3D CAD solid and surface models are the principal means ofcommunicating design ideas and developing new products and systems. 3D parametricmodelling facilitates visual thinking and the design process, and represents a welcomeaddition to the traditional three R's of reading,writing and arithmetic. It stimulates studentsto use their imagination and problem solving skills and helps them to become moretechnologically literate. Worldwide, parametric modelling systems are part of a technologyeducation reform movement that seeks to improve critical thinking and multidimensionalproblem-solving skills, while also inspiring and preparing a growing number of students tobecome the engineers, designers and technologists of tomorrow.

Parametric modelling enables learners to think and create in three dimensions withsophisticated design software typically used by manufacturers. Integrating it intotechnological subjects will inspire more students to become the innovators of tomorrow bychoosing careers in product design, engineering and technology. The integration ofparametric modelling into the technological subjects will give these subjects a great futureand make them even more relevant to the needs of society. It will modernise these subjectsand bring excitement, interest, and vibrancy to them and facilitate the realisation of theirpotential.

What is Parametric Modelling?

Parametric modellers are often referred to as Mechanical CAD (MCAD) modellers and can bedescribed as parametric, feature-based, solid and surface modelling design tools. Let uslook 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 drivethe geometry, as opposed to the geometry driving the dimensions, which is the case in 2Dand traditional 3D solid modellers. Therefore when you change a dimension value, this causesthe model size to change. In addition, the relations or constraints used to create thefeatures of a part are also captured in the model.

Features

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



dimensions that apply to it are specified. The modeller stores these properties and uses themto 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 insuch a manner that design changes are captured automatically. In essence, feature-basedmodelling captures the designer's intent. If an element of the feature, or a related part of themodel, changes, the modelling software re-generates that feature in accordance with thedefining properties assigned to it. For example, an edge that is defined to be tangent to anarc will move to preserve the tangency constraint if the size of the arc is changed. Featurescan be classified into two main types, namely sketched features and applied or placedfeatures.

Sketched Features

A sketched feature requires a 2-D sketch that is then transformed into a feature in one offour main 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 volumeinformation. This means that a solid model can provide such information as the massproperties of a part and interference checking between parts in an assembly.

Associativity

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 anddrawings. This means that your drawings are always correct as they are based on the partsand 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 rangeof ways 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, assembliesand drawings.3D systems provide easier design revisions. Changes can be made at the level of eachindividual sketch and feature. If a sketch is not the required size, it can be easily editedby selecting the relevant dimensions. Similarly, the definition of individual features canbe edited by changing their properties.3D systems are more motivational, interesting and appealing for today's students whohave never 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 themodel was created. This is an invaluable tool for learning modelling strategies fromexisting models and is also very useful for assessing student workNot alone is the modelling sequence captured in a parametric system but modellingerrors are highlighted 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 andoverall model proportions are more easily understood and defined in 3D. In essence, 3Dsystems provide better design visualization.3D systems better capture design intent. This essentially relates to how the modelshould behave when design changes are made.3D systems provide automated drawing production.Within industry, 3D systems provide better integration with downstream applications andreduced engineering cycle time. The accuracy and completeness of the design definitionin the CAD database make the models suitable for use in analysis and for transfer torapid prototyping and manufacturing machinery.

The Parametric Solid Modelling Process

The starting point for a parametric solid model is a sketch that need only be the approximatesize and shape of the part or feature being created, as dimensions can be added later tochange the size and shape of the geometry. While a parametric solid model is an intelligentrepresentation of a part, it is important to analyse and plan every part before modelling todetermine the most efficient sequence for creating the features. Poor modelling strategies willresult in parts that take longer to create and that are difficult to edit. Features should becreated to allow for maximum part flexibility and variation. Rather than perceiving the finishedsolid model as a large solid mass, it needs to be viewed as a composition of features that arelikely 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 ofthe 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 andright planes in space, each of which passes through the origin, which is the zero point inspace.

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 themost appropriate sketch plane on which to create this first sketch so that the final modelwill have the correct orientation when viewed pictorially. The sketch geometry should becreated by capturing constraints as you sketch, and then dimensioned to fully define thegeometry. Although sketches do not have to be fully defined to create features, normally itis better to do so to avoid possible later model distortion. A fully defined sketch is black andis 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.As noted previously, sketches can also be turned into solid features through a sweep or loftprocess. Extrusions pull the sketch normal to the sketch plane, while a revolved featurerotates the sketch around an axis. Sweeping moves the sketch along a path made up ofstraight or curved geometry, while lofting uses multiple sketches to transition from one shapeto another. Each sketch is linked to its resulting feature. If you go back and change thesketch, the feature will update to reflect the change. Normally each sketched feature willrequire its own sketch. Fillets and rounds can be added to the model to round sharp cornersthat would be inappropriate to do in a sketch. From the finished solid model you can create adrawing file with standard dimensioned orthographic and isometric views.



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