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# Parametric Modelling Basics

Nowadays, 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 addition to 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 also inspiring 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 will inspire 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 relevant to 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 most significant 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 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 of individual parts, a part file is made up of individual elements called features. Each feature has intelligent 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 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 a manner 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 properties assigned to it. For example, an edge that is defined to be tangent to an arc will move to preserve the 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 four 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 volume information. This means that a solid model can provide such information as the mass properties of a part and interference checking between parts in an assembly.

## Associativity

3D modelling software can automatically update related parts of the model when design changes are 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.

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 of ways to view the model, including rendered and animated views.
• 3D modelling software can automatically update related parts of the model when design changes 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 individual sketch and feature. If a sketch is not the required size, it can be easily edited by selecting the 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 have 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 the model was created. 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 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 and overall model proportions 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 should behave 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 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 approximate size and shape of the part or feature being created, as dimensions can be added later to change the size 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 take longer 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 solid mass, 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 for creating 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 descriptive profile for the first sketch for the base (first) feature of the model. You then select the most appropriate 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. 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 black and 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. As noted 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.

University of Limerick

Department of Manufacturing
and Operations Engineering