Key terms:
2D
3D
Features
Sketch based features
Extrude
Revolve
Sweep
Blend
Parametric Modelling
Cartesian coordinate system 

 

A three dimensional – 3D – model and environment is ‘simulated’ on a two dimensional – 2D – screen.

Entities in the 3D environment are called features. We start with a base feature, say a cube, and then develop it by creating further features which either add or subtract volume from the the base feature – ie. we may subtract a cylindrical shape from our cube to create a hole.

Recognising how your intended design can be broken down into these ‘primitive’ elements is the key to successful CAD modelling.

In main stream CAD packages these primitive are sketch based features, that is, they generally start with a 2D sketch. This sketch forms the cross section of the feature.

 

The primary feature creation methods are:

Extrusion: A 2D sketch is developed along a linear path to a specified distance to create a 3D form.

Revolve: A 2D sketch is rotated around an axis through a specified angular distance to create a 3D form.

Sweep: A 2D sketch is developed along a non linear path to create a 3D form.

Blend [Loft]: A 3D form is created by ‘blending’ multiple parallel 2D sketches

 

Feature Matrix

Cross Section or Section or Xsec – the shape you get when you slice a solid

Trajectory – the path through space the section travels to form the solid

Section Trajectory
Extrude Constant Linear
Revolve Constant Circular
Sweep Constant Non Linear
Blend Varying Varying

 

Solids?

Entities in the modelling environment are in three broad categories;

Reference geometry – frameworks and scaffolding use to build our products on – curves, planes, points and axis – see Reference Geometry

Construction geometry – looks like a product or elements of it or even just illusions to it, but it has no mass properties, it has no thickness, it is not solid.  It is used as a basis for fundamental geometry – see surface modelling

Solid geometry – once a group of surfaces have been joined together to form a closed volume they can be designated by the system as a solid – 6 square surfaces can be joined to form a solid cube, remove one surface and the surfaces become construction geometry.  Closed groups of surfaces are often reffered to as watertight or manifold.

 

Boolean operations

Solid features describe a volume in space, that volume can add or subtract material from existing volume – see Boolean operations.

Creating material is often referred to as a protrusion, removing material as a cut. If one protrusion intersects another they will automatically be merged.

Model display

Models are displayed by default with no perspective – this can make them look distorted when interpreted by our normal visual perceptive cues.  This default setup is to save processing power on the computer graphics card.

Perspective can be applied to a model at any time and allows you to better visualise how the model may look in ‘real life’. Creating some simple elements and construction lines and printing the result in perspective view can also be a useful basis for 2D hand sketching with correct vanishing points.

 

Parametric Modelling

Any entity in the 3D space has parameters to describe its position relative to a default point or relative to another entity.

The three dimensions X, Y and Z [hence 3D] are described using the Cartesian coordinate system – 0,0,0 – a dimension for X, a dimension for Y and then a dimension for Z.

You will rarely have to interact with these coordinates but you must be aware of this concept.

In LDS, our preferred 3D environment orientation is the XY plane as the floor and +ve Z up. Creating models as they would sit in ‘real life’ with respect to this orientation will simplify the design process – particularly when we start creating assemblies