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1.4 EDITING AND CREATING

• Each CAD package allows us to manipulate the geometrical model using various interactive techniques.

• Editing Geometry depends upon the representation the geometry is stored with.

• If an elemental (remember: lines, circles, arcs, etc.) geometric model is used, then the methods are much different than a B-Rep model.

• The major editing methods used are for Elemental, Surfaces, and CSG.

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1.4.1 2D Curves and Lines

• A number of functions must be provided to allow editing of 2D geometrical entities, such as lines, circles, arcs.

• Some of the basic editing functions are listed below,

- Basic Entity Creation (lines/ circles/ arcs/ etc)

- create using exact coordinates

- two screen points for line ends, circle radius/centre/diameter/etc

- Line Trimming

- trim lines back to intersection

- extend lines to intersection

- trim line to perpendicular point

- cut a circle/arc on one side of an intersection

- Point Creation

- screen position

- exact numerical coordinate

- nearest tangent of line to an arc

- nearest end of a line

- midpoint of nearest line

- centre of nearest arc

- nearest grid point

- Arc Creation

- intersection of circle with another line

- Special Techniques

- offset of a line

- extend lines to intersection

- trim line to perpendicular point

- delete entities

- etc

• There are a number of ways (philosophies) for creating drawings using the basic elemental editing techniques. A few popular methods are listed below.

- Construction Lines - A set of construction lines are set up, then segments of the lines are selected for the actual drawing

- Trimming - The construction lines are all drawn, then the unwanted parts are trimmed off

- Navigation - A line figure is built up using successive line segments.

- Parametrization - Objects such as rectangles, circles, arcs, etc. are created using their dimensions, then positioned with traditional methods.

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1.4.2 Surfaces

• Most surface modelling packages rely on the elemental definition of lines, and points.

• There are a number of basic philosophies for creating surfaces,

- Swept profiles - a profile, and a path in space are used to sweep out a surface.

- Rotated Profiles - a Profile is created then swept about an arbitrary axis

- Extruded Profiles - a profile is created, then grown in one direction.

- Skins (Splines) - a direct creation of points, then the splines that connect them

- Polygon Approximation - polygons are defined which join up to define a surface

- Sections - sections are defined for different points along a path, which then allow generation of complex transition geometries.

• Once surfaces have been created, they may be operated on by boolean operations.

• This method is often used as a preliminary stage to CSG editing.

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1.4.3 CSG

• This is by far the simplest method

• Solid Primitives are progressively cut and joined to form new shapes.

• Primitives may come from,

- Traditional Sources - Blocks, Spheres, Wedges, etc.

- Surfaces - A Volume is assigned to a surface model

- Previous operations

- others ?

• CSG editing requires storage of the results of operations. This is because a part may be used many times to cut another part, for example a chamfered hole for a sunken screw.

• The fundamental CSG operations are,

Union - both parts joined as one

Intersection - Only where two parts overlap

Subtraction - only where parts do not overlap, One of the parts is typically discarded.