page 120
•There are other features of great value to a CAD system,
-Don’t forget the manuals, They can help get through the tough times. If not, you may spend a lot of money calling help lines.
DI:15.4 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.
DI:15.4.1 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
page 121
-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
Step 1: Construct a basic set of lines to lay out the shape to be drawn.
Step 2: Pick parts of the construction lines to darken.
- Trimming - The construction lines are all drawn, then the unwanted parts are trimmed off
page 122
Step 1: Construct a basic set of lines to lay out the shape to be drawn.
Step 2: Pick parts of the construction lines to delete.
- Navigation - A line figure is built up using successive line segments.
Step 1: Starting from one point, select the type of element, and build the shape in sequence.
- Parametrization - Objects such as rectangles, circles, arcs, etc. are created using their dimensions, then positioned with traditional methods.
page 123
DI:15.4.2 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.
Polygons/skins
Swept/Extruded
Rotated
sections
•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.
DI:15.4.3 CSG
•This is by far the simplest method
•Solid Primitives are progressively cut and joined to form new shapes.