page 134
Analysis,
Evaluation, Etc.
Design |
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Display |
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A Geometric |
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(Data Entry & |
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Representation |
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(Data Interpretation |
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(Data Storage) |
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and Rendering) |
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Manipulation) |
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User
•The Geometric Representation determines what a system can do.
•There are a number of methods of representing geometry, 1D, 2D, 2.5D, 3D 2D - All in one dimension, such as a side view of a part
2.5D - Points have an x, y coordinate, and height. This is a special case which can describe raised surfaces like buildings.
3D - Full spatial dimensions
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2D |
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2.5D |
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3D |
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• 2D models are quickly becoming obsolete, 2.5D models were an early compromise, but 3D models are quickly becoming the standard.
DQ:16.2 GEOMETRIC MODELS
• There are many ways to model a part, the major categories are,
page 135
-Elemental (using lines and points like drafting)
-Surfaces (such as polygons used in ES 206)
-Solids - Swept
-Solids - BRep
-Solids - CSG
-Features
•If describing a block with a hole in it, each of the methods above will result in different descriptions
Elemental: |
14 Lines |
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2 Circles |
Surfaces: |
4 Square polygons for sides |
Many rectangular polygons for inside hole 2 Odd shaped polygons for top and bottom
Solids Swept: |
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Base block |
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Circular profile |
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Straight path to sweep the cutting circle |
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through the block |
page 136
Solids BRep: |
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Faces |
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Vertices |
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14 Edges |
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Solids CSG: |
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Block primitive |
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Cylinder primitive |
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Equation: Block - Cylinder |
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Features: |
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Block from stock |
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Bored hole |
•Which is best???? all of them in the right situations.
•Each method has its particular advantages, and disadvantages.
•The best software and hardware supports a combination of all methods.
•It is assumed that other information is used to describe the geometries above, like,
-Position
-Orientation
-Dimensions
•The geometries can also be used to associate other information,
-Materials
-Tolerances
-Finishes
-Inspection information
BLOCK
HOLE
page 137
HOLE = {
radius = .5 inches height = 3 inches x_position = 1.5 inches y_position = 1.5 inches x_rotation = 90 degrees
radius_tolerance = 0.001 inches
}
DQ:16.2.1 Elemental Depiction:
•Depicted with the simplest of details (lines, points arcs, etc.)
•Advantages,
-very easy to store and alter
-well suited to line based problems
-does not require a powerful computer
-easy to perform traditional drafting
•Disadvantages,
-not capable of carrying complex information
-ambiguous
-complex items require long time to model
-requires repetition
-very hard to connect to programs for FEM, etc.
•Typically used in older CAD systems like AUTOCAD, CADKEY, etc.
•A classic demonstration of the arbitrary nature is shown below,