## ENG 004 Lecture 7, Oct 18, 2012

### Announcements

• Homework #3 is due Tuesday
• Finish reading Chapter 3
• No office hours today

### Topics

Coordinates

Primitives

Constraints/Conditions

## More On Coordinate Systems

### Absolute Coordinates

Coordinate values that always refer back to the origin.

### Relative Coordinates

Coordinate values that refer back to a previously defined point or feature.

## More On Coordinate Systems

### World Coordinates (WCS)

"Stationary" or "Fixed", never changes position or orientation relative to the model.

### Local Coordinates (LCS)

Coordinate system defined by the designer or CAD system. May be located and oriented anywhere and in any direction within the World Coordinate System

## Geometric Elements: Primitives

Primitives are the building blocks used to create drawings and CAD models

### Points/Nodes

No size, shape, width, height, or depth

Location only

### Locus

All possible positions of a point

### Lines

Length and direction, no thickness

### Curves

Generated by point moving in constantly changing direction

## Curves from intersections

Curves can be generated by intersecting surfaces

## Interpolated Curves

Various order polynomials fit through every point

Splined: different polynomials used for different sections

## Constraints

Constraints define the geometry through geometrical relationships

Implicit: software assumes constraints exist in a sketch

Explicit: user must define the constraints

### Level of constraint

• Fully constrained: just enough constraints to define all of the geometry
• Underconstrained: not enough constraints to fully define the geometry
• Overconstrained: too many constraints that are conflicting

## Conditions

Conditions are similar to constraints but they are not enforced. Conditions are simply the geometric (or kinematic) state of the feature.

Constraints remain enforced until the designer releases them. Conditions are simply the state of the feature at a given moment or location.

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