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Forging new generations of engineers
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Parametric Parametric ModelingModeling
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• Types of computer design parameters
• Review of geometric constraints
• Parametric constraints
• Creation of parametric equations that maintain geometric proportions
Presentation OverviewPresentation Overview
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3D CAD programs use parameters to define a model of a design solution.
A parameter is a property of a system whose value determines how the system will behave.
ParametersParameters
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• Geometric Constraints (review)
• Parametric Constraints
• Assembly Constraints (discussed later)
Types of ParametersTypes of Parameters
3D CAD programs typically have three types of user defined parameters:
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Non-numerical geometric relationships that the user assigns to sketched elements.
Examples:
Review of Geometric ConstraintsReview of Geometric Constraints
• Making two lines parallel
• Making two arcs concentric
• Making a line horizontal
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Perpendicular, Parallel, Tangent, Coincident, Concentric, Collinear
Horizontal, Vertical, Equal, Fix, Symmetric
Review of Geometric ConstraintsReview of Geometric Constraints
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• Are used to control the size and location of geometry.
• May take the form of simple numeric values such as 2 inches or 25 degrees.
• May take the form of abstract algebraic formulas such as (d2*d0)/d5.
Parametric ConstraintsParametric Constraints
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• Can be tied to spreadsheets that allow for more complex mathematical formulas.
Parametric ConstraintsParametric Constraints
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Algebraic equations that use variables can be substituted for individual numeric values.
The resulting dimensional value may change, but the formula will remain constant.
Parametric EquationsParametric Equations
Symbols: + - * /add subtract multiply divide
d7 = ((d2*d0)/d5)+2 in
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Scenario: A child’s proportions are similar to those of an adult. A chair could be dimensioned in such a way that a change in the seat height could scale all the other chair features uniformly.
Parametric EquationsParametric Equations
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Each dimension is given a designation, starting with d0.
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d1d0
All location and size dimensions are given designations. Geometric constraints, such as the perpendicular and parallel edges, do not have designations.
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d2
d3
Extrusion and taper angle values are also given designations.
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d1d0
Problem:
The Overall Plate Depth (d0) and the Overall Plate Width (d1) must maintain a constant ratio. This means, if the plate were scaled up or down, the overall dimensions would remain proportional to each other.
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5 in
If the Overall Plate Depth and Overall Plate Width must maintain a constant ratio, then the current dimensional values can be used to establish the ratio:
Parametric EquationsParametric Equations
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5 in
If the Overall Plate Depth and Overall Plate Width must maintain a constant ratio, then the current dimensional values can be used to establish the ratio:
5 : 3 or 5/3 or 1.666673 : 5 or 3/5 or .6
Parametric EquationsParametric Equations
Note: unitless values
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5 in
If dimension d0 is the only linear dimension that will have a numeric value, then it must be used to develop an equation that will maintain proportionality:
d1 = d0 in*(5/3) d1 = d0 in/(3/5)or
5 in = 3 in x 1.66667 5 in = 3 in .6
Parametric EquationsParametric Equations
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5 in
Both equations work, so either may be used in the CAD program as a parametric equation for dimension d1 to maintain proportionality.
d1 = d0 in*(5/3) d1 = d0 in/(3/5)or
5 in = 3 in x 1.66667 5 in = 3 in .6
Parametric EquationsParametric Equations
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d5
d4d6
d7
Each parametric equation must tie back directly (i.e., d0/2) or indirectly (i.e., d1*.8 = (d0*(5/3))*.8) to a dimension that has a true value. In this case, dimension d0 has a true value of 3 inches.