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Parametric Parametric ModelingModeling

• Types of computer design parameters

• Review of geometric constraints

• Parametric constraints

• Creation of parametric equations that maintain geometric proportions

Presentation OverviewPresentation Overview

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

• 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:

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

Perpendicular, Parallel, Tangent, Coincident, Concentric, Collinear

Horizontal, Vertical, Equal, Fix, Symmetric

Review of Geometric ConstraintsReview of Geometric Constraints

• 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

• Can be tied to spreadsheets that allow for more complex mathematical formulas.

Parametric ConstraintsParametric Constraints

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

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

Each dimension is given a designation, starting with d0.

d1d0

All location and size dimensions are given designations. Geometric constraints, such as the perpendicular and parallel edges, do not have designations.

d2

d3

Extrusion and taper angle values are also given designations.

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.

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

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

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

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

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.