parametric modeling. presentation overview types of computer design parameters review of geometric...

Parametric Modeling

Presentation Overview

• Types of computer design parameters

• Review of geometric constraints

• Parametric constraints

• Creation of parametric equations that maintain geometric proportions

Parameters

• 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

Types of Parameters

• 3D CAD programs typically have three types of user-defined parameters:– Geometric Constraints (review)– Parametric Constraints– Assembly Constraints (discussed later)

Review of Geometric Constraints

• Non-numerical geometric relationships that the user assigns to sketched elements.

• Examples– Making two lines parallel– Making two arcs concentric– Making a line horizontal

Review of Geometric Constraints

Perpendicular, Parallel, Tangent, Coincident, Concentric, Collinear

Horizontal, Vertical, Equal, Fix, Symmetric

Parametric 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 Constraints

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

Parametric Equations

• Algebraic equations that use variables can be substituted for individual numeric values

• The resulting dimensional value may change, but the formula will remain constant

d7 = ((d2*d0)/d5)+2 in.

Symbols: + - * /add subtract multiply divide

Parametric Equations

• Scenario: A child’s proportions are similar to those of an adult

• A chair could be dimensioned so that changing the seat height uniformly scales all other chair features

Each dimension is given a designation, starting with d0.

Parametric Equations

d1d0

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

Parametric Equations

• Extrusion and taper angle values are also given designations

Parametric Equations

Parametric Equations

• Problem– The Overall Plate Depth (d0) and the Overall

Plate Width (d1) must maintain a constant ratio

– If the plate were scaled up or down, the overall dimensions would remain proportional to each other.

Parametric Equations

• 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 in.

Parametric Equations

• 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

Note: unit-less values

5 in.

Parametric Equations

• 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

5 in.

d1 = d0 in.*(5/3) d1 = d0 in./(3/5)or

5 in. = 3 in. x 1.66667 5 in. = 3 in. .6

Parametric Equations

• Both equations work, so either may be used in the CAD program as a parametric equation for dimension d1 to maintain proportionality

5 in.

d1 = d0 in.*(5/3) d1 = d0 in./(3/5)or

5 in. = 3 in. x 1.66667 5 in. = 3 in. .6

Parametric Equations

• Each parametric equation must tie back directly (e.g., d0/2) or indirectly (e.g., 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 in.