Parametric Modeling © 2012 Project Lead The Way, Inc.Introduction to Engineering Design

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:+-*/ addsubtractmultiplydivide

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

d1 d0 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

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 : 3or5/3or : 5or3/5or.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 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 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.

Parametric Equations in Inventor Dimension parameters are named in sequential order beginning with d0. The first dimension you place will be labeled d0, the next d1, and so on.

Parametric Equations in Inventor You may type equations into the dimension dialog box (instead of a number)

Parametric Equations in Inventor You may assign the parameter name to any dimension

Parametric Equations in Inventor You may change the display mode of dimensions by right clicking Choose “Name” as the dimension display in order to display the parameter names instead of the numeric value

Parametric Equations in Inventor The parameter table displays parameter names, units, equations, and numeric values of dimensions. Access to the parameter table is provided under the Manage tab.

Parametric Equations in Inventor You may revise parameter names and equations and add comments within the parameter table The dimensions will automatically update when you depress Done