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: + - * /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.
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