# Bright Hub Engineering

## Why 3D Cad Modeling Software is Parametric

written by: Suvo • edited by: Lamar Stonecypher • updated: 5/21/2011

Have you ever wondered why 3D CAD modeling software, especially Pro Engineer, is parametric in nature? This article discusses geometric and dimensional sketcher constraints in 3D applications.

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There are two types of features in Pro Engineer, and most of the other 3D CAD software. The first type, which consists of majority of them, are based on sketch and the second type does not require sketch to build the feature. Sketcher constraints, as the name suggests, is used only for the first category of constraints. Two broad categories of sketcher constraints are possible:

o Geometric sketcher constraints

o Dimensional sketcher constraints

Now before going in details of those two types of constraints, let me explain what is meant by constraints. Constraint is a kind of relation between the entities (which you are creating in the sketch) and the references. All the elements of a sketch are called sketch entities. Similarly for creating any sketch entities, you need to refer to some already existing entities, and these are called reference entities.

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### Geometric sketch constraints:

In Pro Engineer (and other 3D CAD software) in sketch mode, to locate any sketch entities you will have options of putting some geometric relations with other entities. For example, you want to draw a line perpendicular to an existing surface, you can use the perpendicular constraint instead of putting the value of angle to 900.

To use geometric sketch constraints in Pro Engineer, go to sketch → constraint or click on the constraint tool in sketch mode. The following nine different types of geometric constraints can be used in Pro Engineer:

1. Horizontal: Use this constraint to make any entity horizontal. You can make use of this constraint for line and two points.
2. Vertical: Similar to the horizontal constraints but make the entities vertical.
3. Perpendicular: Makes two entities perpendicular. Entities may be two lines, one line and one circle etc.
4. Tangent: Make two entities tangent to each other.
5. Mid-Point: This constraint place a point at the mid of a line.
6. Coincident: Make two entities coincident. It can be used for two lines, one line and one point or two points.
7. Symmetric: This constraint makes two entities symmetric about a center line. You need to create center line and constraint it before using the symmetric constraint.
8. Equal: Two similar entities could be made equal by this constraint. Two lines, two circles etc could be make equal by using this constraint.
9. Parallel: This constraint makes two lines parallel to each other.
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### Dimensional sketch constraints

You will normally use dimensional sketch constraints in the case where either you can’t use geometric sketch constraints- or you feel that you have to vary the distance in the later stages of your design.

1. To place a dimension in your sketch, either go to sketch → dimension → normal or use the dimension icon of the Pro Engineer sketch window.
2. To lock a particular dimension click on the dimension, then right click and click lock. Locked dimensions will not be changed while editing other entities of the sketch.
3. To make a reference dimension go to sketch → dimension → reference.
4. To make a dimension strong select the dimension/dimensions and press Ctrl+T.
5. To modify dimension double click on the dimension and enter the new value. If you want to modify a group of dimensions together then either follow edit → modify or click on the suitable icon on the Pro Engineer sketcher window.