Copyright Goodheart-Willcox Co., Inc. Chapter 6 Model Extrusions and Revolutions 139 Surface modeling variables can be set on the Create panel in the Surface tab of the ribbon. See Figure 6-11. When the Surface Associativity button is on, the SURFACEASSOCIATIVITY system variable is set to 1. When the NURBS Creation button is on, the SURFACEMODELINGMODE system variable is set to 1. Extruding Surfaces from Objects with Dimensional Constraints When the Mode option is set to create a surface, you can use the Expression option to create a parametric relationship between a dimensional constraint and the extruded surface. For example, the circle in Figure 6-12 has been given a dimensional constraint named dia1. To create an extruded surface cylinder with a height that is always one-half of the diameter, enter the Expression option. Then, at the Enter expression: prompt, enter: dia1/2 In this manner, using appropriate formulaic expressions, you can create an extruded surface that has a mathematical relationship to a 2D object that has a dimensional constraint. CAUTION CAUTION When you enter an expression while creating a solid, the result is calculated, but the dimensional constraint is removed. Therefore, the parametric aspect of using an expression is lost. Pick to create a NURBS surface Pick to associate the surface with the original object Figure 6-11. Surface modeling variables can be set on the Create panel in the Surface tab of the ribbon. B A Figure 6-12. A—The circle has been given a dimensional constraint named dia1. B—The circle is extruded using an expression to create a cylinder surface with a height that is one-half the distance of dia1. When the diameter is changed, the height changes to maintain the relationship.