SolidWorks 2010 Bible

Chapter 9: Using Equations  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

In this case, a more sophisticated equation has not been implemented to account for the diameter

of the holes possibly interfering with one another when there are a large number of holes. In other

words, because there are two values that need to be calculated (the spacing and the gap), you need

to create two equations. Because the gap dimension is always half of the spacing, the spacing needs

to be calculated first, as follows:

Spacing = Length / ((Instances-1)+1)

The Instances –1 term stands for the number of spacings. If you have two holes, then there is only

one spacing. The +1 term stands for the two half-spacings for the two ends. The second equation is

simpler and looks like this:

Gap = Spacing / 2

The order of the equations is important. SolidWorks solves the equations in the order in which

they are listed in the Equations dialog box. Because the gap is dependent on the spacing, the

spacing must be calculated before the gap. If it is done the other way around, you can get into a

situation where it takes two rebuilds to finalize a set of equations, or even a situation where in

every rebuild all the numbers change. This is called a circular relation, and is a common error in

order or history dependent functions, not just in SolidWorks, but in many CAD applications.

Figure 9.5 shows the resulting set of equations.

FIGURE 9.5

Equations for the hole pattern

Before beginning to build the equation, you should first display the dimensions that you need to

use to create the equation. You can add dimensions to the equation by clicking them from the

graphics window. To do this, right-click the Annotations folder at the top of the FeatureManager,

and select Show Feature Dimensions. You should also select the Display Annotations option if it is

not already selected. When you have done this, all the dimensions that you need to create every

feature are displayed. Also, be sure to turn on the Show Dimension Names option by choosing

Tools ➪ Options ➪ General.

Tip
For models that have more than a few features, showing all the dimensions in the entire model may overload
the screen with information. In this case, you can double-click a feature from the FeatureManager to show all
the dimensions on that feature. n