SolidWorks 2011 Parts Bible - Matt Lombard [146]
FIGURE 10.3
Using the FeatureManager Filter to filter dimension names
Building the equation
When creating an equation in SolidWorks, it is often a good idea to write it out on paper first to make sure you have the concept correct. Examine the part shown in Figure 10.4, where the relevant dimensions have been named and displayed. The number of holes — called Instances here — is the driving variable. From that number, the spacing of the holes is calculated over the length of the part. There is also a gap on each end of the pattern of holes. This gap (measured between the center of the last hole and the end of the part) always needs to be half of the spacing between the holes. The sigma symbols to the left of the dimensions indicate that an equation is driving it. Dimensions driven by equations cannot be directly edited.
FIGURE 10.4
Variables for the hole pattern
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 10.5 shows the resulting set of equations.
FIGURE 10.5
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, as shown in Figure 10.6. 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 View⇒Dimension Names.
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.
To build the equation, first click the Equation button on the Tools toolbar to open the Equations dialog box. Then click the Add button to display the Add Equation dialog box. To add dimensions to the equation section, just select the dimension. You can use the keypad on the dialog box or on your keyboard to add operators and syntax. All standard rules of syntax apply for the order of operations, use of parentheses, and driving versus driven sides of the equation.
FIGURE 10.6
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