[mplanchard]

A SolidWorks 2013 you can define the spiral equation in terms of r, theta.

The biggest issue when you are new to entering equations is "Syntax". Make certain you have the right number of (). Order of operations is important. Marie

To add an equation in the Equations View:

Do one of the following:
Click Equations (Tools toolbar).
Click Tools > Equations.
Right-click the Equations folder in the FeatureManager design tree, and select Manage Equations.
Select the Equations View.
In the Equations section, click an empty cell in the Name column.
Click a dimension in the graphics area.

The SolidWorks software does the following:
Propagates the dimension name to an empty cell in the Name column and encloses it in quotation marks.
Moves the cursor to the Value/Equations column and inserts = (equal sign).
Displays a flyout menu with options for starting the equation.

After = (equal sign), add a term to the equation by doing one of the following:
Type a number or a conditional statement.
Select a Global Variable, Function or File Property from the flyout menu.
Select Measure ... from the flyout menu and use the Measure Tool to create the term.
A appears in the cell to indicate that the syntax is valid.

Type + (plus), - (minus) or another mathematical symbol.
Add another term to the equation.
When the equation is complete, click OK.