Examine the Parameters.
Display the parameter definition table to examine the parameters on the part:
1. Click Parametric and then, in the Show group, click 
Current.
Current.The Current Constraints dialog box opens.
2. Click Advanced.
Assuming that the top part is active, there should be five parameters defined:
Bwid | Gives the width of the bars between the air intakes. Set to 20. |
Inrad | Gives the inner radius of each air intake. Set to 25. |
Outrad | Gives the outer radius of each air intake. Set to 100. |
Pt1 | A reference point. Pt1 is a geometry parameter, so its value will always correspond to the top corner of an air intake, no matter how this part is resized. |
Pt2 | A reference point. The value of Pt2 will always correspond to the bottom corner of Pt1's air intake. |
To prove that the parameters work as we say they do, try them:
1. Change the value for "Out_rad" to 120. Click the "Outrad" entry in the table and type 120.
2. Click Parametric and then, in the Solve group, click 
Preview.
Preview.Notice how the hole configuration changes.
3. Restore the parameters to their original values when you've finished experimenting.
Take a look at the parameters on the exhaust ports now.
4. Make "slot_grid" the current part.
5. Preview a few variations of this part by inserting new values for the "Newrad" parameter and solve using Preview in the Solve group on the Parametric tab.
Preview in the Solve group on the Parametric tab.6. Set "Newrad" to 5 and solve by using 
No Keep in the Solve group on the Parametric tab.
No Keep in the Solve group on the Parametric tab.7. Make the top part current.
The configuration of the parameters should give you an idea of the strategy we'll use to accomplish our task. We can easily vary the size and configuration of the four air intakes in the top part by changing the parameter values for "Bwid," "Inrad," and "Outrad." On the "slot_grid" part, we can vary the width of all three exhaust ports by entering a new value for the "Newrad" parameter. All that needs to be done now is to translate the area defined by "Bwid", "Inrad", and "Outrad" to a radius that can be fed to "Newrad". This is exactly what our macro, Do_it, does.