Creating Intrusions (Negative Extrusions)

COMSOL has a very user-friendly interface for taking objects from 2D to 3D using the "extrusion" feature. But suppose one wants to design an object which contains screw holes or some other indentation. One method would be to use the usual "Boolean Difference" function. However, this method leaves one with many new undefined object faces, which may force one to redefine a bunch of new work planes -- needless to say, a potentially messy strategy. Instead, I've found that a Layered-Extrusion method to Intrusions is a much safer method, especially because it forces the user to systematically update the object with appropriate work planes. The result is an orderly, easy-to-modify object.

This layered method is best described through example:

1) First, create the planar projection in a work plane:

http://nodus.ligo.caltech.edu:8080/40m/100712_110254/Screen_shot_2010-07-12_at_10.51.22_AM.png

2) Extrude the first layer only in the regular fashion:

http://nodus.ligo.caltech.edu:8080/40m/100712_110413/Screen_shot_2010-07-12_at_10.51.28_AM.png

3) Add a new work plane which is offset in the z-direction to the deepest point of the intrusion.

http://nodus.ligo.caltech.edu:8080/40m/100712_110446/Screen_shot_2010-07-12_at_10.52.08_AM.png

4) Now, create the shape of the intrusion in this new work plane.

http://nodus.ligo.caltech.edu:8080/40m/100712_110518/Screen_shot_2010-07-12_at_10.53.53_AM.png

5) Use the Boolean "Difference" to let COMSOL know that, on this plane, the object has a hole:

6) Extrude once more from the second work plane to complete the intrusion.