MLA Citation: Bloomfield, Louis A. "Question 1297"
How Everything Works 23 Oct 2017. 23 Oct 2017 <http://howeverythingworks.org/print1.php?QNum=1297>.
1297. A center punch is a device used in extricating people from cars. You put tape over the glass window you want to pop, place the center punch in the corner of that window, and simply press inward. This does something that causes the glass to crack in a spider web pattern. The glass sticks to the tape, you push in enough glass to get your hand through, and knock the rest outward. This technique works on any window except the front windshield, which is 3 layers(glass, plastic, glass). Can you explain it? — RSG, Boston, Massachusetts
A center punch is a common tool used to dent a surface prior to drilling. The drill bit follows the pointed dent and the hole ends up passing right through it. But in the situation you describe, the center punch is being used to damage the surface of a car window. When you push the handle of the center punch inward, you are compressing a spring and storing energy. A mechanism inside the center punch eventually releases that spring and allows it to push a small metal cylinder toward the tip of the punch. This cylinder strikes the tip of the punch and pushes it violently into the glass. The glass chips.

In normal glass, this chipping would be barely noticeable. But the side and rear windows of a car are made of tempered glass—glass that has been heat processed in such a way that its surfaces are under compression and its body is under tension. Tempering strengthens the glass by making it more resistant to tearing. But once an injury gets through the compressed surface of the tempered glass and enters the tense body, the glass rips itself apart. The spider web pattern of tearing you observe is a feature of the tempered glass, not the center punch. Any deep cut or chip in the tempered glass will cause this "dicing fracture" to occur.


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