The chuck is held in place by a friction fit in the spindle. The interference fit only has to overcome gravity forces on the chuck and when the drill is used to drill in a material it pushes itself in the spindle increasing the friction fit.
The chuck is used to hold the drill fixated in place.
The chuck has 3 jaws wich can open and close, the opening, at it's maximum, is big enough to hold a drill or bit with a 13mm diameter.
It works kinda like a rack and pinion because the chuck translates a rotation into an translation. Using a speciaal tool you can let 2 parts of the chuck move (turn in opposite direction). This wil rotate a disc inside the chuck which is threaded. The rotation of the disk will move the jaws (which are also threaded) in the chuck.
The material of the jaws has to be very hard, harder than the material of the drill, especially with a "cheap" drill for home use. Because the cost of a new drill is way less than the cost of a new pair of jaws or a new chuck. That is why they make the jaws out of steel which they case harden.
Wikipedia about case hardening;
http://en.wikipedia.org/wiki/Case_hardening
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