The best guide to CNC Machining, mold making, and resin casting that I’ve found online is over at coredump.cx. It is thorough and detailed, and definitely worth studying.
CNC machining (computer control milling) is a “subtractive” method for processing raw material with a drill-like rotating cutter with computer controlled movements of the cutting head.
Michal Zalewski points out that if you can make a mold using CNC milling, the cost of making a small part out of high strength,
engineering grade plastic or tough rubber, is extremely cheap.
He gives practical advice on buying a CNC machine – important factors like mill rigidity to take into consideration.
The part on using resin casting to make the part is excellent too! Go read it!