But why the complex shapes and angles on the burrs? Thats easy too, once you know the physics. In particle reduction, the most you can reduce a particle size at any given step is about 1/5th the original size. The answer to getting something finer than 1/5th is to use a series of progressively finer burrs. Each step "cracks" the particle into smaller pieces. Jim shows this clearly in the photo he marks up showing three steps on the burrs. He gets the first step right: bean breaking, but the next two wrong: coarse grind, fine grind. Actually, they are: particle reduction step one, two, and three. Three steps seem to do the trick for coffee. Some materials only need one or two steps, some need five.
So, to wrap this up. the first step takes a coffee bean, lets say it is 0.5" long, and reduces it about 1/5th to 0.1", step two reduces it again by 1/5th to 0.02", and the final step reduces it by 1/5th again to about 0.004". Opening the burr set then allows larger particle sizes to pass through.