Burr grinders have two cylinders that grind coffee beans to a consistent size. The size of the grinding is controlled by the distance between the cylinders. The flat burr has two identical serrated cylinders, parallel to each other, for crushing beans. Flat burrs are used in various grinding machines. The conical burr grinder has two ribbed conical burrs that are used to crush coffee beans. These are usually found on low-speed gear reduction grinders. For both types of burrs, a cylinder usually remains stationary during grinding. The other cylinder is turned by a motor. The beans are crushed to a uniform size by burrs. The flexibility and consistent grinding quality from both types of burr grinders make this type of grinder highly desirable.
The grinder works much like a propeller to chop coffee beans. The longer the grinder works, the finer the powder will be. Unfortunately, blade grinders do not usually grind consistently. Longer grinding times for fine coffee can cause heat to build up. They also tend to give electrostatic charges, making them very messy.