Types of size reduction

There are four ways to reduce a material: impact, attrition, shear, and compression.
Impact - a sudden force occurring when one object collides with another

Attrition - frictional force applied to the material due to the opposing motion of two
hard surfaces

Shear - a force applied when the material is compressed between the edges of two hard
surfaces moving tangentially

Compression - force applied by the faces of hard surfaces moving towards each other

Each of the following types of mill use a combination these types of size reduction.

Hammer mill

• Rotating drum with fixed or swinging blades or knives which use shear, impact, attrition, and compression of materials to reduce size
• Particle size determined by screens

Schematic of hammer mill