Grinding is the last stage of the comminution process. The particles are reduced from a maximum upper feed size range of approximately 9,000 to 10,000 microns (3/8 inch), to some upper limiting product size ranging between 35 mesh and 200 mesh (420 microns and 74 microns). Grinding machines most frequently used are tumbling mills (also called grinding mills). A tumbling mill reduces particle size by applying impact and attrition stresses to the materials to be ground. It is designed to strike the particles with sharp blows of short duration and to produce a rubbing action under as high a unit pressure as possible. Accordingly, grinding mill consists of a horizontal rotating steel shell supported by end bearings on which hallow trunnions revolve. Loose crushing bodies, known as grinding medium, are placed inside the shell. Either steel balls/rods or pebbles are used as grinding medium. They are free to move inside the rotating shell making the particles to break by repetitive blows and by rolling and sliding one over the other. Attrition or shearing forces which result from the application of forces by rolling and sliding bodies tend to produce more fine particles than impact forces applied on particles by repetitive blows. The interior of tumbling mill is lined by replaceable liners primarily to protect the mill body for wear and damage.