Hammer mills with grid, series THG, are tertiary machines suitable for crushing materials, including not very hard limestone. They can produce already calibrated sand (models THG 1 and THG 2) or mixed material with a high percentage of sand (models THG 3 and THG 4). These machines are equipped with a flange type impeller mounted on grease lubricated bearings that are protected by a set of labyrinth flanges as well as free hammers in manganese steel. Internal armours made of manganese steel plates fixed using conical bolts complete the equipment. This type of mill can produce high outputs, a bigger reduction ratio compared to other types of mills, a high percentage of fines in the size distribution curve of the output material and with a controlled maximum size of the produced material. THG 3 and THG 4 models can be supplied with a hydraulic opening system.