A grinding machine (20) for use in grinding materials, the grinding machine (20) comprising: a grinder assembly (22) for receiving material therein; at least one removable hopper (24) attachable proximate to the grinding assembly (22) for dispensing material to the grinder assembly (22); a passage (22) defined in the hopper being cooperatively placed in communication with the grinding assembly (22) for dispensing material from the hopper (24) to the grinder assembly (22); a displaceable shutter (70) being positionnable in an open position when the hopper (24) is placed on the grinder assembly (22) and being positionnable in a closed position when the hopper (24) is removed from the grinder assembly (22); and a slide gate mechanism (42) including a slide gate (42) and a mover linked to the slide gate for controllably opening and closing passage of material from the hopper (24) to the grinder assembly (22) for grinfind material therein.