Device for reducing coffee beans to powder or coffee grinder comprising: at least one primary tank or hopper for containing coffee beans, at least one grinding chamber, having a substantially truncated-cylindrical shape, fed by the coffee coming from said hopper by means of appropriate feed openings, said grinding chamber equipped with grinding surfaces integral with the main body or counter-grinders and housing one or more motorised rotating bodies or rotating grinders, which rotating grinders rotate around an axis substantially coaxial to the cylindrical axis of the said grinding chamber and cooperate with the said chamber in the process of reducing coffee beans or grinding and consequent production of powdered coffee, one or more unloading ducts to unload the powdered coffee from the grinding chamber, which are connected to the latter by means of a connecting element with an opening near to an outlet mouth of said chamber, said ducts intended to channelling said powder coming out of said chamber, a ground coffee collecting zone such as for example a temporary containment tank or a container for immediate infusion with pressurised water, said device further comprising: - means for the manual or automated removal of coffee powder residues and/or accumulations thickened inside said grinding chamber and/or on the surfaces of the members involved in the grinding process by means of gas or gas mixtures preferably containing carbon dioxide introduced by means of at least one duct leading into the grinding chamber with at least one opening preferably in the form of ending part of a nozzle, for the supply of said gas or gas mixtures inside said grinding chamber; said device being characterized in that said gas is preferably directed in the form of a flow substantially tangential to the inner perimeter surface of said chamber and preferably near the zone where said grinders and counter-grinders cooperate and further characterised by comprising at least one control member controlling the flow of said coffee powder to prevent the formation of lumps in the coffee ground by the device, having at least one opening in said connecting element for the introduction of gas for the removal of coffee powder inside the connecting element and inside said unloading ducts and having one opening near said outlet mouth for the introduction of gas near said outlet mouth. - means for the manual or automated removal of coffee powder residues and/or accumulations thickened inside said grinding chamber and/or on the surfaces of the members involved in the grinding process by means of gas or gas mixtures preferably containing carbon dioxide introduced by means of at least one duct leading into the grinding chamber with at least one opening preferably in the form of ending part of a nozzle, for the supply of said gas or gas mixtures inside said grinding chamber; said device being characterized in that said gas is preferably directed in the form of a flow substantially tangential to the inner perimeter surface of said chamber and preferably near the zone where said grinders and counter-grinders cooperate and further characterised by comprising at least one control member controlling the flow of said coffee powder to prevent the formation of lumps in the coffee ground by the device, having at least one opening in said connecting element for the introduction of gas for the removal of coffee powder inside the connecting element and inside said unloading ducts and having one opening near said outlet mouth for the introduction of gas near said outlet mouth.