Metering unit for fine-grained powders including - a hopper for loading the powder, - a support provided with a vertical hole in correspondence with a bottom outlet of said hopper which is mounted on said support , - a volumetric drawer provided with a volumetric metering chamber , said volumetric drawer being mobile back and forth through the support between a filling position, in which said volumetric metering chamber is filled by the powder that descends by gravity from the hopper through said vertical hole in the support , and an emptying position, in which the volumetric metering chamber is emptied of the powder that descends by gravity through a hole formed in a plate on which the support is mounted, and - actuator means that provide the volumetric drawer with said back and forth motion, wherein it further includes - a final metering chamber , placed under said plate and communicating with the volumetric metering chamber through a second hole formed in the plate in correspondence with said filling position, - a metering tube that extends through said final metering chamber so as to protrude in front with its end in correspondence with said emptying position, said metering tube being provided with a slot for the entry of the powder formed in its portion contained in the final metering chamber , - a variable-speed motor that turns the metering tube , - a weighing tray placed in correspondence with the emptying position so as to receive the powder from the volumetric metering chamber and from the metering tube , - a scale that carries the weighing tray and constantly records the exact weight of the powder contained therein; - second actuator means which empty the weighing tray into a discharge pipe , and - a control unit operatively connected to said scale , to both the actuator means and to said variable-speed motor so as to control the operation of the actuator means and of the motor on the basis of the weight of the powder detected by the scale . - a hopper for loading the powder, - a support provided with a vertical hole in correspondence with a bottom outlet of said hopper which is mounted on said support , - a volumetric drawer provided with a volumetric metering chamber , said volumetric drawer being mobile back and forth through the support between a filling position, in which said volumetric metering chamber is filled by the powder that descends by gravity from the hopper through said vertical hole in the support , and an emptying position, in which the volumetric metering chamber is emptied of the powder that descends by gravity through a hole formed in a plate on which the support is mounted, and - actuator means that provide the volumetric drawer with said back and forth motion, wherein it further includes - a final metering chamber , placed under said plate and communicating with the volumetric metering chamber through a second hole formed in the plate in correspondence with said filling position, - a metering tube that extends through said final metering chamber so as to protrude in front with its end in correspondence with said emptying position, said metering tube being provided with a slot for the entry of the powder formed in its portion contained in the final metering chamber , - a variable-speed motor that turns the metering tube , - a weighing tray placed in correspondence with the emptying position so as to receive the powder from the volumetric metering chamber and from the metering tube , - a scale that carries the weighing tray and constantly records the exact weight of the powder contained therein; - second actuator means which empty the weighing tray into a discharge pipe , and - a control unit operatively connected to said scale , to both the actuator means and to said variable-speed motor so as to control the operation of the actuator means and of the motor on the basis of the weight of the powder detected by the scale . <EOS>