In a method of controlling a process in which the particle size of a material is reduced by passing the material through a nip between rollers at least one operational figure, such as the motor current, of at least one motor associated with at least one roller is continuously measured and the nip is adjusted by controlling the operational figure so as to match a target operational figure. A machine for reducing the particle size of a material by passing the material through a nip between rollers has a device for measuring at least one operational figure, such as the motor current, of at least one motor associated with at least one roller and a device for adjusting the nip on the basis of the measured operational figure matching a target operational figure. A method of calibrating a machine for reducing the particle size of a material by passing the material through a nip between the rollers has the step of measuring at least one operational figure, such as the motor current, associated with a desired particle size and/or mass consistency and the step of setting the operational figure associated with a desired particle size and/or mass consistency as a target operational figure.