kg at weight arriving at the stop shoulder 32 with rectilinear movement at a speed of 75 m minute and without the stop shoulder being damaged starting from this reference value of kinetic energy the operating parameters are computed for the system for all of the forecast production cases equ1 wherein e 1 value of kinetic energy assumed as the limit safe value parameter which is a pre fixed and constant value m variable mass of the iron sheet to be processed which on the contrary is a variable value the speed at which the impact against the stop shoulder 32 takes place is hence variable as an inverse function of the mass and therefore of the dimensions of the iron sheet 21 with the value of the kinetic energy involved by the impact of the iron sheet 21 against the stop shoulder 32 being constant and pre fixed the stop shoulder 32 can be coated therefore with soft and tough materials commonly available from the market without either they or the sheets 21 being damaged when the impact takes place in case of small size sheets 21 the production speeds can be such that the distance between the sheets 21 does not allow the machine 1 to perform the slowing down cycle by means of the hereinabove disclosed procedure without relative slippage occurring between the surfaces of the incoming iron sheet 21 and of the rolls 3 and 4 whose peripheral.