This paper deals with the conception of a rule based experimental system for the robotization of metal cutting using a 5 kW CO2continuous laser. The image of the melted metal pulsed out of the kerf during the cut has been processed on the fly by a special purpose vision system in order to estimate the cut quality. The reasons for the design of a rule based decision system are analysed. Then, the methods used for the construction of the rules and the global architecture of the system are presented. Finally, an example of the system at work, representing its on the fly behaviour, will be provided.