The assembly of cavity string in the clean room is a tedious work that has noisy and painful steps such as cleaning the taped holes of a part. CEA together with the company INGELIANCE has developed a cobot: a collaborative robot operated by an technician one time and repeating the action without the operator. The cobot can work anytime without any operators: therefore it is working at night reducing the assembly duration by some hours. The cobot consists of a FANUC CRX10 a 6-axis arm on an Arvis cart. At CEA, the cobot is used to blow the flange holes of the cavities and bellows. This allows to reduce the noisy steps that the technicians are exposed to. The process is also more reproducible since the cobot does always the same steps. The cobot is used on ESS cavity string to clean the coupler and cavity flanges. Our activities, results and technical choices for next development will be presented in this poster.