Provided is a mobile machining system for a turning machining of inner surfaces of hollow-cylindrical components, in particular for machining valve seats. The machining system includes a second drive train and the third drive train in each case which preferably have a gear mechanism, in particular a planetary gear mechanism, which is arranged between the respective motor and the associated gearwheel which projects outward from the rotational part. This leads to a simple, inexpensive and space-saving construction.