The design and experimental process of an automatic tracking system applied to local vacuum electron beam welding are dealt with in this paper. When the annular parts of an exactitude apparatus were welded

the centre of rotation of the electron gun and the centre of the annular weld are usually not superposed because of the machining error

workpiece’s setting error and so on. In this teaching process

a little bundle of electron beam is used to scan the weld groove

the amount of the secondary electrons reflected from the workpiece is different when the electron beam scans the both sides and the centre of the weld groove. The difference can indicate the position of the weld

then a computer will record the deviation between the electron beam spot and the centre of the weld groove. The computer will analyze the data and put the data into the storage software. During the welding process

the computer will modify the position of the electron gun based on the deviation to make the electron beam spot centered on the annular weld groove.