Complex sentence: In semi-automatic welding sets the welding head is moved manually along the joint where as automatic welding a separate drive moves either the welding head over the stationary job or the job moves/rotates under the stationary welding head.
Split into simpler sentences: In semi-automatic welding sets the welding head is moved manually along the joint. In automatic welding a separate drive moves either the welding head over the stationary job or the job moves/rotates under the stationary welding head.