An automatic welding machine for lap welding the edges of plastic webs, comprising a movable supporting frame, a heating device for partially melting the plastic webs, at least two opposing, counter-rotating pressing rollers and/or advancing rollers, at least one of which being driven, and at least one driving device comprising a drive motor and at least one gearbox for driving the pressing and/or advancing rollers, wherein the pressing rollers and/or advancing rollers are driven by the driving device by way of at least one flexible shaft. Furthermore, the gearbox can be implemented as a worm gear mechanism comprising a worm that is seated directly on a motor shaft of the drive motor and two counter-rotating worm gears driven by the worm.