severing apparatus 6 to a stacking conveyor 12 39 by rotation of wicketer 3 39 through approximately 180 u00b0 and in a clockwise direction as viewed in fig1 positioned immediately upstream of the wicketer frame 3 is severing and welding station 6 which includes an intermittently driven pair of feed rollers 7 7 39 for advancing a tubular or semitubular plastic web 10 in intermittent steps corresponding to the width or length of a bag the welding apparatus includes an upper welding bar 8 which is adapted to be lifted and lowered relative to a lower welding roller 9 welding bar 8 is provided with a knife edge not shown for cutting the bags from the web and lower welding roller 9 serves as a backup surface in cooperation with welding bar 8 to permit a transverse weld to be made in web 10 feed rollers 7 7 39 are preceded by a pair of transversely spaced web position sensors 11 which are positioned to scan respective side edges of the web the sensors are preceded by a punching means not shown which punches spaced pairs of locating holes along an edge portion of the web at a predetermined spacing inwardly of the edge each of the sensors 11 can be fixed in position transversely relative to web 10 and can be an optical sensor including a plurality of transversely positioned light sources and photocells or the like positioned in opposite sides of the web for sensing the direction and the extent of transverse movement of web 10 relative to an initial position other web sensing means can also be used as will.