surface 16d may be easily pinched together along upper edge 16a as they are fed into sewing machine 18 referring first to fig1 mattress 16 is supported on upstream conveyor 12 and downstream conveyor 14 in preparation for a sewing and taping operation to be performed on upper peripheral edge 16a thereof after mattress 16 has been positioned approximately as shown in fig1 i e with sewing machine 18 disposed about midway along side panel 16d the operator starts the sewing operation specifically as shown in fig2 the appropriate elevating unit 120 122 or 124 is activated to create slack 17 fig1 in the top panel 16c and the sewing machine 18 is properly engaged with upper peripheral edge 16a in the example shown elevating unit 124 is activated and raised at about the same time guide arm 28 may be moved to the position shown in phantom in fig2 referring again to fig1 the operator starts conveyors 12 14 belts 26 and sewing machine 18 while simultaneously pinching together top panel 16c and side panel 16d at peripheral edge 16a mattress 16 moves from left to right as viewed in fig1 and at each corner thereof side rail 24 is automatically activated to turn the mattress 16 during the corner sewing operation when the entire upper peripheral edge 16a has been sewn and mattress 16 has therefore returned to the approximate position shown in fig1 mattress 16 is ready to be flipped over such that the same sewing and taping operation may be performed on the opposite peripheral edge to prepare for the flipping operation sewing machine 18 and guide arm 28 are moved out.