A sewing machine installation for sewing shirt front box hems and having a first underlying belt conveyor for individually conveying shirt front material blanks longitudinally forwardly along a lower main support table through a shirt front edge folder and a second overlying belt conveyor for individually conveying separate box hem material blanks with a liner tape longitudinally forwardly along an upper auxiliary table section through a box hem edge folder and into superimposed preassembled association with the prefolded shirt front blanks; the overlying and underlying conveyors thereafter cooperating for conveying the preassembled shirt front box hems longitudinally forwardly along the main support table through sewing and shearing stations for sewing the preassembled shirt front box hems and shearing excess material from the leading and trailing edges thereof. In an alternative operational configuration of the sewing machine installation, the upper auxiliary table section is inoperative and an entry section of the overlying belt conveyor is lowered into cooperative engagement with the underlying belt conveyor for conveying shirt front blanks along the main support table through a box front hem folder for prefolding a single piece shirt box front hem.