A friction stir welding tool having one or more tapered flats for promoting flow of weld material toward the pin terminus and one or more counter-spiraling channels or threads which for promoting flow of the weld material toward the tool shoulder. The remaining amount detecting unit includes a first electrode member and a second electrode member. When an image abnormality formed on the recording sheet is detected, a controller temporarily suspends the print job. The affiliate and client workstations provide the delivery server with confirmation of delivery of each received envelope and its associated files. The hub is mounted movably in the distal end and selectively moves the sealing plug to make the chamber communicate with the delivery head.