Sheet processing apparatus and sheet processing method

A sheet processing apparatus for executing a hole cutting process on a sheet of paper carrying an image formed thereon, the apparatus comprising:

• a sheet conveying section including a first conveyer roller pair, a second conveyer roller pair, and a third conveyer roller pair arranged sequentially from the upstream side to the downstream side on a sheet conveyance route and adapted to convey a sheet of paper in a first direction directed from the upstream side to the downstream side, the second and third conveyer roller pairs are adapted to pinch the sheet of paper between the rollers thereof;
• a punching unit arranged between the first conveyer roller pair and the second conveyer roller pair and adapted to execute a hole cutting process on the sheet of paper;
• a skew correcting section adapted to stop rotary motion of the third conveyer roller pair before the leading end of the sheet of paper being conveyed on the conveyance route gets to the third conveyer roller pair, and operate the second conveyer roller pair as a slipping roller so as to make the leading end of the sheet of paper being conveyed in the first direction abut, without constraining by, the third conveyer roller pair in order to correct the skew of the sheet of paper;
• and
• a control section to change the position of the third conveyer roller pair according to the size of the sheet of paper, and the hole cutting position for the sheet of paper being defined by the gap between the punching unit and the third conveyer roller pair so as to execute a hole cutting on the sheet of paper being corrected for the skew at a position near the tail end thereof, and control the first and second conveyer roller pairs being driven to rotate after executing a hole cutting so as to deliver the sheet of paper.