Punch unit, sheet post-processing apparatus having the same, and method of punching sheets

A punch unit comprising:

• a punching section configured to punch a sheet transported;
• sheet-skew detecting means which is located on an upstream side of the punching section, which is spaced apart in a line intersecting at right angles with a direction in which the sheet is transported, and which is configured to detect passages of the leading edge and trailing edge of the sheet;
• a first calculation unit which calculates a transverse-direction skew at the leading edge of the sheet, from skew information acquired by the sheet-skew detecting means;
• a second calculation unit which calculates a transverse-direction skew at the trailing edge of the sheet, from skew information acquired by the sheet-skew detecting means;
• a punching-section rotating mechanism which has a rotation center existing outside the maximum width of the sheet and which rotates the punching section back and forth in the direction in which the sheet is transported;
• a punching-section driving mechanism which moves the punching section in the sheet width direction;
• and
• a control unit which detects the lateral edges of the sheet, thereby performs resist compensation of a transverse direction of the sheet, and detects skew at the trailing edge of the sheet of a longitudinal direction parallel to the sheet conveyance direction, thereby the punching-section is inclined.