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

Abstract

A hole punch unit has a punching section, a longitudinal registering mechanism, and a transverse registering mechanism. The registering mechanisms incline the punching section in accordance with a skew of a sheet. The punching section thus corrected in position punches the sheet. The punching section is corrected in position, at both the leading edge of the sheet and the trailing edge thereof, thus increasing the precision of positioning the punching section and shortening the time required to position the punching section.

What is claimed is: 
     
         1 . 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.   
     
     
         2 . The punch unit according to  claim 1 , wherein the control unit which skew of a longitudinal direction parallel to the sheet conveyance direction, thereby the punching-section is inclined. 
     
     
         3 . The punch unit according to  claim 1 , wherein a lack is provided on a side of the punching section, and the punching-section driving mechanism has an adjusting motor for adjusting motion when controlled by the control unit and an idler gear set in mesh with the lack and transmitting the rotation of the motor.