Patent US 6,876,504

A lens apparatus for detecting inaccuracy of machining of a finished workpiece is disclosed. After finishing the workpiece on a machine table in a machining process, remove a tool from a shaft, mount the lens apparatus under the shaft, couple a display to the machine, machine a left side of the workpiece by moving the table to align the left side of the workpiece with a reference point of a lens for determining alignment of the workpiece, initialize data shown on the display if the workpiece has been aligned, adjust the table to align a right side of the workpiece with the reference point for determining the alignment of the workpiece, determine whether there is an inaccuracy of machining of the workpiece by watching updated data on the display, remove the lens apparatus, and replace the tool with a second tool for correcting the machining process if an inaccuracy occurs.