An apparatus for forming a stack of interlocked laminations (disks) that can be used as a rotor in an electric motor. Each disk is severed from a metal sheet that is passed horizontally through a die apparatus. The severed disks are sequentially driven downwardly into a vertical die opening having an upper diameter chosen to grip the disk edges, as a stack is being formed. The lower end of the stack advances into an expandable rotatable collet. When the stack has passed below the upper end of the die opening, the collet is opened to drop the completed stack to a conveyor.A sensor device detects the lowest disk in the descending stack. A signal generated by the sensor device is used to determine the number of disks necessary to finish the stack by dividing the unfinished height of the stack by the thickness of the advancing metal sheet.