An error of this nature is sometimes not detected until we actually have the car welded together and are about to attach a part where there is not a nut for the bolt to fit into. To correct this problem, we simply drilled a hole through the electrode that holds the nut that is attached to the panel in the welding operation. We put a wire through the hole in the electrode, insulating it away from the electrode so as it passes through it will only make contact with the weld nut. Since the weld nut is metal, it conducts electricity and with the nut present, current will flow through, allowing the machine to complete its cycle. If a nut is not present, there will be no current flow.
Informative inspection uses data gained from inspection to control the process and prevent defects.
Traditional SPC is a type of informative inspection. Both successive checks and self-checks in ZQC are also a type of informative inspection. Often inspections are conducted at intermediate stages in the process. Shingo's concern was that the inspections may not occur soon enough after production to give the best information necessary to determine the cause of the quality problem so that it can be prevented in the future. By having each operation inspect the work of the prior operation, quality feedback can be given on a much more timely basis.