In high-volume manufacturing, are zero defects achievable? For every dimension on every part in a complex assembly, probably not. But for key customer characteristics, a qualified yes.
A qualified yes because zero defects require 100% inspection but not all types of inspection can reduce and ultimately eliminate defects. Just working harder will not achieve zero defects…you also have to be very clever about it.
For a statistically stable process with a high capability, the chance for a defect caused by common cause variation has been practically eliminated to less than one chance in the millions. This is a fine achievement. However, no amount of hard work can repeal the second law of thermodynamics. Entropy, or disorder, is always increasing…this is why leaves blow into your garage, not into lawn and garden bags. Defects can and will be produced by even the best processes.
Statistical sampling and control charts cannot guarantee that all defects will be detected. The second law ensures all stable processes will eventually be visited by a special cause and will produce defects. If the special cause is continuous, defective products produced since the last good sample will have to be found and repaired. If a special cause is intermittent, defects may or may not be found in the sampling plan.
Judgment inspection discovers defects and separates the good from the bad. Defects are contained but neither reduced or prevented. A quality system relying on judgment inspection tolerates in-system defects and their effects on downstream processes. Forever. Judgment inspection deserves its bad reputation as a low value added activity.
Informative inspection investigates the cause of defects and feeds the information back to the source. Defects are reduced but the quality system is still tolerant, to a lesser degree, of in-system defects. The effectiveness of informative inspection is proportional to the immediacy of corrective actions: the shorter the feedback loop the better.
Source inspection checks for factors that cause defects. Immediate action corrects problems before a defect can occur. Defects are prevented by controlling their causal factors Because special causes can never be eliminated, only through 100% source inspection and immediate corrective action can zero defects be achieved.
Process Capability = the ability of a process that is in statistical control to consistently meet customer requirements