Steve Beeler

You have a goal…I have a way to get you there.

Zero Defects

Source Inspection for Zero Defects

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As the highest form of error proofing, source inspection for zero defects is such a powerful concept that it merits a close look.

Source inspection is 100% inspection, not statistical sampling, for causal factors that lead to defects. Defects are prevented through immediate action to remove causal factors. While this may sound complicated, it is not. In fact, examples of source inspection are everywhere around us.

source inspection for zero defects

The logic behind source inspection is pretty simple. Before completing an action of some kind, check for causal factors of defects. If causal factors are present, then the action is stopped before completion, and before the defect occurs.  With the causal factor(s) removed, the action can be restarted and completed without creating a defect.

Source Inspection

Everyday examples are all around us:

Automobiles. A little fender bender in the parking lot is a annoying defect. All vehicles with automatic transmissions have a safety interlock that requires the driver to have their foot on the brake pedal before shifting out of park.

On-line forms. In this case, the defect is missing information on an airline reservation, a catalog purchase, or whatever. Possible causal factors: error of omission or typographical error. Source inspection: if a required data entry field is empty, incomplete, or in the wrong format, the reservation or ordering process will not advance to the next screen until the field is filled correctly.

Thermocouples. A residential natural gas explosion would be a very bad defect. Possible causal factor: no pilot light. Gas water heaters, furnaces, fireplaces, and so on have thermocouples. If the pilot light is on, the thermocouple is hot and gas is allowed to flow. If the pilot light goes out, the thermocouple cools down and shuts off the gas flow.

With the explosion of sensors and connectivity in Manufacturing 4.o, there will be more and more opportunities to apply source inspection for zero defects.

Will source inspection make statistical sampling and control charts obsolete?  Absolutely not.  Use statistical process control to reduce causal factor variability to minimize interruptions and increase productivity.

Zero Defects

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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.

zero defects

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.

Terminology

Process Capability = the ability of a process that is in statistical control to consistently meet customer requirements