level of inspection effectiveness unless extreme measures are taken to rectify the tendencies to increase the miss and false alarm rates. “Anecdotal information and
3
research indicates it is not uncom- mon, when specific countermea- sures are not taken, to have a 10% to 20% loss in both the miss rate and the false alarm rate with each successive inspection by the same inspectors,” Schorn said. When companies bring in temporary or other, less well trained inspectors, the second inspection can be far less effective. As inspection effectiveness
decreases over subsequent inspec- tion, the cost penalties increase. Sample scenarios and cases seldom indicate a quality and cost advan- tage to inspections beyond two or, at most, three. Product damage is a significant
factor to consider when weighing the benefits of multiple inspections on overall shipped quality. Schorn has observed damage rates as high as 3% or more with machined aluminum castings. “Generally, people are in a hurry on repetitive sorts, and they tend to handle parts with less care. Ergonomics can be poorer, as well,” he said. Packaging reuse creates further opportunities for product damage. “Parts with delicate features, high surface tex- ture or cosmetic requirements are especially at risk,” he noted. Schorn’s model indicates the
best one can do with the quality of outgoing lots is to approximate the damage rate. This effectively limits inspections to one or two, before the remediation of the lot from continued sorting is smaller than the damage created by continued handling. “Te moral of the story,” accord-
ing to Schorn, “is that every time a decision to accept or reject prod- uct is made, it must be protected to assure its outcome. Tis care is
Fig. 1. The total cost of quality is shown after each inspection with four effectiveness conditions. This graphically represents the data in the chart in Figure 1. The cost case is 1 per Table 2.
January 2013 MODERN CASTING | 49
Results and Conclusions
Te study demonstrated it is unlikely that subse- quent inspections will be performed with an equal
especially necessary for subsequent inspections, as additional vari- ables come into play that must be addressed preventively.” Customers might consider the
mixed benefit of repetitively sort- ing supplied product. The use of such sorting as a punitive measure arising from a demand for error- free lots simply adds cost to an already expensive problem. Repeti- tive sorting holds no guarantee for quality improvement. “The usual glib explanation about the suprem- acy of quality over cost does not apply in reality,” Schorn wrote. “In short, one often gets neither in such situations.” He concluded that manage-
ment must account for the costs of reinspection and invest in quality processes and products that do not depend on sorting to meet outgo- ing quality targets. When both part damage and decreasing effec- tiveness are fully accounted for, quality is more effectively built in by process control, capability
HUMAN ERROR Operator error rates increase with:
• Inadequate supervision. • Inadequate training. • Inadequate or missing proce- dures/instructions.
• Poorly laid out or marked operator/part/equipment interface.
• Environmental factors (heat, humidity, etc.).
• Mental stress. • Fatigue. • Impairments (illness, drugs or alcohol, etc.).
improvement and other up-front actions than by inspection.
Tis article is a followup to “Determining the Worth of Multiple 100% Inspections,” MODERN CASTING, Nov. 2012. Te author will present the paper on which it is based at the 2013 AFS Metalcasting Congress.
Table 2. Example Cost Cases Used to Evaluate the Financial Impact of Inspection Case 1
Inspection cost
Internal reject cost External reject cost
$0.50 $2.00
$20.00
Case 2 $0.50 $0.50
$20.00
Case 3 $0.50 $2.00 $5.00
Case 4 $0.50 $2.00
$50.00
These same cost cases were considered in the previous article. See “Determining the Worth of Multiple 100% Inspections,” MODERN CASTING, Nov. 2012.
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