TESTING 1-2-3
Determining the Worth of Multiple 100% Inspections
A metalcaster developed a cost/benefit model of single and multiple whole-lot inspections for management decision making. A MODERN CASTING STAFF REPORT
tion to confirm the quality of parts prior to shipment, many have noted that visual inspection is not completely effective and often is far from ideal. Tis presents a significant challenge for metalcasters to overcome inspec- tion errors, as managers must optimize cost without sacrificing quality. Metalcasting facilities and their
customers often resort to reinspect- ing accepted product, hoping to confirm good parts and uncover any missed nonconforming parts. It may be asked whether these additional inspections are equally cost effective to the initial examination. Theodore J. Schorn, vice presi-
dent, Enkei America Inc., set out to model the quality of accepted castings after performing a sequence of 100% inspections with a known error. He then linked the model to four cost cases with specific applicability to the metalcasting industry. He evaluated scenarios using the model and established common-sense guidelines for mak- ing management decisions regarding multiple whole-lot inspections of casting shipments.
Question What is the value, in terms of qual-
ity and cost, of improving incoming product quality and the effectiveness of a visual inspection operation?
W 1
hile the casting industry and manufacturers in general rely on attribute visual inspec-
Background
Several researchers have studied the impact of inspec- tion error on effectiveness in two categories: the impact of
inspection error on attribute sampling programs and the problem of initial and subsequent 100% inspections. A 1987 study considered the effective- ness of consecutive 100% inspections where multiple characteristics were evaluated simultaneously and each characteristic had its own inspection error parameters. Researchers inves- tigated the impact on the average outgoing quality level but did not consider cost optimization. Applying this work to metalcasting is difficult. It complicates the funda- mental attribute nature of the visual inspection of castings and divides it into several different characteristics. In addition, the analysis is not greatly useful without knowledge of not only the magnitude of the inspection error, but the specific inspection error associ- ated with each critical characteristic to
Table 1. Example Results
Actual Good Actual Bad Total
Inspector Judges Good 855 15
870
Table 2. Description of Cost Cases Case 1
Inspection Cost
Internal Reject Cost External Reject Cost
$0.50 $2.00
$20.00
Case 2 $0.50 $0.50
$20.00
Inspector Judges Bad 45 85
130
Case 3 $0.50 $2.00 $5.00
Total 900 100
1,000
Case 4 $0.50 $2.00
$50.00 November 2012 MODERN CASTING | 45
be checked. A 1995 study applied this work in a case study and demonstrated the necessity of improving product quality rather than relying on a long series of successive 100% inspections. Yet, this point does not provide much help in direct application of the work to metalcasting. In 2003, researchers considered cost
and multiple 100% inspection systems with inspection error. Tey simplified the application to a single attribute but considered only one level of inspection error, one percentage of defective prod- ucts in the inspection and only one cost scenario. Tis cost scenario, developed for the biomedical industry, applied an exceptionally high average cost of send- ing a bad part to the customer. According to Schorn, what is
needed for the metalcasting plant or quality manager had not yet appeared: a straightforward probability model for quality under inspection error, which combined a realistic variation in incoming defective parts and more normative metalcasting cost scenarios.
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