Attribute Control Chart 不連續管制圖 國立屏東科技大學工業管理系教授 何正斌 博士

不連續管制圖分類 Defective Data Package does/does not leak Lamp does/does not light Go/no-go gauge data p and np charts 樣本數相同 -p or np chart 都可以 樣本數不同 - 只能用 p chart Defect Data Bubbles in a windshield Paint flaws on a casing Errors on an invoice Bad die on a wafer C chart( 缺點數管制圖 ) & u chart( 單位缺點數管制圖 ) 樣本數相同 -c or u chart 都可以 樣本數不同 - 只能用 u chart

p 管制圖 Subgroup size Subgroups are usually quite large (50 to 200 or more!) Ideally, each subgroup should have at least 5 non- conforming units Minimum: 90% of the subgroups must have at least one non-conforming unit Subgroup sizes need not be constant, but should be within ± 25% of the mean subgroup size The lower the number of non-conforming units, the larger the required subgroup size In general, attribute charts need much larger subgroup sizes than the equivalent variable charts

p 管制圖 LCL When the mean proportion defective is small, the LCL can be a negative number. In this case there is no LCL and p=0 is still within the control limits It is advantageous to pick a subgroup size that establishes a lower control limit. This way improvements can also be detected

p 管制圖 -example A plastic molding plant manufactures two-liter plastic bottles for the soft drink industry A common failure mode of the molding process is pinholes in the bottles that will cause long- term seepage. The bottles can be pressure tested for leakage. The test is destructive. A number of bottles from each lot of plastic are tested for leaks and the number of rejects are recorded Construct a p-chart of the defective bottles and evaluate whether the process is in control.

c chart- example Each month, 100 invoices are audited and the total number of mistakes is recorded. In a molding process, there is a problem with pinholes in plastic bottles. Each day, a number of bottles are examined and the number of pinholes are recorded.

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SPC-Charts For All Occasions Variable Data? Rational Subgroups? I & MR chart % Defective or Defects? Subgroup Size >8? Easy to Compute sigma? X-bar & R chartX-bar & S chart Constant Sample Size? p-chart np- or p-chart u-chart c- or u-chart Yes No Defects % Defective No

Demerit Control Chart When several less severe or minor defects can occur, we may need some system for classifying nonconformities or defects according to severity; or to weigh various types of defects in some reasonable manner.

Demerit Systems 1.Class A Defects - very serious 2.Class B Defects - serious 3.Class C Defects - Moderately serious 4.Class D Defects - Minor Let c iA, c iB, c iC, and c iD represent the number of units in each of the four classes.

Demerit Systems The following weights are fairly popular in practice: Class A-100, Class B - 50, Class C – 10, Class D – 1 d i = 100c iA + 50c iB + 10c iC + c iD d i - the number of demerits in an inspection unit

~THE END~ ~ To Be Continued ~