Quality ControlLesson 8

State of ControlProcess in Control a state where Points plotted in the control chart remain within control limitsNo Higher degree of uniformity can be attained with the existing process, BUTOnly Change in the basic process through quality improvements ideas

Natural pattern of variationCentral lineLCLUCL2.5%13%34%

Chart1

1.51

2.5

1.5

3.2

0.8

1.57

0.4

1.49

0.7

1.53

1.44

3

1.48

2.8

1.48

1.52

1.42

1.47

1.57

1.46

1.54

0.3

1.55

1.44

0.6

1.58

1.47

2.7

1.53

2.8

1.54

1.42

1.5

1.46

2.5

1.55

0.8

1.53

2

Y-Values

Sheet1

X-ValuesY-Values

0.11.51

0.22.5

0.31.5

0.43.2

0.50.8

0.511.57

0.60.4

0.631.49

0.650.7

0.681.53

0.691.44

0.73

0.711.48

0.722.8

0.731.48

0.741.52

0.751.42

0.81.47

0.821.57

0.831.46

0.841.54

0.850.3

0.851.55

0.871.44

0.880.6

0.891.58

0.91.47

0.922.7

0.931.53

0.932.8

0.951.54

0.961.42

0.971.5

0.981.46

0.982.5

11.55

1.010.8

0.11.53

0.152

Advantage if process is in controlMore uniform product (less variation)Fewer samples are needed to judge quality reduce inspection costProcess capability/spread of process is easily attained from 6 decide:Product spec.Amount of scrap/reworkTight/loose spec.

Advantage if process is in controlPrediction with highest degree of assurancePermits the consumer to use the producers dataSatisfying performance of operator

Process out-of-controlCondition when a point (subgroup value) falls outside its control limits an assignable cause of variation is present

Process out-of-controlUCLLCLABCCBA

Analysis of Out-of-control ConditionChange or jump in levelX-bar chartIntentional/unintentional changes in the process settingNew or inexperienced operatorDifferent raw materialMinor failure of a machine partR chartInexperienced operatorSudden increase in gear playGreater variation in incoming material

Analysis of Out-of-control ConditionTrend or steady change in levelX-bar chartTool or die wearGradual deterioration of equipmentGradual change in temp/humidityViscosity breakdown in a chemical processBuild-up of chips in a work-holding deviceR chartAn improvement in worker skill (downward trend)A decrease in worker skill due to fatigue, boredom, inattention, etc (upward trend)A gradual improvement in the homogeneity of incoming material

Analysis of Out-of-control ConditionRecurring cyclesX-bar chartThe seasonal effect of incoming materialThe recurring effects of temp.& humidity (cold morning start-up)Any daily or weekly, chemical, mechanical or psychological eventR chart (not so common)Operator fatigue or rejuvenation resulting from breaksLubricant cycles

Analysis of Out-of-control ConditionTwo population (also called mixture)X-bar chartLarge differences in mat.qualityTwo or more machines on the same chartLarge differences in test method or equipmentR chartDifferent workers using the same chartMaterials from different suppliers

Analysis of Out-of-control ConditionMistakes Measuring equipment out of calibrationErrors in calculationsErrors in using test equipmentTaking samples from different populations

Nonrandom Patterns

Quality Control Lesson 9

Process capability & Tolerance6 < USL-LSL6 = USL-LSL6 > USL-LSL

Case 1 :6 < USL-LSL USL-LSL6UCLLCLa. Desirableb. Undesirable but no waste

Case 2 : 6 = USL-LSLUSL-LSL6UCLLCLa. Satisfactoryb. Out of Control & wasteOut of control

Case 3 : 6 > USL-LSLUSL-LSL6USLLSLa. In control & wasteb. Out of control and waste

SolutionDiscuss with engineer the possibility to increase the tolerancePerform 100% inspection to eliminate nonconforming productsChange process dispersion (e.g. reduce std dev. Of raw mat.,more exp. Operator, retraining, etc.)Shift the process average (condition in case 3.b)

Process CapabilityProcedure Calculate sample standard deviation for each subgroup (s)Calculate average sample std devCalculate the estimate of the population std dev

Process capability will equal 6o

Process CapabilityProcedure Calculate range for each subgroup (R)Calculate average rangeCalculate the estimate of the population std dev

Process capability will equal 6o

Capability IndexProcess capability & tolerance are combined to form capability index

Cp = capability indexUSL-LSL = tolerance6o = process capability

AnalysisIf Cp > 1.00 : case 1 desirableIf Cp = 1.00 : case 2If Cp < 1.00 : case 3 undesirable

Process PerformanceCapability index does not measure process performance in terms of the nominal or target value use Cpk

Z(min) yang terkecil dari Z(USL)=(USL-Xbar)/ atau Z(LSL)= (Xbar-LSL) /

Hubungan Cp dan CpkCp value does not change as the process center changeCp = Cpk when the process is centeredCpk is always equal to or less than CpCpk=1.00 product conform to specCpk < 1.00 product not conform to specCp < 1.00 process is not capableCpk = 0 avg equal to one of spec limitCpk negative avg is outside spec

Control ChartSuatu control chart untuk Xbar dan R dibuat terhadap data berat produk pewarna dalam satu batch proses. Setelah 25 subgroup dengan ukuran subgroup 4, didapatkan Xbar=50.08 kg, R=11.82 kg. Asumsikan proses dalam kondisi terkendali, hitung nilai tengah untuk Xbar dan R chart serta batas kendali untuk produksi berikutnya!

JawabJawab

DiketahuiDitanya: n=25 ; g= 4X = 52.08 kg; R= 11.82CL Xbar, CL R,UCLx & LCLx ; UCLR & LCLR

= 2.0832 + (0.729) (0.4728)=2.427= 2.0832 - (0.729) (0.4728)=1.473 =(0 ) ( 0.4728)= 0=(2.282) ( 0.4728)= 1.079

Scrap & ReworkDiameter suatu produk yang diinginkan adalah 12.50mm, dengan toleransi 0.05mm. Jika nilai tengah proses adalah 12.50 mm () dan dispersinya 0.02 mm (), Berapa persen produk yang harus discrap dan berapa persen rework?Berapa nilai tengah proses jika scrap dihilangkan? Berapa persen rework sekarang?

JawabUSL = + 0.05 = 12.55mmLSL = 0.05 = 12.45mmLSLUSLscrapreworkArea 1

Dari Tabel nilai Z Z=-2.50 area 1 = 0.0062 atau 0.62% scrapArea 3Area 2Scrap= 0reworkArea 1

Jumlah scrap = 0 area1=0; dari Tabel nilai Z, yang terdekat dg nilai area 1=0 adalah 0.00017; yaitu dengan nilai Z=-3.50Maka:= 12.52 mm

Persentase rework didapat dari luas area3

Dari Tabel nilai Z: luas area 3 = 0.9332Area 2 = area total area 3 = 1.0000 0.9332 = 0.0668 6.68%Jumlah rework 6.68% yang kurang dari total scrap dan rework (1.24%)

Process capabilitySuatu proses baru dimulai dan jumlah std dev sampel utk 20 subgroup berukuran 4 adalah 84. tentukan process capability ?Jawab

6 = (6) (4.56) = 27.4

Process Capability Diameter suatu produk yang diinginkan adalah 12.50 mm, dengan toleransi 0.05 mm. Setelah inspeksi pada 25 subgroup dengan ukuran subgroup 5, diperoleh nilai tengah proses adalah 12.50 mm dan dispersinya 0.02 mm, dan rerata range 0.1 mmHitung process capabilitySimpulkan kondisi proses

**