Figure 8.1. Engineering drawing of the “steer support”, a critical component of the Xootr scooter. The “height” of the steer support is specified by the dimensions (shown in the lower center portion of the drawing) as falling between 79.900 and 80.000 mm.

Figure 8.2. Steer support within Xootr scooter assembly. The height of the steer support must closely match the opening in the lower handle.

Figure 8.3: Examples for variation types

Time periods

ProcessParameter

Upper Control Limit (UCL)

Lower Control Limit (LCL)

Center Line

Figure 8.4: A generic control chart

Figure 8.5: X-bar chart and R-chart

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

R

79.9

79.91

79.92

79.93

79.94

79.95

79.96

79.97

79.98

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

X-b

ar

Figure 8.6: Control charts for the An-ser case

0

2

4

6

8

10

12

1 3 5 7 9 11 13 15 17 19 21 23 25 27

0

5

10

15

20

25

30

1 3 5 7 9 11 13 15 17 19 21 23 25 27

X-B

ar

R

Figure 8.7: Comparison of three sigma and six sigma process capability

3

Upper Specification Limit (USL)

LowerSpecificationLimit (LSL)

X-3A X-2A X-1AX X+1A

X+2 X+3A

X-6BX X+6B

Process A(with st. dev A)

Process B(with st. dev B)

Bro

wse

rer

ror

Number ofdefects

Figure 8.8: Order entry mistakes at Xootr

Cause of DefectAbsolute Number Percentage Cumulative

Browser error 43 0.39 0.39

Order number out of sequence 29 0.26 0.65

Product shipped, but credit card not billed 16 0.15 0.80

Order entry mistake 11 0.10 0.90

Product shipped to billing address 8 0.07 0.97

Wrong model shipped 3 0.03 1.00

Total 110O

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off

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Pro

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bill

ing

ad

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ng

mo

del

ship

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100

50

Cumulativepercents ofdefects

100

75

50

25

Step 1 Test 1 Step 2 Test 2 Step 3 Test 3

Rework

Step 1 Test 1 Step 2 Test 2 Step 3 Test 3

Figure 8.9: Two processes with rework

Step 1 Step 2 Step 3Final Test

Step 1 Test 1 Step 2 Test 2 Step 3 Test 3

Figure 8.10: Process with scrap

ProcessStep

Bottleneck

Based on labor andmaterial cost

MarketEnd ofProcess

Defectdetected

Defectoccurred

Defectdetected

Defectdetected

Cost of defect

$ $ $Based on salesprice (incl. Margin)

Recall, reputation,warranty costs

Defectdetected

Figure 8.11.: Cost of a defect as a function of its detection location assuming a capacity constrained process

Figure 8.12.: Information turnaround time and its relationship with buffer size

71

2345

68

ITAT=7*1 minute

3

1

2

4

ITAT=2*1 minute

Good unit

Defective unit

Kanban

Direction of production flow

upstream downstream

Kanban

Kanban

Kanban

Authorize productionof next unit

Figure 8.13.: Simplified mechanics of a Kanban system

Inventory in process

Buffer argument:“Increase inventory”

Toyota argument:“Decrease inventory”

Figure 8.14.: More or less inventory? A simple metaphor

Figure 8.15.: Tension between flow rate and inventory levels / ITAT

Flow Rate

Inventory

High

Low

High Inventory (Long ITAT)

Low Inventory (short ITAT)

Now

Frontier reflecting current process

Reduce inventory(blocking or starvingbecome more likely)

Increase inventory(smooth flow)

New frontier

Path advocated byToyota production system