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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nu
mb
er o
ut
off
seq
uen
ce
Pro
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hip
ped
, bu
tcr
edit
car
d n
ot
bill
ed
Ord
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ym
ista
ke
Pro
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hip
ped
to
bill
ing
ad
dre
ss
Wro
ng
mo
del
ship
ped
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