Rekayasa Kualitas
Materi ke – 2
Dasar Rekayasa Kualitas
ekop2003@yahoo.com
Contents

Basics of Quality by Design



Quality
Quality Loss Function
Quality and Process Capability

Process capability
Basics of Quality by Design

Introduction



A large amount engineering manufacture
products in conducting in experiments
Quality by design is one of such engineering
methodology
Customer’s perception of quality
( Sony television case in Japan and USA )
Basics of Quality by Design
Colour density
distribution of
Sony America
and Japan
(April, 1979)
Basics of Quality by Design
Basics of Quality by Design


Taguchi’s definition of quality
“ the quality of a products is the ( minimum )
loss imparted by the product to society from
the time the products is shipped “
What is loss – and who pays for it


Loss as additional manufacturing cost
Loss be paid by society
Basics of Quality by Design

Interpretation of loss

Quality loss function (QLF) is quantitative
evaluation of quality loss due to functional
variation
m = target
y = hasil pengukuran
Δo = toleransi
Ao = jumlah kerugian
Basics of Quality by Design
Basics of Quality by Design

Interpretation of loss


A quality characteristics is the object of interest
of a product or process
Three types quality characteristics
• Nominal the best ( NTB )
• Smaller the better ( STB )
• Larger the better ( LTB )
QLF - Nominal the best ( NTB )
( One piece )
•
A measurable characteristics with a specific userdefined target value. ( Sony TV : 115 ± 20 volts )
•
QLF for NTB
QLF - Nominal the best ( NTB )
QLF - Nominal the best ( NTB )
( Many pieces )
QLF – Smaller the better ( STB )
( One piece )
QLF - Smaller the better ( STB )
( One piece )
Example
QLF - Smaller the better ( STB )
( One piece )
QLF - Smaller the better ( STB )
( Many pieces )
QLF – Larger the better ( LTB )
( One piece )
QLF – Larger the better ( LTB )
( One piece )
Example
QLF – Larger the better ( LTB )
( Many pieces )
1
L( y )  k x
n
 1 



2 

i 1  y i 
n
QLF – Asymmetric
Quality and Process Capability
Capability process
Quality and Process Capability
Capability process
Quality and Process Capability

Spec limit : fixed engineering limit for the product
dimmensions

Cp index

Cpk index
 Cpk is an important measure
 Cpk as a measure of quality
Quality and Process Capability
Quality and Process Capability
Quality and Process Capability
Cpk as a measure of quality

The convesional methods
Loss-by-defect = numbers of defect x cost of product
= proportion out of spec x total numbe x cost
of product

The quality loss function methods
Loss-by-dispersion =
product

A0 2
2


(
y

m
)
2

x numbers of
Quality and Process Capability
Cpk as a measure of quality
Example ( Sample 100.000 unit )
Quality and Process Capability
Cpk as a measure of quality
Quality and Process Capability
Cpk as a measure of quality
Quality and Process Capability
Cpk as a measure of quality
Case Study ( Tugas – 2 )
A spring is used in the operation of a camera shutter. The manufacturing process
suffers from a degree of variability, in terms of the spring constant (measured in
oz/in), which significantly effects the accuracy of the shutter times. The functional
limits for this spring constant are m±0.3oz/in (m=0.5oz/in), and the average cost for
repairing or replacing a camera with a defective spring is $20.
What is the loss function? Hence, what is the loss associated with producing a spring
of constant 0.25oz/in versus the loss associated with one at 0.435oz/in.
Next Week

The Design Process
 Principle of robust design in the
production process
 Robust design process
 Parameter design