statistical quality control

1

1

Lecture 6: SPC tools and

techniques– part 1

Chapter 4

2

Magnificent seven quality tools

 Process flow diagram

 Pareto diagram

 Cause and effect diagram

 Check sheet

 Scatter diagram

 Histogram/stem-and-leaf chart

 Control charts/run chart

3

Process flow diagram

input

Task 1 Decision Task 2

Document

Output

4

Three types of process flow diagram

 Top down

 Logical — decision making is included

 Deployment or integrated — responsible person is included

5

Top down process flow diagram

 Simplest in form

 Contains major process steps with a list of sub-process steps

 Does not include how the process works but simply lists the components

6

Example: top down

Powder-Based Metal Alloy Process

Get mold

fill Clean up

Get crucible

Add powders mix

Get powders

Get scale weigh

Put in press

press

Weight Powders

Mix Powders

Fill Mold

Press to shape

2

7

Example: top down (cont.)

Powder-based Metal Alloy Process

Specimen

Measure strength Measure density

Extract part

Machine polish

Mold in oven

Heat oven Hold

Cool down

Sinter

Machine

Test

8

Logical process flow diagram

 How does a process work?

 Details of a sub-process

 Major steps in a complex

 example: getting a company car to go on a trip

9

10

Deployment/integrated flow diagram

 Steps in a process plus who is responsible for each step

 Example: software support

11

12

3

13

Cause-and-effect diagram

 Also known as

 Ishikawa diagram

 Fishbone diagram

 Visualization of the possible causes of process phenomena

14

Constructing cause-and-effect diagrams

 Choose a quality characteristic

 Write primary category causes as “big bones”

 Fill in secondary and tertiary causes as medium-size and small bones

15

Cause-and-effect diagram

Quality control

Materials Methods

Machines People

Basic

16

Cause-and-effect diagram

 Example: drilling process

 Computing software: Minitab has a function for cause-and-effect diagram

 You can construct them by hand

Fish-bone diagram for drilling operations

17 19

Cause-and-effect diagram

 Process-analysis type: particularly useful when looking at a product going through multiple procedures

4

20

Pareto chart

 Bar chart with bars drawn vertically and positioned in decreasing order of height, also giving information to separate majority from minority

 Principle and purpose

 80% of the problems are attributable to 20% of the causes

 Separate the vital few from the trivial many

21

Pareto chart construction

 What is to be investigated?

 How to collect data?

 Operational definition

 Implementation of data collection

 Summarize and chart data

 Interpret data

22

Pareto chart by phenomena

 Quality

 Complaints, defects, repairs

 Cost

 Amount of loss, expenses

 Safety

 Accidents, lost time

23

Pareto chart example Number Category Freq.

1 Splater 141

2 Runs 434

3 Overspray 109

4 Other 50

5 Lt. Spray 582

6 Drips 227

7 Blister 212

8 Bad Paint 126

Practice on Pareto chart

# Category Freq.

Freq.

%

Cum. Freq

%

1 Lt. Spray 582 (582/1881)*100=31% 31%

2 Runs 434 (434/1881)*100=23% 31+23=54%

3 Drips 227 (227/1881)*100=12% 54+12=66%

4 Blister 212 (212/1881)*100=11% 66+11=77%

5 Splater 141 (141/1881)*100=7% 77+7=84%

6 Bad Paint 126 (126/1881)*100=7% 84+7=91%

7 Overspray 109 (109/1881)*100=6% 91+6=97%

8 Other 50 (50/1881)*100=3% 97+3=100%

Total 1881

24 25

Pareto chart example

 Create the following columns given frequency number

 Frequency by percentage

 Cumulative frequency by percentage

5

26

Instruction of Excel for constructing Pareto chart

1) Sort data from high to low

2) Add columns of cumulative frequency, cumulative frequency, percentage, cumulative percentage, and construct the data value

3) Highlight “category”, “frequency”, “cumulative percentage”

4) Construct a column chart.

5) Right click on the chart of cum. Per., select “change type” to line chart

6) Make sure that the line data is clicked,

Add “secondary vertical axis” in “Format” or “Layout” tab, click “Format selection”

7) Add chart title in “Layout” tab

8) Add vertical axis label in “Layout” tab

27

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0

100

200

300

400

500

600

700

C u

m u

la ti

v e p

e rc

e n

ta g

e

F re

q u

e n

c y

Pareto Chart for Painting Noncomformities

This is not a Pareto chart

0%

5%

10%

15%

20%

25%

30%

35%

0

100

200

300

400

500

600

700

Freq.

percentage

How to interpret the Pareto chart

 Main nonconformity categories are Lt spray, runs, and drips

 The three categories account for 66%

 Improve strategy: find causes of the three big categories

29

30

Quality improvement

 The first step involves building knowledge about your process and its sub-process

 These tools seem simple and nothing more than common sense

 Simple, common sense tools can be very powerful

Inclass Practice 1— Pareto chart

Category Frequency Frequency %

Cum. Freq %

Scratch 42

Pin hole 20

Scuff 104

dent 14

stain 6

crack 10

other 4

31

6

32

Pareto chart by percentage data Pareto Chart

0.0

10.0

20.0

30.0

40.0

50.0

60.0

sc uf

f

sc ra

tc h

pi n

ho le

de nt

cr ac

k

ot he

r

st ai n

Category

F re

q u

e n

c y

0.0

20.0

40.0

60.0

80.0

100.0

120.0

C u

m u

la ti

v e

fr e q

u e n

c y

Percent.

cumulative percent

33

Assignment today

 InClass Practice 4