chapter 5 statistical quality control, 7th edition by douglas ......• statistical process control...

Chapter 5 1 Statistical Quality Control, 7th Edition by Douglas C. Montgomery.

Copyright (c) 2013 John Wiley & Sons, Inc.



Basic SPC Tools

• If a product is to meet or exceed customer expectations,

generally it should be produced by a process that is stable or

repeatable.

• More precisely, the process must be capable of operating with

little variability around the target or nominal dimensions of the

product’s quality characteristics.

• Statistical process control (SPC) is a powerful collection

of problem-solving tools useful in achieving process stability

and improving capability through the reduction of variability.



Basic SPC Tools (“Magnificent Seven”)

Stem-and-Leaf Diagram

Steps for Constructing a Stem-and-Leaf Diagram

Stem-and-Leaf Diagram

Histograms

A histogram is a more compact summary of data than a

stem-and-leaf diagram. To construct a histogram for

continuous data, we must divide the range of the data into

intervals, which are usually called class intervals, cells, or

bins. If possible, the bins should be of equal width to

enhance the visual information in the histogram.

Histograms



Check Sheet

• In the early stages of process improvement, it will

often become necessary to collect either historical or

current operating data about the process under

investigation.

• A check sheet can be very useful in this data

collection activity.

• Usually a basis for performing further calculations or

used as a worksheet for data entry into a computer.



Check Sheet



Pareto Chart

• Pareto chart is a variation of the histogram. Data

usually represent different types of defects, failure

modes, or other categories of interest to the

analyst.

• The categories are ordered so that the category

with the largest number of frequencies is on the

left, followed by the category with the second

largest number of frequencies, and so forth.



Pareto Chart



Pareto Chart

• Pareto chart does not automatically identify the most important

defects, but only the most frequent.

• For example, casting voids occur very infrequently (2 of 166

defects, or 1.2%). However, voids could result in scrapping the tank,

a potentially large cost exposure—perhaps so large that casting

voids should be elevated to a major defect category.

• When the list of defects contains a mixture of those that might have

extremely serious consequences and others of much less importance,

one of two methods can be used:

1. Use a weighting scheme to modify the frequency counts.

2. Accompany the frequency Pareto chart analysis with a cost or

exposure Pareto chart.

Chapter 5 16



Cause-and-Effect Diagram

• Once a defect, error, or problem has been identified

and isolated for further study, we must begin to

analyze potential causes of this undesirable effect.

• In situations where causes are not obvious

(sometimes they are), the cause-and-effect diagram

is a formal tool frequently useful in unlayering

potential causes.



Cause-and-Effect Diagram



Defect Concentration Diagram

• A defect concentration diagram is a picture of the unit,

showing all relevant views. Then the various types of

defects are drawn on the picture, and the diagram is

analyzed to determine whether the location of the defects

on the unit conveys any useful information about the

potential causes of the defects.

• When defect data are portrayed on a defect concentration

diagram over a sufficient number of units, patterns

frequently emerge, and the location of these patterns often

contains much information about the causes of the

defects.



Defect Concentration Diagram



Scatter Diagram

• The scatter diagram is a useful plot for identifying

a potential relationship between two variables.

• The shape of the scatter diagram often indicates

what type of relationship may exist between the

two variables.



Scatter Diagram



Scatter Diagram • Correlation does not necessarily imply causality.

• This apparent relationship could be caused by something quite

different. For example, both variables could be related to a

third one, such as the temperature of the metal prior to the

reclaim pouring operation, and this relationship could be

responsible for what we see in the previous figure. If higher

temperatures lead to higher metal recovery and the practice is

to add reclaim flux in proportion to temperature, adding more

flux when the process is running at low temperature will do

nothing to enhance yield.

• The scatter diagram is useful for identifying potential

relationships. Designed experiments must be used to verify

causality.



Implementing SPC in a Quality Improvement Program

• SPC is most effective when it is integrated into an overall,

companywide quality improvement program.

• Management involvement and commitment to the quality

improvement process are the most vital components of SPC’s

potential success.

• A team approach is also important.

• The basic SPC problem-solving tools must become widely

known and widely used throughout the organization. Ongoing

education of personnel about SPC and other methods for

reducing variability are necessary to achieve this widespread

knowledge of the tools.




• The objective of an SPC-based variability reduction

program is continuous improvement on a weekly,

quarterly, and annual basis.

• SPC is not a one-time program to be applied when the

business is in trouble and later abandoned. Quality

improvement that is focused on reduction of

variability must become part of the culture of the

organization.



An Application of SPC

• Improving quality in a copper plating operation at a printed

circuit board fabrication plant

• This process was characterized by high levels of defects such

as brittle copper and copper voids and by long cycle time. The

long cycle time was particularly troublesome, as it had led to

an extensive work backlog and was a major contributor to poor

conformance to the factory production schedule.

• The DMAIC process was used

• During the define step, the team decided to focus on reducing

flow time through the process

• During the measures step, controller downtown was

recognized as a major factor in excessive flow time

chapter 5 statistical quality control, 7th edition by douglas ......• statistical process control...

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