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Unformatted text preview: , D4 = 2.575)
= 2.575 * 0.1423 = 0.366423
Rounded off to 0.366 (one more decimal place after the measured values).
There is no need to consider LCL because n = 3 (<6).
Step 8. Write in the control limits
Get a sheet of paper specially designed for control. Draw the vertical and horizontal axes on
it. In the case of –-R control charts, position – control above R control charts. Scale the
graduations so that the distance between UCL and LCL is about six times as long as the
distance between sub-groups shown on the horizontal axis. There is no need to put an LCL
on R control charts when n< = 6. Therefore, set the bottom graduation on the horizontal
axis to zero. This value is considered a substitute for LCL. Denote the central line with a solid
line (–). In control charts used for analysis and management, indicate the upper control limit
(UCL) and lower control limit (LCL) with broken lines (...).
Step 9. Write marks in the control charts
Calculate the “x” and “R” values for sub-groups and plot these on the control charts. Use
different marks “.” and “x” to plot – and R values so that the differences will be clear.
Indicate marks that went outside the control limits as
to make it easier to identify
Step 10. Write in the necessary items
Write “x” and “R” on the left edge of the control charts, and “n” on the top-left corner of –
control charts. In addition, give the quality characteristics, characteristic units, the names of
those who entered data, technical standards, process names, and sampling methods.
Step 11. Study management conditions
Note the following in Figure 9.8d on page 28:
1. Both – control charts and R control charts exhibit deviations from control limits in a
chain of eight successive values. This process is “out of control”.
2. There is a need to investigate causes of deviations from control limits that took place
on the 3rd, 5th, 16th and 22nd of the month.
3. Both – control charts and R control charts exhibit a chain of eight successive values
from the 23rd to the 30th of the month. This situation is abnormal. However, this is a
welcome abnormality. Once the causes of the abnormality are established and they
prove satisfactory from the standpoint of intrinsic technology, it will be possible to
reduce the amount of steam used.
4. Once causes of the abnormality after the 23rd day are identified and process
conditions (location, equipment, employees, temperature etc.) clearly differ from
those prior to that date, the central line and control limits must be drawn again using
data from the lst to the 22nd of the month. A Roadmap to Quality 27 Unit 9 - Problem solving 05-87581_unit 9.qxd 09/09/2005 11:55 Page 28 Figure 9.8d Control chart for the amount of steam consumed in drying synthetic rubber B 0.8 April 19XX n=3 0.7
0.6 EUCL = 0.5844 – 0.5
0.4 – = 0.4388
LCL = 0.2932 0.3
0.2 chain of eight
successive values 1.0
UCL = 0.366
1 5 10 15 20 25 30 35 – = 0.1423
(Date) Value for sub-group Discussion
Study this text carefully, and discuss how you could make use of control charts in your
company. Action plan
Draw up an action plan to introduce the use of control charts in your company. You might
like to use the outline described in the 6-Point Structure. Alternatively you may choose to
prepare one action plan that draws on your discussions of several texts. Unit 9 - Problem solving 28 A Roadmap to Quality 05-87581_unit 9.qxd 09/09/2005 11:55 Page 29 9.9 The QC Story I
1. Sometimes we over-react to problems. We try to solve them immediately in whatever way
we can. But over-hasty reactions often get us nowhere. We will handle problems much
more effectively if we approach them systematically. For this we need to have a basic
problem-solving procedure, and to follow it step by step. A QC Story is a good
procedure for solving problems scientifically and rationally.
2. A QC Story consists of the following eight steps:
a. Select a theme to work on.
b. Clarify the problem and set targets.
c. Get a clear understanding of the effects that the problem has caused.
d. Analysis: investigate the causes.
e. Devise and implement recurrence prevention measures.
f. Confirm the effects of these measures.
g. Standardize the new methods.
h. Reflect on the problems left unsolved and consider future countermeasures.
3. QC stories were originally used in reports to explain past problem-solution cases in a
way that was easy to understand. However, it was then recognized that the QC Story
activities could themselves provide a neat problem-solving procedure - one that was
clear and easy to follow, and that kept people on target. QC stories are now solidly
established as the basic procedure for solving problems at work.
(Text 11.3.5 in Unit 11, Statistics, shows how the QC tools can be applied to the QC Story.) Select themes
4. To select a theme, first of all identify all the problems in your workplace, compile them
on a chart, evaluate them and...
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