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SPC
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| Statistical process control (SPC)
monitors specified quality characteristics of a product or
service so as: |
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| a) To detect whether the process has
changed in a way that will affect product quality. |
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| b) To measure the current quality of products or services. |
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| Control is maintained through the use of control charts. The
charts have upper and lower control limits and the process
is in control if sample measurements are between the limits. |
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WHY STATISTICAL PROCESS CONTROL
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Efficiency of any process depends on the quality of decision
taken by the persons who handles it. He uses the data and
interpret it to make decision. In many cases the common
sense does not help for interpreting the data. That’s where
the role of Statistical Process Control comes into play. |
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The aim of statistical process control system is to make
economically sound decision about the actions affecting the
process i.e. providing statistical signal when special cause
of variation is present and to avoid false signal when they
are not present thus preventing over control (taking actions
when it is not necessary) and under control (failing to take
action when it is require). |
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The process is said to be in statistical control when only
source of variation are from the common causes. |
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Control charts are the statistical techniques used to
measure and reduce variability of a process. They can be
used as an on going measurement of various key parameters of
a process so that action can be taken as soon as a problem
is indicated. Control charts effectively direct attention
towards special cause of variation when they appear and
reflect the extent of common cause of variation that must be
reduced for process improvements. |
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PROCESS CAPABILITY
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| Process capability is calculated to find out whether the process
can meet the specification. |
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| Capability is measure of the inherent variation (from common
causes) of a machine or a process. The capability of a machine
or a process can be determined by conducting capability studies.
These studies involve the analysis of a small sample of output
to enable an estimate to be made of the overall variation likely
to occur in longer run. The capability study can be of two
types: |
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SHORT TERM STUDY
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| Short term study is based on the data collected for short period
i.e. one operating run of the m/c etc. In this study the
external influence is kept minimum (same operator, same lot,
same setting etc.). This study is being normally to find out the
m/c capability. |
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LONG TERM STUDY
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| Long term study is based on measurement collected over a period
of time. The data shall be collected for long enough time and in
such a way as to include all expected source of variation. Many
of these source of variation may not have been observed during
short term study, to assess the ability of a process to satisfy
customer requirements over a long period of time with many
source of possible variation included. |
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CONTROL CHART
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| Control charts permits plotting of actual data as they occur
from time to time. A set of criteria known as control limits,
defines the area within which variation may normally be expected
to fluctuate. The most common type of variable control chart is
X – R control charts. |
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