| SPC
Tools - Process Capability |
Process Capability
is also another important concept in SPC. Process capability
examines
- the variability in process
characteristics
- whether the process is capable
of producing products which conforms to specifications
Process capability studies
distinguish between conformance to control limits and
conformance to specification limits (also called tolerance
limits)
- if the process mean is in
control, then virtually all points will remain within control
limits
- staying within control limits
does not necessarily mean that specification limits are
satisfied
- specification limits are
usually dictacted by customers
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Process
capability: concepts |
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The
following distributions show different process scenarios. Note
the relative positions of the control limits and specification
limits. |
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In control and
product meets specifications.
Control limits are within specification limits |
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In control but
some products do not meet specifications.
Specification limits are within control limits |
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Process
capability: relative capability |
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Data from process
with low capability |
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Data from process
with medium capability |
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Data from process
with high capability |
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Process
capability: capability index |
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The capability index
is defined as:
Cp
= (allowable range)/6s
= (USL - LSL)/6s
The capability index show how
well a process is able to meet specifications. The higher
the value of the index, the more capable is the process:
- Cp < 1 (process
is unsatisfactory)
- 1 < Cp <
1.6 ( process is of medium relative capability)
- Cp > 1.6 (process
shows high relative capability)
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Process
capability: process performance index |
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The capability index
- considers only the spread
of the characteristic in relation to specification limits
- assumes two-sided specification
limits
The product can be bad if the
mean is not set appropriately. The process performance index
takes account of the mean (m)
and is defined as:
Cpk
= min[ (USL - m)/3s,
(m
- LSL)/3ss
]
The process performance index
can also accommodate one sided specification limits
- for upper specification
limit: Cpk = (USL - m)/3s
- for lower specification
limit: Cpk = (m
- LSL)/3s
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Process
capability: the message |
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The message from
process capability studies is:
- first reduce the variation
in the process
- then shift the mean of the
process towards the target
This procedure
is illustrated in the diagram below:
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