Transcript Capability - Northrop Grumman OASIS

Capability
What it is
• Capability is a process which gives a set of indexes which tell us how
capable we are in meeting customer specifications, in the short term and
in the long term.
• In manufacturing (and commercial) areas, “capability” is used generically,
to describe these elements:
Measurement
System
Analysis
Evaluation of
Capability
indexes
Process in
Control
(SPC)
Process
Capability
Study
Corrective Actions
- using PDCA or
DMAIC
• Measurement System Analysis (MSA) looks at the accuracy and
precision (variation) of a measurement system : gauges, people, and the
process of taking measurements.
• SPC (Statistical Process Control) is a statistical technique which
compares current process output against a historical template. The
template is usually based around a centre value with ± 3 standard
measures of variation (of the characteristic being measured). The
objective is to keep the current process stable, within these ± 3 control
limits and to limit variation within them.
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Capability
What it is
• SPC (contd): Industry-standard rules are adopted: If data from the
current process approaches these control limits or forms non-random
patterns within the limits, then appropriate action to control the process is
taken.
• Process Capability looks at short term capability and long term
performance of a process with regard to customer specifications
• Standard indexes are used: Cp, Cpk, Pp and Ppk. These can be
converted into ppm.
• Caution:
Cpk is an index much misused: It is only applicable to normally
distributed variable data that is measured in the short term. Conversions
from Cpk to ppm and sigma assume that all defectives are occurring at one
tail of the distribution with an average sigma shift of 1.5 sigma. (industry
standard). Cpk MUST be seen as part of a package of information.
Capability studies for non-normal data (Weibull) do not give Cpk values.
• Corrective Actions: If a process is “not capable” (refer to following
pages for details), corrective action will follow standard continuous
improvement PDCA (Plan-Do-Check-Act) or step-change D-M-A-I-C
(Define-Measure-Analyse-Improve-Control) procedures.
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Capability
How to use it
• Because most people use “capability” to describe any of the above
elements, a practical approach is to stick to standard definitions and
approaches, and to examine these elements in more detail
• Use the correct language and do not try to do everything at once.
• Follow the process outlined in block form, above:
How it helps
• The principles of Capability can be adapted for non-manufacturing
processes and for attribute data.
References
• ASG aTQ training modules:
• 3.3, 3.4, 3.5 : (MSA) Measurement System Analysis
• 4.1, 4.2, 4.3, 4.5 : Process Capability
• 8.4, 8.5, 8.6, 8.7 : SPC (and non-SPC control)
• ASG “seniors” aTQ training module 1.5 : Capability
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