Using
Quality Analyst to produce Cpk and DPMO reports
Do you need to provide
customers with CpK (Process Capability) and DPMO (Defects Per Million
Opportunities) reports? Does the paperwork constantly bog you down?
Are you going for ISO,
TS16949 and AS9000 accreditation, and need to show evidence of a Quality
Control Procedure?
Are you looking to reduce
variation, and need to identify the most cost effective improvements to
make?
If the answer to any of these questions is yes, then you need
a customised SPC solution. NWA Quality Analyst allows
you to automatically produce Cpk and DPMO reports to effectively analyse
the quality of your data, as well as comply with ISO9000, QS9000 and TS16949
standards.
 Cpk
Reporting
The rapid pace of change in
highly competitive industrial environments requires speedy and efficient
decisions and actions, and an automated statistical method which minimises
sampling during these studies hands any organisation which uses them a
great advantage - Cpk reporting can result in 50% sampling savings when
used regularly with other forms of analysis and charting.
Cpk reporting is used for Variable
Data, which is quanititative and can be divided into (Discrete) count
data and (Continuous) data. Examples are:
- Length
- Weight
- Volume
- Time to complete
- etc.
Studies
are usually performed whenever:
- The capability of a process to meet customer specifications needs
to be determined
- engineering tolerances are reviewed against the observed variability
of the process and/or new equipment is evaluated
- process changes and/or improvements need to be evaluated.
A
definition of Cpk:
The process potential index,
or Cp, measures a process's potential capability, which is defined as
the allowable spread over the actual spread. The allowable spread is the
difference between the upper specification limit and the lower specification
limit. The actual spread is determined from the process data collected
and is calculated by multiplying six times the standard deviation, s.
The standard deviation quantifies a process's variability. As the
standard deviation increases in a process, the Cp decreases in value.
As the standard deviation decreases (i.e., as the process becomes less
variable), the Cp increases in value.
 By
convention, when a process has a Cp value less than 1.0, it is considered
potentially incapable of meeting specification requirements. Conversely,
when a process Cp is greater than or equal to 1.0, the process has the
potential of being capable.
Ideally, the Cp should be as
high as possible. The higher the Cp, the lower the variability with respect
to the specification limits. In a process qualified as a Six Sigma process
(i.e., one that allows plus or minus six standard deviations within the
specifications limits), the Cp is greater than or equal to 2.0. However,
a high Cp value doesn't guarantee a production process falls within specification
limits because the Cp value doesn't imply that the actual spread coincides
with the allowable spread (i.e., the specification limits). This is why
the Cp is called the process potential.
The process capability index,
or Cpk, measures a process's ability to create product within specification
limits. Cpk represents the difference between the actual process average
and the closest specification limit over the standard deviation, times
three.
By convention, when the Cpk is less
than one, the process is referred to as incapable. When the Cpk is greater
than or equal to one, the process is considered capable of producing a
product within specification limits. In a Six Sigma process, the Cpk equals
2.0.
The Cpk is inversely proportional
to the standard deviation, or variability, of a process. The higher the
Cpk, the narrower the process distribution as compared with the specification
limits, and the more uniform the product. As the standard deviation increases,
the Cpk index decreases. At the same time, the potential to create product
outside the specification limits increases.
Cpk can only have positive values.
It will equal zero when the actual process average matches or falls outside
one of the specification limits. The Cpk index can never be greater than
the Cp, only equal to it. This happens when the actual process average
falls in the middle of the specification limits.
DPMO
Reporting
From DPMO data, you can identify
and prioritise assembly process improvements. DPMO is an invaluable reference
for enhancing the design for manufacturing (DFM) process and predicting
yield at design stage.
DPMO
is used for Attribute Data, which is purely binary in nature. It is for:
DPMO reporting takes place when
checking your final product. It factors in product complexity and allows
comparing assembly performances for different designs, different processes
and different types of assembly environments. DPMO is the reference of
choice for measuring and comparing process quality achievements because
it is a robust and portable metric.
A definition of DPMO:
DPMO is the total number of
defects divided by the total number of opportunities for a defect, multiplied
by 1,000,000. To operate at Six Sigma level means a process operates at
not more than 3.4 defects per million opportunities.
Defects are any critical characteristic
of a sampled entity that fails to meet customer expectations.
Opportunities are critical characteristics
of a sampled entity that could either meet or fail customer expectation.
In many manufacturing situations there may be multiple opportunities per
entity, whereas transactional situations often either meet the customer's
requirements or they don't. When evaluating potential opportunities (and
defects) it is important to work at the customer-significant functional
level. Defect opportunities must be critical to the customer, be
independent of each other, and only increase numerically with increased
complexity. For example, in a book, defects could be misspelled words,
grammatically incorrect sentences, missing text or pages and so on - however,
readers may regard the 'sentence' as the functional entity of interest,
and a defect is a spelling or grammatical error in each sentence.
For more information on improving
yields with Statistical Process Control, click here.
This link acts as a gateway to the NWA website, an external site.
If you have any further questions
about using Quality Analyst to produce Cpk and DPMO reports, please contact
our Quality Analyst team.
To download a free 30-day
Quality Analyst demo, click here.
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