Process Capability

1
Process Capability
What it is

• 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.

• A simple view of Process Control and Process
  Capability
• process control - refers only to the voice of
  the process - looking at the process using an
  agreed performance measure to see whether the
  process forms a stable distribution over time.
• process capability - measures the goodness of a
  process - comparing the voice of the process
  with the voice of the customer. The voice of
  the customer here is the specification range
  (tolerance) and/or the nearest customer
  specification limit.

The objective is to get as close to the
theoretical best that your process can achieve by
eliminating special causes of variation, so that
only common (natural) causes are acting on the
process, and then to reduce these to a minimum,
whenever possible.
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Process Capability
What it is
Process Control
Note - no reference to specs !
In Control (Special Causes Eliminated)
TIME (continued below)
Out of Control (Special Causes Present)
Process Capability
In Control and Capable (Variation from Common
Causes Reduced)
Lower Spec Limit
Upper Spec Limit
TIME (continued from above)
In Control but not Capable (Variation from Common
Causes Excessive)
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Process Capability
What it is
Cp or ENTITLEMENT is the theoretical best hat a
process can achieve.
4
Process Capability
What it is
The 4 capability Capability Indexes explaining
the terms . . .
Not (necessarily) in Control common causes and
special causes acting (possible drifts or shifts
in the process)
long term
short term
In Control only common causes acting (no sudden
drifts or shifts in the process)
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Process Capability
What it is
The 4 capability Capability Indexes explaining
the terms . . .
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Process Capability
What it is
The 4 capability Capability Indexes explaining
the terms . . .
USL
LSL
Cp, Pp compares specification range (tolerance)to
process width regardless of where the process is
centred. It is the ratio of the specification
width to 6 x process standard deviation.
sP pooled standard deviation sT
total standard deviation
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Process Capability
What it is
The 4 capability Capability Indexes explaining
the terms . . .
Cpk, Ppk measure how close the process centre is
to the nearest customer spec. It is (difference
between the process average and the nearest
specification) (3 x the process standard
deviation).
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Process Capability
How to use it

• Use Minitab output whenever possible, since it
  combines the indexes with a graphical
  representation of the process actual sample,
  estimated short term capability and estimated
  long term performance
• Also use as a rule of thumb the following chart

(Cpk x 3) 1.5 sigma
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. Capabil
ity studies for non-normal data (Weibull) do not
give Cpk values
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Process Capability
Example Using Minitab
10
Process Capability
Example Using Minitab, with non-normal data
Note For Weibull Capability Analysis, data must
have positive values, and LONG TERM metrics
only are given. NO values for Cpk are given,
because the data is non-normal
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Process Capability
How it helps

• A Process Capability study is the end result
  of the analysis done on process output data.
• A Process Capability study reflects what our
  customer sees.
• It indicates what our short term and long term
  situation looks like.
• Recognise however that our processes need to
  be stable (in statistical control) as well as
  capable.
• Before anything, we must have faith in our
  measurements, hence the need for a measurement
  system analysis.

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