2. Process capability is the repeatability and consistency of a
manufacturing process relative to the customer requirements in terms
of specification limits of a product parameter. This measure is used to
objectively measure the degree to which your process is or is not
meeting the requirements.
PROCESS CAPABILITY
3. WHY STUDY PROCESS CAPABILITY
⢠Process capability studies provide a baseline for us to how to understand
the process is operating relative to the specifications.
⢠It is the first, thorough look at how variability is affecting the process,
and gives us Metris for quantifying that variability.
⢠Such studies also provide information regarding what the process
could do under best conditions, and thus give a performance target to
shoot for.
5. ⢠Capability information helps designers to set realistic specifications
limits.
⢠Planning the interrelationship of sequential process.
⢠Selecting between the competing vendors.
⢠Reducing the variability of the manufacturing process.
⢠Predicting the extent of variability that process will exhibit.
Principle of Process Capability
6. EFFECT OF CP VALUES ON PROCESS
VALUE OF CPK CAPABILITY ACTION
Less than 1 Incapable Improve by reducing common
causes of variation in process
variables. Use 100% inspection.
Between 1 and 3 Capable Do nothing or some process
improvement. Dependent on
sample size.
Greater than 3 Very capable Do nothing or reduce
specification limits. No
inspection necessary.
7. MEASUREMENT OF PROCESS CAPABILITY
⢠There are several statistics that can be used to measure the capability of a
process: Cp, Cpk, and Cpm.
⢠Most capability indices estimates are valid only if the sample size used is
"large enough". (50 independent data values).
⢠The Cp, Cpk, and Cpm statistics assume that the population of data values is
normally distributed.
⢠Assuming a two-sided specification, if Îź and Ď are the mean and standard
deviation, respectively, of the normal data and USL, LSL, and T are the upper
and lower specification limits and the target value, respectively, then the
population capability indices are defined as follows.
9. IT GRADES
⢠IT Grade refers to the International Tolerance Grade of an industrial
process defined in ISO 286.
⢠The specific tolerance for a particular IT grade is calculated via the
following formula
where:
ď T is the tolerance in micrometres [Âľm]
ď D is the geometric mean dimension in millimetres [mm]
ď ITG is the IT Grade, a positive integer.
10. ⢠An industrial process has an IT Grade associated with it, indicating how precise it is.
⢠Decide the level of precision of the level known as the resilience level.
⢠It should be noted the tolerance values given in the table are the full tolerance zone
(full range), therefore the value should be divided by 2 when it is written as a
bilateral tolerance.
Example: Write the following dimensions with bilateral tolerances using
IT12: 6, 50, 150 mm
6 Âą 0.06 , 50 Âą 0.125 , 150 Âą 0.2