The slide is about process capability and IT grades used. Hope the material will be useful for Design of Manufacturing.

1. PROCESS
CAPABILITY
AND IT
GRADES
Presented
by
Hiran Gabriel D J.

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.

4. DISTRIBUTION
CHARTS FOR
VARIOUS
PROCESS

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 μ and σ 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.

8. Formula used

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:
 T is the tolerance in micrometres [µm]
 D is the geometric mean dimension in millimetres [mm]
 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 ± 0.06 , 50 ± 0.125 , 150 ± 0.2

13. CALCULATED
VALUES
BASED ON
DIAMETERS

14. FOR
SHAFT

15. FOR
HOLES

17. Surface finish
vs
production
time

18. COST VS
TOLERANCE

20. Process
Tolerance vs.
Feature Size

21. REFERENCE
• http://www.syque.com/quality_tools/toolbook/Procap/how.htm
• https://www.itl.nist.gov/div898/handbook/pmc/section1/pmc16.htm
• http://www.cobanengineering.com/Tolerances/FundamentalDeviatio
n.asp