Process Capability

1
Process Capability

• Training and Explanation

2
Purpose of EasyTools

• There are lots of Quality tools around that can
  help improving many situations of the daily
  quality life. The problem with many tools is that
  you need training and experience to use them
  effectively.
• EasyTools is a collection of easy to use and
  mostly self explaining tools that everybody can
  use.
• No need for weeks of training or years of
  experience.

3
Purpose and Contents

• Purpose
• To get a basic feeling on capability studies.
• Contents
• Normal Distribution
• Scattering
• Process Potential
• Machine Capability
• Process Capability
• Preliminary Process Capability
• Formulas for capability and potential
• Customer and internal requirements

4
The Normal distribution
Assume you have a chicken farm and you measure
the weight of the eggs. Their weight is around
50g means some eggs have only 45g but some have
55g. If you make a distribution curve where you
show how many eggs have what weight, you will get
following curve.
m 50
Sigma 5
5
Capability
The Capability is the ratio of Tolerance to
Scattering
Tolerance
Scattering
6
How to measure scattering?
Assume you have a normal distribution then 99
of all data lie within - 3 sigma.
The capability is 1 if 99 of the data lie
within the tolerance (- 3 sigma).
The capability is better than 1 if more than 99
of the data lie within the tolerance.
-Sigma
-2 Sigma
-3 Sigma
7
Potential versus capability
The better the data lie in the middle of the
tolerance the better the capability.
The potential is what we could achieve if the
scattering is the same but the data would lie in
the middle.
8
Different capability types
Cmk - Machine capability
Short term study. Take a sample of e.g. 50 parts
as they are produced, measure the characteristic
you are interested and calculate the Cmk.
Cpk - Process capability
Long term study. Take samples over a long time
(e.g. 3 month e.g. each week 5 parts), measure
the characteristic you are interested and
calculate the Cpk.
Ppk Preliminary Process capability
Mid term study. Take samples over a mid time
(e.g. 3 weeks e.g. each day 3 parts), measure
the characteristic you are interested and
calculate the Ppk.
9
Capability formulas
The formulas for Cpk and Ppk are the same as for
Cmk.
10
Potential formulas
USL - LSL
Cm
6 s
The formulas for Cp and Pp are the same as for Cm.
See visualisation of capability
A comprehensive article about capabilities can be
found at http//www.itl.nist.gov/div898/handbook/p
mc/section1/pmc16.htm
11
Capability requirements
Our Customers requirements are Cpk gt 1.00 Cpk
for safety items gt 1.33
Our requirements should be Cpk gt 1.33 Cpk for
safety items gt 1.67
Sigma Cpk ppm
3 1,00 2700
4 1,33 64
5 1,67 0,5
6 2,00 0,002