RME 085 TQM unit-3 _part 4

RME-085
Total Quality Management
Topic: Process capability
By:
Dr. Vinod Kumar Yadav
Department of Mechanical Engineering
G. L. Bajaj Institute of Technology and Management
Greater Noida
Email: vinod.yadav@glbitm.org

Process capability : How a process will satisfy USL/LSL
Note: The contents used in this slide is being used for academic purposes only, and is intended only for students registered in B.Tech Mechanical Engineering at AKTU
Lucknow in VIII semester 2019-20, and is not intended for wider circulation.
- How capable is the process in meeting customer’s need
Process
Y
Z
X
Output
Source: CLEANPNG [1]
Noise/vibration
(Factors may
contribute)
X, Y, Z - Inputs
6σ Framework
D – Define
M – Measure
A – Analyze
I – Improve
C - Control
Process is not capable
Process is not capable
Process improvement steps [2]
1. Measure (External / Internal)
2. Analysis
3. Control (Modify the process)
4. Improve (Minimize variation)
5. Innovate (Redesign)
2σ – 68 %
4σ – 95.5%
6σ – 99.7%

Process capability : How a process will satisfy USL/LSL (Contd.)
Process is not capable Process is not capable
Process improvement steps [2]
1. Measure (External / Internal)
2. Analyze
3. Control (Modify the process)
4. Improve (Minimize variation)
5. Innovate (Re-design)
Tolerance band: Product is okay
Process variation
Process capability:
- The inherent variability of process output
relative to the variation allowed by the
design or customer specification [2].
- Process under tolerance limit: Capable
process
2σ – 68 %
4σ – 95.5%
6σ – 99.7%

Process capability (PC) : How a process will satisfy USL/LSL (Contd.)
Process variation
Process capability analysis
Vs
Control chart analysis
- PC: Focus on improvement not on control
- PC: Valid for variables and Not applicable
for attributes
- PC: Checks the range of individual output
(However, control charts checks the range
of sample measures)
- PC: assumes that the output is normally
distributed
- PC: Requires process first to be in
statistical control (and is under the
influence of chance cause like
(noise/vibration) only and not under
assignable causes.
2σ – 68 %
4σ – 95.5%
6σ – 99.7%

Process variation
Why to check process
capability ???
- PC: For customer’s satisfaction
- Process variability must be minimized
- Process capability – Specifications limits (USL
and LSL) are important compared to control
limits (UCL and LCL) as used in control charts.
- In control charts control limits (UCL/LCL) are
used to stabilize the process. It ensures that the
process is under statistical control.
- Control charts removes assignable causes.
USL/LSL: The range that the customer will
accept.
2σ – 68 %
4σ – 95.5%
6σ – 99.7%
If we fix 2σ (When normally distributed) – 68% of
the products will lie in that range.
Six Sigma (6σ):
USL = µ + 3σ
LSL = µ - 3σ
Property of normal
distribution

Natural range of Process capability
- The range in which all the products are acceptable
- Best way to control the process is to target the average
or mean value
Ref: [3]
Ways of capability improvement:
1. Redesign the process
2. Adopt alternate process
3. Conduct 100% inspection
4. Examine USL and LSL and change (if
possible) – in consultation with customer.
(Last option) best is 1

Process capability ratio Cp
Cp =
𝑻𝒐𝒍𝒆𝒓𝒆𝒏𝒄𝒆 𝑹𝒂𝒏𝒈𝒆
𝑷𝒓𝒐𝒄𝒆𝒔𝒔 𝑹𝒂𝒏𝒈𝒆
Or
Cp =
𝑺𝒑𝒆𝒄𝒊𝒇𝒊𝒄𝒂𝒕𝒊𝒐𝒏 𝒘𝒊𝒅𝒕𝒉
𝑷𝒓𝒐𝒄𝒆𝒔𝒔 𝒘𝒊𝒅𝒕𝒉
Or
Cp =
𝑼𝑺𝑳−𝑳𝑺𝑳
𝟔𝝈
If Cp = 1 i.e.
𝑼𝑺𝑳 −𝑳𝑺𝑳
= 𝟏
Ref [4]
Okay but not best
Cp = 1 is okay but slight variation will cross specification limits
Cp =2 is best – slight variation will not cross specification limits
Cp < 1 is not acceptable

Process capability ratio Cp
Cp =
𝟔𝝈
This expression do not have mean (µ) – which is important as
the centre of normal curve must coincide the mean value for
better control.
Process capability Index is used
Cpk = Minimum of (
𝑿−𝑳𝑺𝑳
𝟑𝝈
) or (
𝑼𝑺𝑳− 𝑿
𝟑𝝈
)
Where, 𝑿 = Average of all Xi values
Cpk > 1 is good
Cpk < 1 is not acceptable – process out of control
One more parameter
Ppk = Minimum of (
𝑿−𝑳𝑺𝑳
𝟑𝒔
) or (
𝑼𝑺𝑳− 𝑿
𝟑𝒔
) Where, s = sample standard deviation (Local Std. dev)
Ref. [5]
Control charts are used to identify abnormal variables – They may provide wrong information about
abnormalities. However, under control process indicates internal ability of the process Improving the
process capability – deals with changing the mean and reducing normal variability
Ref: [6]

References:
[1] https://www.cleanpng.com/png-six-sigma-standard-deviation-normal-distribution-t-2356470/ (Accessed on 03.04.2020)
[2] https://www.youtube.com/watch?v=mwpsjfFup54 (Accessed on 03.04.2020)
[3] http://www.presentationeze.com/presentations/statistical-process-control/statistical-process-control-full-details/
(Accessed on 03.04.2020)
[4] http://process--0.blogspot.com/2018/12/67-tutorial-r-process-capability-pdf.html (Accessed on 03.04.2020)
[5] https://slideplayer.com/slide/4673456/ (Accessed on 03.04.2020)
[6] https://www.six-sigma-material.com/Process-Capability-Indices.html (Accessed on 03.04.2020)

RME 085 TQM unit-3 _part 4

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to RME 085 TQM unit-3 _part 4

Similar to RME 085 TQM unit-3 _part 4 (20)

Recently uploaded

Recently uploaded (20)

RME 085 TQM unit-3 _part 4