This presentation tells about Process capability and its concepts, calculations, process stability, Cp Cpk,Pp Ppk,

1. PSG COLLEGE OF TECHNOLOGY
DEPARTMENT OF
AUTOMOBILE ENGINEERING
REX MATTHEW P
22MS01
21AE22 DESIGN FOR MANUFACTURING AND ASSEMBLY
TOPIC
Process capability – relation between yield and number of parts in assembly under 6σ

2. PROCESS CAPABILITY
• Process capability is a statistical measure that quantifies the ability of a process to consistently
produce output within specified tolerance limits.
• Process capability is the measured, inherit variation of the product turned out by a process.
• We are often required to compare the output of a stable process with the process specifications
and make a statement about how well the process meets specifications.
• Process refers to the unique combinations of machines, tools, methods, materials and people
engaged in production.
• Capability refers to an ability, based on test performance, to achieve measurable results.

3. THE BASIC CONCEPT
• When the manufacturing process is being defined, our aim is to ensure that the parts produced
fall within the Upper and Lower Specification Limits (USL, LSL).
• Process Capability measures how consistently a manufacturing process can produce parts within
specifications.
• We want our manufacturing process to be:
• Centered over the nominal desired by the design engineer.
• With a spread narrower than the specification width.
• Cp measures whether the process spread is narrower than the specification width.
• Cpk measures both the centering of the process as well as the spread of the process relative
to the specification width.

5. BASIC CALCULATIONS:
1: Plot the Data: Record the measurement data, and plot this data on a chart, histogram, etc.
2: Calculate the Spec Width: Plot the Upper Specification Limit (USL) and Lower Specification Limit (LSL) on the chart, and
calculate the Specification width.
Spec Width = USL — LSL
3: Calculate the Process Width: Similarly, we will also calculate the Process Width. The simplest way to think about the process
width is "the difference between the largest value and the smallest value this process could create".
Process Width = UCL — LCL
4: Calculate Cp: Calculate the capability index as the ratio of the spec width to the process width.
Cp = Spec Width / Process Width

7. A Simple Analogy
Imagine a driver trying to park a car in a garage. If the car is too wide, it won't fit, also if it's narrower than the garage
opening, but if it's not centred, it won't make it in, it will likely hit/scrape one of the sides. Hitting one of the sides of the garage is
equivalent to producing a defective part.
But if the car is narrow enough and well-centered, the car will fit. That is our goal. We want a manufacturing process
width that is narrow and well-centred relative to the specification limits.

8. A More Realistic Analogy

9. PROCESS STABILITY
• Before we begin a process capability analysis, we
must check to ensure your process is stable.
• If your process is stable, the short-term
behaviour of the process (during the initial run),
will be a good predictor of the long-term
behaviour of the process (i.e. you can predict
future performance with confidence).
• Process behaviour - both short-term and long-term
- is characterized by the average and the standard
deviation.
• A process will be considered stable when it's
average and standard deviation are constant over
time.

10. DIFFERENCE BETWEEN Cp, Cpk and Pp, Ppk
[Potential] Process Capability Analysis (Cp, Cpk):
• A process capability study uses data from a sample to PREDICT the ability of a manufacturing process to produce parts conforming to
specifications.
• Cpk uses "short-term" to predict the behaviour of the process.
[Actual] Process Performance Analysis (Pp, Ppk):
• A process performance study is used to EVALUATE a manufacturing process and answers the question: “How did the process actually
perform over a period of time”.
• Ppk uses "long-term" to evaluate the behaviour of the process.
If the process is stable, Ppk = Cpk, (i.e. the actual performance will match the predicted potential performance). However, if the process is
unstable (i.e. if it shifts or drifts over time we will find Ppk << Cpk)

11. Process capability can be categorized under two categories:
Short Term Capability:
• Potential performance of a process, under control at a point in time such that there is no external influence
on the process.
• Short term capability represents the true process capability. Short term capability indicates the technology
of the process.
Long Term Capability:
• The actual performance of a process over a period of time such that external factors can influence the
process.
• Long term capability represents both the Technological capability combined with the controls that you
exercise.

12. What is Process Sigma?
• Process Sigma is a measure of the capability of a Six Sigma process, which has a theoretical
defect rate of 3.4 defects per million (DPM). Six Sigma practitioners use statistics, financial
analysis, and project management to improve quality control by identifying and correcting
mistakes or defects in existing processes.
• It is a measurement to evaluate the output of a process against the set performance standard.
• Higher the process sigma, better the process capability.
• Sigma measure gives us a common platform to compare different process that is otherwise
being measured differently.
• The five phases of the Six Sigma method, known as DMAIC, are defining, measuring,
analyzing, improving, and controlling.

13. PROCESS SIGMA CALCULATION:
• Unit – An Item being processed
• Defect – Failure to meet a customer Requirement or a Performance standard
• Opportunity – Any product / service characteristics which is measured to a standard
• Defective – A unit that has a defect
• Defects Per Million Opportunity – Number of defects that would arise given a million opportunity
DPMO Calculation:
• Defects Per Opportunity
DPO = D / (O*U)
D = Total No of defects
O = Opportunity for defects per unit
U = Total No of Units
• DPMO (Defects Per Million Opportunity)
DPMO = 1,000,000 * DPO = 1,000,000 * D/(O*U)

14. SIMPLE EXAMPLE:
• For any Six Sigma process, the calculation will always result the process to have only 3.4
defects per million opportunities (DPMO).
• For example, if a process had only 2 Defects, 18 Opportunity for Defects per Unit and Total
number of units to be 32500, the DPMO calculation will be as follows:
DPO = 2 / (18*32500) = 0.0000034188
DPMO = DPO * 1000000 = 3.4

15. Yield:
• Yield is generally defined as the output of a process.
• Different types of fulfilment can impact the quality level we measure in our processes.
• Yield can be understood as Classical Yield, First Pass Yield and Rolled Throughput Yield.
GENERAL WORKFLOW WITH MULTIPLE PASSES:

16. Classical Yield (YC):
• Classical yield is the proportion of correct items (conforming to specifications) you get
out of a process compared to the number of raw items you put into it.
• Units Passed / Final Units Tested = 65/70 = 0.93
65
70

17. First Time Yield (FTY):
• Unlike traditional yield, it captures the harsh reality of the effectiveness of the
process.
• Units Passed / Units input for First time = 249/352= 0.707

18. Rolled Throughput Yield (RTY):
Six Sigma quantifies the complexity of a system is to count the number of processes
involved combined together to create an overall process structure for accomplishing
complex tasks.

19. SIX SIGMA CAPABILITY METRIC
1. The metric must have a scale, such as frequency of occurrence, rate of occurrence, units produced correctly over time,
number of defects, and dollars. To be effective, the measurement scale must be meaningful, valid, and reliable.
2. The metric must have a goal or standard. If we are measuring injuries, the goal could be no more than 2 missed days of
work per person per year from injury on the job.
3. Alternatively, the goal is to have an injury rate of no more than 0.05% each month for the entire workforce. In other words,
99.95% of the workforce in any given month has not experienced an injury that caused an absence of more than one day
from work.
4. Compensation and other forms of recognition must be attached to actual performance compared to the goal. If we measure
safety, we must also monetarily reward and recognize people for improving safety. This is where many companies fail to
support their measurement systems. While they may claim to value customer satisfaction, they do not compensate employees
for improving customer satisfaction. While they have a scale of measure and a goal, they do not reward or recognize actions
supporting the goal.

20. 5. The metric is reported and reviewed vertically and horizontally throughout the organization on a regular basis. There
is a system in place to distribute performance data to all executives, managers, and employees who impact the metric.
6. High-level metrics are cascaded down through the business, operations, and process levels of the enterprise as a
family of supporting measures. In turn, they are monitored and reported up, down, and across the organization as
needed.
7. The metric must be able to be pooled horizontally and vertically within the enterprise. This means that the metric
must have meaning and impact across various functions and at many levels of the organization.
8. The metric must be strongly correlated with one or more of the 12 dimensions of quality at the business, operations,
and/or process level of the organization.

21. THANKYOU