SPC - Statistical process control

SPC

For Internal Use Only Presentation By Senthilkumar RM

Today Learning's !!

1.Product Characteristics
2.Source of Process
3.Process Types
4.Variation & Its Causes
5.Tolerance
6.Standard Deviation
7.Interpretation of Stdev with Distribution
8.Calculating Std deviation
9. SPC ,Control Chart Selection & It interprets
10.Cp,CpK,Pp,Ppk


Product Characteristics

• 2 Types of Product Characteristics
• Variable
• Attribute

• Variable – A Characteristics measured in Physical units
eg : Millimeter, Volts
• Attributes – A characteristics measured by comparison
eg : Good and Bad


Source of Process


Process Types

All Process have
• Natural Variability (due to Common cause)
• Un-Natural Variability (due to Special cause)

• Stable Process – Variation in Output is by only
common cause
• Unstable Process – Variation in Output is by both common &
Special cause


Variation & Its causes

Variation : Inevitable changes in output over time

Two causes of Variation
• Common Cause : Implemented in process due to design
of process
• Special Cause : Not Present in process arises in
certain circumstances

Answer :
• Poor design ?
• Broken part ?


Tolerance

Tolerance : Allowed Maximum variation in dimension

Otherwise called as “Engineering Tolerance”


Quick recap

1. Two types of Product characteristics?
2. 5 source of Process?
3. Two types of Process?
4. Two Cause of variation ?
5. What is Engineering Tolerance ?

How Process speaks ?
What is Within control and Out of control in a process?


Standard Deviation (σ)

Standard Deviation is a measure of Variability . It is denoted by
Greek letter σ . In Other words Data dispersion around mean

What is Population and Sample?


Distribution & Standard Deviation and 3σ Level


Practical example to understand Distribution

Now, we can create an example of our own and understand
2. What is Observation X
3. What is Mean (X bar)
4. How to calculate std deviation?
5. What it interprets in distribution?
6. Histogram or Frequency Chart? Explanation
7. How bell curve is derived? Explanation
8. What is 6 σ ? What it tells? % derivation?


What is SPC

SPC : Statistical Process control
• Statistical tool
• Inspects random sample from a process
• To determine whether the process is working properly or not
• Forms Charts with data (Control charts) and Process speaks
• Developed by Walter Schewart


Why We need to care about SPC ?

Suppose Supplier A parts are with in Spec
Supplier B parts are within Spec, But

DON’T FIT TOGETHER WHY?


Why We need charts

Data Charts
Information

Tools


Control charts

It is a Chart, named as control charts because it interprets to
Us whether the process is in control or not

Remember, Two Types of data
• Variable data
• Attribute data

Based on data, We need to select the control charts !!

There should be some common factor also

Sample size


Selection of Control charts

Based on Sample size & Data Type – Selection of chart done

• Variable data

• Attribute data


I – IR- MR Charts
Sample size = 1

What : “I” stands for Individual; R Stands for Range; MR
stands for Moving Range
When : Situation like data are limited such as production low,
testing or Homogeneous Batches (Heat Treatment)
How : By Plotting Individual Measurements and Moving range
in separate graph and adding Center line


X bar – X bar R – X Bar S chart Sample size = 3 ~5

What : “X Bar” stands for average; R Stands for Range;
When : Situation like data are limited such as production High,
How : By Plotting average Measurements and range
in separate graph and adding Center line with spec
(Sub Group-->>>>>)


Attribute Charts

Attributes data – are “ Count data” where each data is classified in one of two
categories eg., Good or Bad ; Pass or Fail

Defects – is “ Non conformity ” eg., No.of defects in that unit / board / piece

Defectives– is “ Non conformance ” eg., That unit / board / piece which has one
or more defects is called defectives

What are Defects in item
Which of the following is the source of
Attribute data’s Cap
Loose
1. Cap loose (one of defect)
2. Label wrong
•Testing – Why ? 3. Breakage
•Productivity – Why ?
Label
•Caliper measurement – Why ? wrong What is Defective here
•Ring gage – Why ? Breakage
•Answer paper– Why ? WATER BOTTLE (unit)

19

Chart Types


Selection of Chart : Attribute

See Sample size first
Constant Sample size – C chart or nP chart
Variable Sample size – P chart or U chart
See type of data
Defects – C chart or nP chart
Defectives – P chart or U chart


Now Tell


Short Term (Sst) & Long Term (Slt) Standard Deviation


Cp,Cpk,Pp,Ppk

Cp – Process Capability –
• Ability of a production process to meet desired spec
• Common Cause capability
• Short –Term
Pp – Process Performance –
• Ability of a production Process to meet desired spec
• Includes special cause
• Long – Term

Cpk, Ppk are indexes – Numbers of Cp, Pp


Calculate Cp, Cpk

Cp = Engineering Tolerance / 6 σ

Engineering Tolerance = USL – LSL

Engineering Tolerance with Std Deviation = USL- Avg/ 3 σ
= Avg – LSL/ 3 σ

Cp = Engineering Tolerance / 6 * Sst – (see next page)
(Question here)

Cpk = Smaller of Engineering Tolerance with Std Deviation
(Question here)


Cp,Cpk,Pp,Ppk


Data for calculating Cp.. Illustration

Observations =

Customer Spec = 60 +- 5

6. n=
7. Avg =
3.Std dev =
4.Eng Tolerance =
5.Cp =
6.Cpk =

Lets do this XL


Higher the better


SPC - Statistical process control

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