© absl power solutions 2007 © stm quality limited stm quality limited statistical process control...

© ABSL Power Solutions 2007© STM Quality Limited

STM Quality Limited

STATISTICAL PROCESS CONTROL

TOTAL QUALITY MANAGEMENT

Introduction to S.P.C.

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What is meant by Statistical Process Control?


STM Quality LimitedThis is a control system which uses

statistical techniques for knowing, all the time, changes in the process.

It is an effective method in preventing defects and helps continuous quality improvement.

Statistical Process Control (S.P.C.)


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What does S.P.C. mean?

Statistical:Statistics are tools used to make

predictions on performance.There are a number of simple

methods for analysing data and, if applied correctly, can lead to predictions with a high degree of accuracy.


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Process:The process involves people,

machines, materials, methods, management and environment working together to produce an output, such as an end product.


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Control:Controlling a process is guiding it

and comparing actual performance against a target.

Then identifying when and what corrective action is necessary to achieve the target.


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S.P.C.

Statistics aid in making decisions about a process based on sample data and the results predict the process as a whole.


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The Process

People Machines Material

Management Methods Environment

Output


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S.P.C. is statistical analysis of the predictability and capacity of a process to give a uniform product.

Definition


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The Aim of S.P.C. - Detection Strategy

Detection: This focuses on identification of

problems after production, by 100% inspection or by customer complaints.

It is a historically-based strategy.


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The Aim of S.P.C. - Detection Strategy

Detection Drawbacks: Production is already made. Customer dissatisfaction. Inflated costs - rework;

inspection. Repetitive problems. Neglected improvements.


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The Aim of S.P.C. - Prevention Strategy

Prevention: This focuses on in-process

production and identification of problems through analysis of process capability.

It is a future-orientated strategy.


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The Aim of S.P.C. - Prevention Strategy

Prevention Benefits: Improved design and process

capability. Improved manufacturing quality. Improved organisation. Continuous Improvement.


STM Quality LimitedThe S.P.C. has to be looked at as

a stage towards completely preventing defects.

With stable processes, the cost of inspection and defects are significantly reduced.

S.P.C. as a Prevention Tool


STM Quality LimitedAssesses the design intent.

Achieves a lower cost by providing an early warning system.

Monitors performance, preventing defects.

Provides a common language for discussing process performance.

The Benefits of S.P.C.


STM Quality LimitedProcess Element Variable Examples

Machine………………………….Speed, operating temperature, feed rate

Tools………………………………..Shape, wear rateFixtures…………………………..Dimensional accuracyMaterials…………………………Composition,

dimensionsOperator…………………………Choice of set-up,

fatigueMaintenance…………………Lubrication, calibrationEnvironment…………………Humidity, temperature

Process Variations


STM Quality LimitedNo industrial process or machine is

able to produce consecutive items which are identical in appearance, length, weight, thickness etc.

The differences may be large or very small, but they are always there.

The differences are known as ‘variation’. This is the reason why ‘tolerances’ are used.

Process Variations


STM Quality LimitedCommon causes are the many sources of

variation that are always present.A process operates within ‘normal

variation’ when each element varies in a random manner, within expected limits, such that the variation cannot be blamed on one element.

When a process is operating with common causes of variation it is said to be stable.

Stability


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The process can only be termed ‘under control’ if it gives predictable results.

Its variability is stable over a long period of time.

Process Control


STM Quality LimitedGraphs and charts have to be

chosen for their simplicity, usefulness and visibility.

They are simple and effective tools based on process stability monitoring.

They give evidence of whether a process is operating in a state of control and signal the presence of any variation.

Process Control Charts


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Data InterpretationConsider these 50 measurements

Bore Diameter 36.32 ±0.05mm (36.27 - 36.37mm)

1 36.36 11 36.37 21 36.34 31 36.35 41 36.362 36.34 12 36.35 22 36.37 32 36.35 42 36.373 36.34 13 36.32 23 36.34 33 36.36 43 36.374 36.33 14 36.35 24 36.35 34 36.37 44 36.355 36.35 15 36.34 25 36.34 35 36.34 45 36.376 36.33 16 36.34 26 36.35 36 36.36 46 36.367 36.33 17 36.35 27 36.36 37 36.38 47 36.358 36.34 18 36.33 28 36.33 38 36.34 48 36.349 36.35 19 36.32 29 36.36 39 36.35 49 36.3510 36.35 20 36.35 30 36.38 40 36.35 50 36.34


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Data Interpretation

As a set of numbers it is difficult to see any pattern.

Within the table, numbers 30 and 37 were outside the tolerance – but were they easy to spot?

A way of obtaining a pattern is to group the measurements according to size.


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Data Interpretation – Tally Chart

The tally chart groups the measurements together by size as shown.

The two parts that were out of tolerance are now easier to detect (36.38mm).

36.39

36.38

36.37

36.36

36.35

36.34

36.33

36.32

36.31

36.30

36.29

36.28


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Tally Chart - Frequency

The tally chart shows patterns and we can obtain the RANGE - 36.32mm to 36.38mm.

The most FREQUENTLY OCCURRING size is 36.35mm.

36.39

36.38

36.37

36.36

36.35

36.34

36.33

36.32

36.31

36.30

36.29

36.28

2

6

7

16

12

5

2


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Tally Chart - Information

The tally chart gives us further information:

The number of bores at each size;The number of bores at the most

common size;The number of bores above and below

the most common size (36.35mm) -number above 36.35mm is 7+6+2=15number below 36.35mm is 12+5+2=19


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Histogram

We can redraw the frequency chart as a bar chart known as a histogram:

16

14

12

10

8

6

4

2

0 36.31 36.32 36.33 36.34 36.35 36.36 36.37 36.38 36.39


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Smoothed FrequencyIf we now draw a smooth curve through the top

of each box we get a bell-shaped pattern:

16

14

12

10

8

6

4

2

0 36.31 36.32 36.33 36.34 36.35 36.36 36.37 36.38 36.39


STM Quality LimitedThe bell-shaped pattern is fairly typical

of most industrial processes.There is a central value at the highest

point of the curve and the pattern of results spread out equally on both sides of the central value.

The further we move from the central value, the fewer values we will find.

This bell-shaped pattern is known as the ‘NORMAL DISTRIBUTION’.

Distribution


STM Quality LimitedThe bell-shaped pattern known as

‘Normal Distribution’ is one that we would expect to see where the process is running in a stable condition.

Where you work in a manufacturing environment, why not perform the exercise on 50 parts for yourself and see if your process is in a stable condition!

Normal Distribution


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S.P.C. is a very effective method in preventing _______.

Self-test (1 of 9)

defects

S.P.C. uses statistical techniques for knowing _______________.process variation


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Controlling a process is guiding it and comparing ________________ against a target and identifying when and what ______________ is necessary to achieve the target.

Self-test (2 of 9)

actual performance

corrective action


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The process may involve a combination of people, machines, ________, methods, management and ___________ working together to produce an output such as an end product.

Self-test (3 of 9)

materials

environment


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Self-test (4 of 9)

uniformpredictabilityS.P.C. is a statistical analysis of

the __________ and capacity of a process to give a _______ product.


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Self-test (5 of 9)

historicallyalready made

Disadvantages of 100% detection inspection are that production is ___________ and that it is __________ based.


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Self-test (6 of 9)

future

quality

An advantage of prevention inspection is that _______ is improved.

It is a strategy that is _____ orientated.


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Self-test (7 of 9)

variation

identical

No industrial process is able to produce consecutive items which are _______.

There are always differences, known as _______.


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Self-test (8 of 9)

stable

predictableThe process can only be termed

‘under control’ if it gives __________ results.

Its variability is ______ over a long period of time.


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Self-test (9 of 9)

normal

histogramA frequency chart drawn with

bars is known as a _________.

A smooth curve drawn in a bell-shaped pattern is known as the ______ distribution curve.

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Documents

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continuous quality improvement

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definition slide

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aim of