spc sample

8/12/2019 SPC Sample

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2009 Factory Strategies Group LLC. All rights reserved.

Statistical Process ControlQuality Series


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2 2009 Factory Strategies Group LLC. All rights reserved. 2009 Factory Strategies Group LLC. All rights reserved.

Outline

What is Statistical Process Control

Variability

Control Charts

Process Capability Cp

Measuring Instruments Repeatability & Reproducibility


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Statistical Process Control (SPC)

Invented by Walter Shewhart at Western Electric

Distinguishes between common cause variability (random)

special cause variability (assignable)

Based on repeated samples from a process


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VariabilityThe world tends to be bell-shaped

Most

outcomes

occur in the

middle

Fewer

in the

tails

(lower)

Fewer

in the

tails

(upper)

Even very rare

outcomes are

possible

(probability > 0)

Even very rare

outcomes are

possible

(probability > 0)


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Variability

Add up the dots on the dice

0

0.05

0.1

0.15

0.2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Sum of dots

P

robability

1 die

2 dice

3 dice

Here is why:Even outcomes that are equally likely (like dice),

when you add them up, become bell shaped


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Normal bell shaped curve

Add up about 30 of most things

and you start to be normal

Normal distributions are divide

up into 3 standard deviations

on each side of the mean

Once your that, you know a lot

about what is going on


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Causes of Variability

Common Causes: Random variation (usual)

No pattern

Inherent in process

adjusting the process increases its variation Special Causes

Non-random variation (unusual)

May exhibit a pattern

Assignable, explainable, controllable

adjusting the process decreases its variation


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9 2009 Factory Strategies Group LLC. All rights reserved.

Control Chart

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

UCL

LCL

Sample number

Mean

Out of

control

Normal variation

due to chance

Abnormal variation

due to assignable sources

Abnormal variation

due to assignable sources


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Process vs. control limits

Distribution of averages

Control limits

Process limits

Distribution of individuals

Specification limits

Variance of

averages

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