Higher National Certificatein Engineering

Unit 36Lesson 1 - Statistical Process

Control

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Statistical ProcessControl

• Learning Outcomes– LO 1 – Understand the basic types, variations and

characteristics of statistical techniques used in process control– LO 2 – Be able to select data, construct process control charts

and initiate a control programme for a specified application– LO 3 – Be able to evaluate process capability against a given

product or component quality requirement using modified control chart limits

– LO 4 – Be able to analyse types of variation within a process and record information on that variation

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Learning Outcome 1• LO 1 – Understand the basic types, variations and

characteristics of statistical techniques used in process control.– 1.1 evaluate the two basic types of inspection used in sampling for

process control– 1.2 describe the significance of natural and assignable causes of

variation– 1.3 use selected data to construct frequency distribution and calculate

mean, range and standard deviation– 1.4 relate the characteristics of the normal curve to the distribution of

the means of small groups

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Learning Outcome 1.1

• LO 1.1: evaluate the two basic types of inspection used in sampling for process control

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So What is SPC?• Statistical: the collection, representation and interpretation

of data.– Statistics provide a means of assessing risks and predicting

results– SPC actually involves the handling of data and understanding

a basic chart.• Process: any activity is a process

– An activity (and hence a process) is a combination of method, people, materials, environment and equipment

• Control: controlling the activity

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So What doesControl look like?

• Processes (activities) are initially brought under control and then improved by reducing the variability about the nominal or, if appropriate, reducing the level of rejects to zero.

• Control means avoiding fire-fighting: the instantaneous reaction to problems as and when they occur

• Control means planning and prediction.• It should be recognised that when a process is brought into

control it is then the job of management to continue to improve that process! (IMPROVEMENT SHOULD NEVER STOP!)

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Some History

• Fischer - agriculture• Dr Walter Shewhart – Bell telephone labs

– Application during WW 2• Dr W. Edwards Deming

– Re-building of Japan

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So What doesControl look like?

• https://www.youtube.com/watch?v=GHvnIm9UEoQ&index=2&list=PLFF3iDLW3ARBV3MRiGdDUvfauXJO22Rvn&spfreload=10

• https://www.youtube.com/watch?v=3pmQGTJQ_nc&list=PLFF3iDLW3ARBV3MRiGdDUvfauXJO22Rvn&index=18&spfreload=10

• https://www.youtube.com/watch?v=578gaOpT6PU&index=21&list=PLFF3iDLW3ARBV3MRiGdDUvfauXJO22Rvn&spfreload=10

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Why introduce SPC?• External Pressure

– Demand by customers• Internal Benefits – see next slide• Survival

– Perceived competitive advantage• Quality Costs

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Why introduce SPC? NDGTA

Quality costs as a percentage of sales

5

10

15

20

PreventionAppraisal

Internal Failure

External Failure

Benefits of SPC• Less downtime• Improved performance• Fewer defectives• Fewer production interruptions• Fewer complaints• Increased capacity• Less scrap• More involvement from operatives• Less trouble at next operation• Savings in time

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Detection System NDGTA

Method People Material

Environment Equipment

OUTPUT

PROCESS

Information on

performance

Action on output

Detection System• Traditional based on inspection• 100% inspection is only about 80% efficient• Repeated inspection see this figure actually reduce!• Inspection based systems are expensive• Inspection based systems can be demotivating• Focus is on scrap, rework, recycling, warranty claims

and concessions

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But how ‘good’is inspection?

• If Alfred Fields had though of it first, few of them would have listened and, of those, fewer would have cared because that’s the effect Alfred has on all of us.

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But how ‘good’is inspection?

• If Alfred Fields had though of it first, few of them would have listened and, of those, fewer would have cared because that’s the effect Alfred has on all of us.

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See if your Inspectionhas improved with feedback!

• First of all she followed all the rules of the game (as suggested by her friend). But if her friend had offered to find the fundamental principle before she began, her understanding of what was expected of her would have been greatly enhanced before the off.

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Did you achieve 100% !!

• First of all she followed all the rules of the game (as suggested by her friend). But if her friend had offered to find the fundamental principle before she began, her understanding of what was expected of her would have been greatly enhanced before the off.

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So how good is Inspectionas a tool to ensure your Quality?

• Statistics show that 100% quality inspection is only, at best, about 95% effective.

• So why not inspect things twice! Would you identify 100% of the defects if you inspected things twice?

• Statistics show that inspecting things twice the chance of finding all the defects fall to 88%. Why do you think this is?

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Quality Inspection

• The key point therefore is that you cannot depend upon inspection to identify all your quality defects!

• Why therefore are you wasting your time?

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The Traditional Approach to Quality

• Inspection• Detection of Defects• Rejection• Rectification or Scrap• This approach tolerates poor quality – it

does nothing to avoid it!

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To remain viable, we must therefore…

• …continually strive to reduce costs.• So what does traditional approach to

quality cost?– To gain an understanding of what quality

costs in your company, try answering the following questions…

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The Cost of Administering Traditional Quality

• How many Inspectors do you have?• How much does its cost for their services?• What is the cost of rework?• What is the cost of scrap?• What is the cost associated with your workforce

reworking / scrapping defects?• What are your warranty costs?• Why are companies adding all this expense to

control ensure quality. In effect you are throwing away money?

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Statistical ProcessControl (SPC)

• Fortunately there is an alternative to the traditional approach to quality, that based on Statistical Process Control.

• The basis of this approach is..– Collect information– Record– Analyse– Act

• SPC is an approach based upon prevention rather than detection!

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Prevention System NDGTA

Method People Material

Environment Equipment

OUTPUT

PROCESS

Information on

performance

Action on output

Information on process

Action on process

Prevention System• Requires operatives to be given ‘tools’ to interpret the

process• Requires operatives to become more involved in finding

solutions to problems which has serious implications for the way companies so business.

• Has implications for the suppliers which spills over to regarding the process upstream as you internal supplier and you becoming an internal customer of the process downstream.

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