Statistical process control
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Statistical Process Control. (SPC). What is Quality?. Fitness for use Conformance to the standard. Quality Improvement. Quality improvement processes that involve statistical method; Incoming Quality Control Statistical Process Control Outgoing Quality Control. Incoming & Outgoing QC.

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Statistical Process Control

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Statistical Process Control

(SPC)


What is Quality?

  • Fitness for use

  • Conformance to the standard


Quality Improvement

Quality improvement processes that involve statistical method;

  • Incoming Quality Control

  • Statistical Process Control

  • Outgoing Quality Control


Incoming & Outgoing QC

Acceptance Sampling

  • Lot by lot sampling plan for attributes

  • Acceptance sampling by variables


Lot by Lot Sampling Plan For Attributes

Types Of Sampling Plan

  • Single Sampling Plan

  • Double Sampling Plan

  • Multiple Sampling Plan

  • Sequential Sampling Plan

    MIL STD 105E (ISO 2859)


Acceptance Sampling By Variables

Types Of Sampling Plan

  • Plan that control the lot/process fraction defective/nonconforming.

  • Plan that control the lot/process parameter

    MIL STD 414 Tables


Statistical Pocess Control

Chance and Assingable Cause Of Quality

Statistical basis for control chart

  • Basic Principles

  • Choice Of Control Limit

  • Sample Size & Sampling Frequency

  • Subgroups

  • Analysis Of Patterns On Control Charts


Control Charts For Variables (Univariate)


  • Statistical basis of the charts

  • Development and use of the charts

  • Interpretation of the charts

  • The operating characteristics function

  • Average Run Length (ARL) for the mean chart


  • Constuction & Operation of the charts

  • Control charts with variable sample size


Control Charts For The Attributes

Control Charts for the fraction non-conforming

  • Development & Operation

  • Variable sample size

  • OC and ARL

    Control Charts for non-conformities

  • Procedure with constant sample size

  • Procedure with variable sample size

  • OC and ARL


Process Capability Analysis

  • Using histogram

  • Using probality plots

  • Process Capability Ratio (PCR)

    • Cp

    • PCR for an of center process

    • Normality and PCR

    • Confidence Interval & Test on PCR

    • PCR using control charts


Chance and Assingable Cause Of Quality

  • 2 types of variation

    • Natural variability (chance)

    • Assignable causes

  • Process with assignable causes is said to be out of control.


Basic Principles

  • Control charts consist of

    • Center line

    • Upper control limit

    • Lower control limit

      These limits is chosen so that when the process is in control, almost all the sample points will fall within the control limits.


Choice Of Control Limit

2 types of control limits

  • Three-sigma limits.

    • The distance between CL and the UCL/LCL is 3 sigma.

  • The 0.001 probability limits chart (use 3.09 sigma).

    • The distance between CL and the UCL/LCL is 3.09 sigma.

      Note:-

      3-sigma limits popular in US.

      0.001 prob. limits popular in UK & Western Europe.


Sample Size & Sampling Frequency

  • Larger samples easier to detect.

    • Use large sample if the shift of interest is small and small sample if the shift of interest is large.

  • Frequent sampling is better.

    Current practice favour small but more frequent samples.


Analysis Of Patterns On Control Charts

Process out of control if

  • One or more points fall outside the control limits.

  • Points exhibit some non-random pattern.

  • Exhibit a cyclic behaviour.


Pattern recognition

Process is out of control if

  • One point plots outside the control limit.

  • Two out of three consecutive points plot beyond the two sigma warning limits.

  • Four out of five consecutive points plot at a distance of the one sigma or beyond from the center line.

  • Eight consecutive points plot on one side of the center line.


Control Charts For Variables (Univariate)


Control Charts For Attributes

  • Control Chart For Fraction Nonconforming

  • Control Chart For Nonconformities


Process Capability

Calculated using

  • Process capability ratio (PCR), Cp.

  • Probability ;

    *need to know process std deviation


Process standard deviation

Process standard deviation is calculated by

*use to estimate process capability


Process Capability Ratio (PCR), Cp

And is estimated by


Interpretation

What does it mean if

  • Cp < 1

  • Cp = 1

  • Cp > 1


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