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Statistical Process Control

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Statistical Process Control

(SPC)

- Fitness for use
- Conformance to the standard

Quality improvement processes that involve statistical method;

- Incoming Quality Control
- Statistical Process Control
- Outgoing Quality Control

Acceptance Sampling

- Lot by lot sampling plan for attributes
- Acceptance sampling by variables

Types Of Sampling Plan

- Single Sampling Plan
- Double Sampling Plan
- Multiple Sampling Plan
- Sequential Sampling Plan
MIL STD 105E (ISO 2859)

Types Of Sampling Plan

- Plan that control the lot/process fraction defective/nonconforming.
- Plan that control the lot/process parameter
MIL STD 414 Tables

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

- 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 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

- 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

- 2 types of variation
- Natural variability (chance)
- Assignable causes

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

- 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.

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.

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

- 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.

Process out of control if

- One or more points fall outside the control limits.
- Points exhibit some non-random pattern.
- Exhibit a cyclic behaviour.

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 Chart For Fraction Nonconforming
- Control Chart For Nonconformities

Calculated using

- Process capability ratio (PCR), Cp.
- Probability ;
*need to know process std deviation

Process standard deviation is calculated by

*use to estimate process capability

And is estimated by

What does it mean if

- Cp < 1
- Cp = 1
- Cp > 1