Page 217 - DMGT524_TOTAL_QUALITY_MANAGEMENT

Total Quality Management

Notes Control charts work on the basis of “6 sigma control”. The six-sigma value is derived from
looking at distance ± 3σ from the mean (µ), for a range of 6σ total.
When applying control charts it is common practice to establish the control limits based on the
process capability study and then use fix limits on the chart during production. They should be
reevaluated regularly depending on production volume, but at least once every 3 months to
minimise error in control chart analysis.
Control limits can be managed to prioritize the efforts of operators and help allocate limited
resources on the shop floor. This is done by setting control limits at 3 sigma for critical
characteristics and perhaps 4 or 5 sigma for less critical characteristics.

14.4.2 Patterns in Control Charts

One Point Outside Control Limits: Indicates variation due to a special cause like a sudden
power surge, a broken tool, measurement error, or an incomplete or omitted operation in
the process.

Sudden Shifts in the Process Average: Indicates sudden shifting of the process average.
Cycle: Short and repeated patterns in the chart with alternating high peaks and how
valleys called cycles are the result of cause that come and go on a regular basis.

Trends: A trend is the result of some cause that gradually affects the quality characteristics
of the product and causes the point on a control chart to gradually move up or down from
the centre line.
Hugging the Centre Line: It occurs when nearly all the points fall close to the centre line
indicating that the control limits are too wide.

Hugging the Control Limits: The pattern shows up when many points are near the control
limits with few in between.
Control charts are designed to be used by production operators rather than by inspectors or
quality control personnel. The use of control charts allows the operators to react quickly to
special cause of variation.
Control charts help to identify key input variables causing the process to shift and aid in the
reduction of the variation. Control charts are used to detect changes in the process. They are also
used as part of a capability study to demonstrate that the process is stable or consistent.

Elements of Typical Control Chart

1. Horizontal axis for sample number

2. Vertical axis for sample statistics e.g. mean, range, standard deviation of sample
3. Target Line
4. Upper control line
5. Upper warning line
6. Lower control line

7. Lower warning line
8. Plotting of sample statistics
9. Line connecting the plotted statistics

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