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Quality Control

Quality Control. PowerPoint presentation to accompany Besterfield Quality Control, 8e. Chapter 5- Control Charts for Variables. Variation. The variation concept is : A law of nature in that no two natural items in any category are the same. Variation.

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Quality Control

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  1. Quality Control PowerPoint presentation to accompany Besterfield Quality Control, 8e Chapter 5- Control Charts for Variables

  2. Variation The variation concept is : A law of nature in that no two natural items in any category are the same.

  3. Variation • The variation may be quite large and easily noticeable • The variation may be very small. It may appear that items are identical; however, precision instruments will show difference • The ability to measure variation is necessary before it can be controlled

  4. Variation There are three categories of variation in piece part production: • Within-piece variation: Surface • Piece-to-piece variation: Among pieces produced at the same time • Time-to-time variation: Difference in product produced at different times of the day

  5. Variation (Perbezaan) Sources of Variation in production processes: Measurement Instruments Operators Methods Materials PROCESS INPUTS OUTPUTS Human Inspection Performance Tools Machines Environment

  6. Variation Sources of variation are: • Equipment: • Toolwear • Machine vibration • Electrical fluctuations etc. • Material • Tensile strength • Ductility • Thickness • Porosity etc.

  7. Variation Sources of variation are: • Environment • Temperature • Light • Radiation • Humidity etc. • Operator • Personal problem • Physical problem etc.

  8. Variation There is also a reported variation which is due to the inspection activity. Variation due to inspection should account for one tenth of the four other sources of variation.

  9. Variation Variation may be due to chance causes (random causes) or assignable causes. When only chance causes are present, then the process is said to be in a state of statistical control. The process is stable and predictable.

  10. Control Charts • Variable data • x-bar and R-charts • x-bar and s-charts • Charts for individuals (x-charts) • Attribute data • For “defectives” (p-chart, np-chart) • For “defects” (c-chart, u-chart)

  11. What is Control Chart? • Control charts are used to determine whether your process is operating in statistical control. • Basically a control chart is a run chart (described earlier) that includes statistically generated upper and lower control limits. • The purpose of a control chart is to detect any unwanted changes in your process.

  12. Control Charts Continuous Numerical Data Categorical or Discrete Numerical Data Control Charts Variables Attributes Charts Charts R P C X Chart Chart Chart Chart

  13. Control Charts for Variables The control chart for variables is a means of visualizing the variations that occur in the central tendency and the mean of a set of observations. It shows whether or not a process is in a stable state.

  14. Control Charts Figure 5-1 Example of a control chart

  15. Control Charts Figure 5-1 Example of a method of reporting inspection results

  16. Variable Control Charts The objectives of the variable control charts are: • For quality improvement • To determine the process capability • For decisions regarding product specifications • For current decisions on the production process • For current decisions on recently produced items

  17. Control Chart Techniques • Procedure for establishing a pair of control charts for the average Xbar and the range R: • Select the quality characteristic • Choose the rational subgroup • Collect the data • Determine the trial center line and control limits • Establish the revised central line and control limits • Achieve the objective

  18. Quality Characteristic • The Quality characteristic must be measurable. • It can expressed in terms of the seven basic units: • Length • Mass • Time • Electrical current • Temperature • Substance • Luminosity • as appropriate.

  19. How’s to construct? • Select the quality characteristic. • Choose the rational subgroup. • Collect the data.

  20. 4. Determine the trial center line. (CL) Central Lines are obtained using:

  21. 5. Determine the control limits for Xbar and R chart After get the value, then you draw both chart and determine the assignable causes.

  22. 6. Establish the revised central line and control limits • for both chart.

  23. 7. Construct the new chart after get the new value

  24. Achieve the Objective Figure 5-7 Continuing use of control charts, showing improved quality

  25. State of Control When a point (subgroup value) falls outside its control limits, the process is out of control. Out of control means a change in the process due to a special cause. A process can also be considered out of control even when the points fall inside the 3ơ limits

  26. Lets do exercise!

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