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Using a control chart in six sigma manufacturing

A control chart is one method used to see variations in a process. A successful control chart will measure the performance of a process while allowing you to see any changes or deviations in the process. Six sigma manufacturing will find control charts particularly helpful when it comes to tracking data.

How is a control chart used?

Control charts will help to determine process variation and identified processes as either in control or out of control. There are two main types of variation, categorized in the following ways:

  • Assignable cause. This type is also known as Special Cause. Characteristics of this variation include meaningful factors of process, although they may not always be present. The cause can also be avoided, but it should be investigated. In addition, Special Cause variations are not normal to the overall process. For example, a special cause could occur when you are painting or drawing, and someone bumps your arm and causes an error. This type of variation is not normal to the overall process.
  • Unassignable cause. This is also referred to as Common Cause or Chance Cause. These variations are always present and caused by chance. They are typically unavoidable in a process, but they are considered normal and should be expected.
  • Control charts used in six sigma manufacturing have three main components:
    • A center line. Typically, this is the mathematical average of all the samples plotted on the chart.

    • Upper and lower statistical control limits. These points define the specific constraints of common cause variations.

    • Process values, plotted on the chart over time.

    • Each process you put into a control chart will vary. A process can be defined as In Control if each process value is plotted within the upper and lower control limits of the chart with no real tendency. Processes plotted outside of these control limits, however, or those that show a particular tendency, are then referred to as Out of Control.
  • These control limits are determined using the following statistical formula: (Average Process Value) (3 x (Standard Deviation)) where the standard deviation is due to unassigned process variation only.
  • Using a control chart in six sigma manufacturing

  • Making a control chart for your manufacturing processes typically involves the following steps:
    • Select the process you would like to chart. You can chart multiple processes, but you will need to make a separate chart for each.

    • Determine your process sampling plan

    • Collect data from your process

    • Calculate the control chart specific statistics

    • Calculate your control limits using the formula stated above

    • Make your control chart. You can do this through a variety of different means and software programs.

    • Once you have made your control chart, you can then go about looking for variation and for special causes. There are a number of ways this can be done, but the most common is by tracking a data point that falls outside of the control limit. Even if your process is within the control limits, you should still be working to remove the special causes so your process can become more stable.
  • Using a control chart is six sigma manufacturing can be useful in a number of ways. This mathematical chart can help you to identify variations and special causes within your processes, and then determine when your processes need adjustment and what should be done.

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