Control charts – 7QC Tools

Control charts are tools used to determine if a manufacturing or business process is in a state of statistical control. It is a line Graph used to monitor the process variation and helps understand how a process changes over time. By capturing the current data from the process and comparing it with the historical data we can understand whether the current process has only common cause variation and does not have any special cause variation. The process with only common cause variation is a good process and is predictable.


Let us understand about variation in detail.

Any process will have variation. The variation is of basically two sources Common cause variation and Special cause variation.

Common cause variation: The variation that is inherent of the process is identified as common cause variation. Common cause variation can be reduced but cannot be eliminated.

Example: Voltage measured in the welding process will have variation. This variation cannot be eliminated but however can be reduced.

Special cause variation: The variation that occurs due to specific reasons is called as special cause variation. Since the reason for the variation is  known the variation can be eliminated.

Example: Diameter oversize in a machining process due to a broken fixture clamping bolt is a special cause variation. since broken clamping bolt is the reason the same can be eliminated by replacing it.

Objectives of using a Control Chart

  • To understand the current variability in the process and control the process to continue operate within this variability.
  • To improve the process continually by tracking down the sources of variability and reducing variability to the minimum level thereby keeping the process at the economic minimum level.

Benefits of using a Control Chart

  • Preventive inspection and corrective action.
  • Decision based on facts rather than subjective judgment
  • Improved inspection methods
  • Improved flow of material
  • Quality consciousness

Few books recommended to read on Control charts are given below

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