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Understanding Process Control Charts

Learn about Control Charts, a technique for online process monitoring to determine process capability and identify control limits. Discover the components of Control Charts and the formulas used to calculate control limits, center line, and ranges. Find out how to identify processes that are out of control and need restructuring.

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Understanding Process Control Charts

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  1. Process Control Charts By: Brian Murphy

  2. Control Charts are an on-line process-monitoring technique. • Used to determine if a process is capable or out of control. • In control processes operate with only chance causes of variation. • Out of control processes have assignable causes of variation and can be restructured when identified.

  3. Control Chart Contains • A center line (usually the arithmetic mean or the average of a collection of sample ranges) • A Lower and Upper Control Limit (LCL and UCL) • Points beyond the control limits indicate that the process is out of control.

  4. Formulas: • xave = grand average of x = (x1+x2+…+xm)/m Or R=xmax-xmin R1, R2,…,Rm: ranges of m samples Rave = (R1+R2+…+Rm)/m

  5. Control Limits for R chart: • UCL = D4*Rave • Center Line = Rave • LCL = D3*Rave • Control Limits for x chart: • UCL = xave + A2*Rave • Center Line = xave • LCL = xave –A2*Rave *See next slide for values of D3, D4 and A2

  6. *n denotes sample size

  7. Rules for determining out of control process:

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