STATISTICAL PROCESS CONTROL OVERVIEW

Manetek, Inc. is committed to maintain manufacturing processes in a state of Statistical Process Control, ensuring the machining of products meet specified requirements.

What is SPC?
Statistical Process Control is a set of techniques that are used to monitor processes in order to provide feedback. The feedback is used to maintain and improve the capability of the process. SPC, like traditional inspection, is an approach used to ensure product conformance. However, SPC is also used to control the process by signaling when adjustments may be necessary. Some techniques associated with SPC include histograms, and control charts.

A control chart is the tool developed to monitor the variation in a process. The control chart indicates if the process is in a state of control or going out-of-control. The control chart is basically the histogram turned on its side with the observations stretched out over time. This allows the operator to monitor the trends going on in the process. The control chart has upper and lower "out-of-bounds" limits. These limits and the center line are used to help evaluate trends in the process. The value of the upper control limit is calculated by adding the process average to 3 times the process standard deviation or upper part tolerance in thousands of inches. Upper Control Limit (UCL) is a line drawn all the way across the chart at this value. The Lower Control Limits is the reciprocal calculation of the (UCL) process. The control chart uses simple rules to signal when conditions may have changed. Some commonly used rules are:

  1. Any point on or outside a Control Limit
  2. A trend of consecutive points moving in the same direction, either upwards or downwards
  3. Consecutive points above or below the centerline
  4. Non-random pattern of points.

This tool is a prime force in discovering and acting to remove assignable causes to continuously improve processes. Once it has been determined that a process is acting out of control, efforts are made to determine what the cause is. This is usually handled by a team of operators, supervisors, and management.


 

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