Precontrol or Narrow Limit Gaging Sometimes known as StopLight Gaging

Precontrol is a control technique develop by Dorian Shannin in the 1950's to force process setups to fall within the middle half of the allowed specification tolerances, followed by frequency checks to assure continued conformance to requirements.

Preconditions:

  1. Process must be governed by the Normal Distribution
  2. The process should have a Capability Index of at least 1.33 to minimize resetting the process. A CI of 1.66 is highly recommended.

 Rules:

  1. Divide the specification tolerance band with PC lines at 1/4 and 3/4 of the tolerance.
  2. Start job
  3. If piece is outside specification limit, reset
  4. If one piece is inside specification limit but outside a PC line, check next piece
  5. If second piece is also outside same PC line, reset.
  6. If second piece is inside PC line, continue process and reset only when two pieces in a row are outside a give PC line.
  7. If two successive pieces show one to be outside the high PC line and one below the low PC line, action must be taken to reduce variation.
  8. When five successive pieces fall between the PC lines, frequency gaging may start. 
  9. While waiting for five, if one piece goes over a PC line, start count over again.
  10. When frequency gaging, let process alone until a piece exceeds a PC line.  Check the very next piece and proceed as in 6 above.
  11. When machine is rest, five successive pieces inside the PC line must again be made before returning to frequency gaging.
  12. If the operator checks more pieces than 25 times without having to reset, the gaging frequency may be reduced so that more pieces are made between checks. 
  13. If, on the other hand, the operator must reset before 25 checks are made, increase the gaging frequency.  An average of 25 checks to a reset is indication the gaging frequency is correct.

 
Return