Trying to set up a serious PPM system - Parts per Million
We are trying to set up a serious PPM system to control the quality of our suppliers. there seem to be various definitions of 'the number of opportunities for error'. Does anyone know of a standard definition that all our suppliers can recognise. eg a hole can be wrong place, wrong diameter, wrong depth, wrong finish, out of circular, out of perpendicular, wrong bottom profile etc etc. This mus be laid down somewhere??
Looking into the outlines for training I have given in the past, I have given the following for a definition of Parts Per Million or PPM:
One nonconforming (or defective) item in a collection of 1 million items is called one part per million. A nonconforming or defective item is classified this way after the first anomoly that causes the item to be out of acceptable limits/tolerances.
So based on your question, if a part has 7 defects, any one of them alone causes the part to be defective - all seven would tell you that there is not one root cause but probably many you would need to go after in order to reduce the rate. There is no absolute scale by which a component/parts
quality can be evaluated. The quality levels required depend on the complexity of the systems built from the components/parts. Although your part has seven identifiable defects, the first one found caused it to be
classified as defective or nonconforming. The rest of the defects found, although indicative of a larger problem in any given process, do not count when you are calculating ppm since you are looking at more than one part produced and the production rate of conforming parts. If you are looking
for defect rate regardless of number of parts or number of defects found on a single part, this would change. Now the 1 million parts would be counted by adding up all the possible defects for a single part then dividing 1 million by that number to determine how many parts it would take if all
defects were found in every part to reach a million. That number of parts would constitue the total number needed and each would be inspected for each defect. Now you are looking at ppm but for defect found and not part produced.
As far as documenting these values - the number of defects allowed for 1 million parts produced or the total number of defects found in a sample group that would represent 1 million defects if all are found - yes. How
would people know what the goals are or if they are achieving those goals if
they are not published/documented somewhere. This would be part of a continuous improvement program.
Trying to set up a serious PPM system - Parts per Million
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