0001// Why part OOS still can meet functional requirement?

As a manufacturing engineer, I believe that, you will notice some of the parts can meet functional requirement even the part is out of specification (OOS).

Why part OOS still can meet functional requirement such as alignment, mating, air flow, thermal conduction and etc?

Below shown the explaination for this phenomenon (this explaination only applicable to alignment and mating).

Assume that, shaft require to assemble at desired position (in this case, position tolerance zone) to ensure assembly meet functional requirement. At the same time, assume shaft fabricated at perfect form.

First figure shown that, hole located exactly at true position. (which designer like to see). So, the shaft can mate with the hole and meet the functional requirement because the geometrical configuration of the hole and shaft is within the tolerance which designer expected.

Second figure shown that, hole located at wrong position. (which designer not willing to see). However, size of the hole is so huge (assume still meet feature of size specification) allow shaft located at position same as first figure. We known that, as long shaft located at desired position, it meet functional requirement. Therefore, shaft and hole assembly in second figure still meet functional requirment.

Third figure, the hole located same position as second figure. However, this time, our assembly technician unable to assemble the shaft to desired position. As as result, this shaft and hole assembly cannot meet functional requirment.

Conclusion,

Part which meet spec is expected to meet functional requirement all the time;

Part which out of spec may not expected to fail functional requirement all the time.

In other words,

Behavior of part which meet spec is predictable;

Behavior of part which not meet spec is not predictable.