Errare humanum est to err is human. But if mistakes can be avoided, so much the better.
Very often, it is hands-on experience and learning from our own mistakes that enable us to prevent similar issues from occurring again.
One such experience took place a few years ago when we were asked to manufacture a component designed by a customer that featured a blind hole (Ø6 mm × 12 mm deep).
For dimensional inspection, we considered the use of a GO/NO-GO plug gauge sufficient. This type of gauge is commonly used to verify whether a hole—threaded or unthreaded—falls within the dimensional tolerances specified on the drawing.
During production, however, an unexpected situation emerged. The hole exhibited a slight taper, with the larger diameter located at the bottom of the blind hole. This subtle geometric deviation went undetected because the GO/NO-GO gauge only verified the acceptable diameter limit at the gauging point and could not identify diameter variations along the depth of the hole.
So, what was the consequence?
The component was designed to receive a press-fitted dowel pin. However, the required interference fit existed only at the entrance of the hole, while the internal taper left the pin free to move once inserted deeper into the bore.
The result was that the hole appeared to comply with the specified tolerance, yet the interference fit was insufficient for the intended application.
Looking back, we realized that, in cases like this, a GO/NO-GO plug gauge is neither the most appropriate nor a sufficient inspection method. A more comprehensive inspection—using, for example, a dial bore gauge or a coordinate measuring machine (CMM)—would have been necessary to measure the bore diameter at three different depths, making it possible to detect taper or other geometric irregularities.
As is often the case, an engineering assessment should never focus on a single characteristic alone. It must consider the complete functional context and evaluate all the factors that may affect the component’s performance.
When it comes to dimensional inspection, the selection of the measuring instrument should always be based not only on the component’s functional requirements but also on the specified tolerances, ensuring an accurate and reliable verification process.
To achieve this,
clear communication between the customer (or design engineer) and the supplier is essential to avoid misunderstandings and to identify the most appropriate inspection method from the very beginning.
This experience was one of many that reinforced the importance of carefully selecting the right inspection strategy, even for checks that may initially appear straightforward. It also highlighted the level of expertise, critical thinking and responsibility required of quality engineers to ensure that components meet not only their dimensional specifications, but also their intended functional performance.
Dimensional inspection: a lesson learned in the field
As is often the case, an analysis should not be limited to a single parameter but must take the overall context into account, carefully evaluating all variables that could influence the final result.
