5 GD&T Mistakes

A drawing without GD&T is a wish. A drawing with wrong GD&T is a lawsuit waiting to happen.

And in MNC interviews, GD&T errors are one of the fastest ways to lose credibility.  

Here are 5 GD&T mistakes that immediately filter out candidates:  

1. Using +/− tolerances on critical features: If a feature needs orientation control, no amount of bilateral tolerance will help. This is a Position or Profile callout problem.  

2. Confusing Concentricity with Position: Concentricity is rarely the right call. It’s costly to inspect and almost always replaceable with Position or Runout. MNC senior engineers will catch this in 10 seconds.  

3. Datum stacking errors: Saying "Datum A B C" without understanding the degrees of freedom each one constrains — this is a beginner mistake that screams inexperience.  

4. MMC and LMC misuse: Most engineers know what these symbols mean. Few know when to use which. The interviewer’s favorite trick: "Why MMC here and not LMC?"  

5. Over-tolerancing: Tighter tolerance is not better engineering. It’s lazy engineering and expensive manufacturing. Over-toleranced drawings reveal that the engineer doesn’t understand process capability.  

Here’s the truth most engineers don’t want to hear:  GD&T is not about memorizing 14 symbols. It’s about understanding which functional intent each symbol communicates — and choosing the cheapest, most informative one.