Understanding GD&T in CNC Machining: Why It Matters for Functional Parts

In precision manufacturing, dimensions alone aren't enough to ensure a part will fit and function. That’s where GD&T—Geometric Dimensioning and Tolerancing—comes in.

GD&T is a symbolic language used on 2D drawings to define how a part feature must behave, not just what it measures. It controls things like straightness, roundness, and how one feature relates to another—critical in die-cast + CNC machined components that interface with mating parts.

🧠 Key GD&T Symbols and What They Mean:

• True Position (⭘) – How far a feature can deviate from its perfect location

• Flatness (⏥) – Ensures a surface is flat within a tolerance zone

• Perpendicularity (⊥) – Controls how square one feature is to another

• Circularity (○) – Ensures round features stay round, even after machining

• Profile (⌒) – Used for complex shapes or surfaces that must match a form

✅ Why GD&T Is Important:

1. Communicates Functional Intent – It tells the machinist what matters most about a feature

2. Improves Inspection Consistency – GD&T defines clear zones for quality checks

3. Supports Interchangeability – Ensures parts fit into assemblies without trial-and-error

4. Reduces Over-Engineering – You don’t have to tolerance everything tightly—just the functional zones

At LVIO Precision, we interpret GD&T callouts to ensure that machining and CMM inspection align with your product’s intended function. We’ll advise when a tolerance is unnecessarily tight—or when a critical relationship might benefit from added GD&T control.