GD&T for CNC Machining: Practical Guide
Geometric Dimensioning and Tolerancing (GD&T) explained from a machinist's perspective. What symbols matter most, how they affect cost, and when to use them — or not.
GD&T is one of those topics where engineering schools teach the theory, but production shops live the reality. The difference between a drawing that uses GD&T well and one that uses it poorly can be 30-50% in machining cost. Here's what actually matters in practice.
The Six GD&T Controls That Matter Most in CNC
| Symbol | Control | What It Measures | Impact on Cost |
|---|---|---|---|
| ⏤ | Flatness | Surface deviation from a perfect plane | Low — grinding pass |
| ◎ | Concentricity | Axis alignment between features | High — single setup critical |
| ⟂ | Perpendicularity | Squareness of a feature to a datum | Moderate — fixture dependent |
| ⏀ | Position | Hole location relative to datum | Moderate — CMM verification |
| ⌂ | Surface Profile | 3D surface deviation from CAD | High — 5-axis + CMM required |
| ↻ | Circular Runout | Roundness variation during rotation | Moderate — between centres |
The Machinist's Take on GD&T
The most common GD&T mistake we see is over-specification. Someone puts a surface profile tolerance of 0.05mm on a face that's just a datum reference. That tolerance requires 5-axis machining + CMM inspection, when the part could be made on a 3-axis machine to ±0.1mm flatness for half the cost.
Rule of thumb: only use GD&T when the feature's function requires it. A sealing face needs flatness. A bearing bore needs concentricity. A clearance hole does not need true position — just call it out with ±X/Y dimensions.
The second most common issue is inconsistent datum structure. Pick one or two datums and dimension everything from them. We've seen drawings where the primary datum changes between views — that's a recipe for measurement disputes.
And here's something the textbooks don't tell you: CMM programs take time to write and verify. A part with 5 GD&T callouts might need 30 minutes of CMM programming. A part with 50 callouts might need 4 hours. That programming cost shows up in the quote whether you see it as a line item or not.
When You Can Skip GD&T
For simple brackets, plates, housings, and non-critical components, a title block tolerance of ±0.1mm or ±0.05mm and coordinate dimensions on the drawing are perfectly adequate. GD&T adds cost. Use it where the function demands it, and save it where it doesn't.
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