CNC Machining Tolerances Explained: IT Grades, GD&T Basics, and What ±0.01mm Really Means (2026)
CNC Machining Tolerances Explained: IT Grades, GD&T Basics, and Cost Impact (2026)
Key fact: Standard CNC machining achieves ±0.05mm to ±0.01mm (IT7–IT9) without special effort. Tightening from ±0.05mm to ±0.005mm can double your part cost. Understanding tolerance grades and how to specify them correctly is the single biggest cost lever in CNC machining.
IT Grade Tolerance Chart for CNC Machining
| IT Grade | Tolerance at 30mm | Tolerance at 100mm | Typical Process | Cost Level |
|---|---|---|---|---|
| IT5 | ±0.0045mm | ±0.0075mm | Precision grinding | $$$$$ |
| IT6 | ±0.0065mm | ±0.011mm | Precision grinding, honing | $$$$ |
| IT7 | ±0.010mm | ±0.017mm | High-end CNC, cylindrical grinding | $$$ |
| IT8 | ±0.016mm | ±0.027mm | Standard CNC (tight control) | $$ |
| IT9 | ±0.026mm | ±0.043mm | Standard CNC machining | $ |
| IT10 | ±0.042mm | ±0.070mm | CNC turning, basic milling | $ |
| IT11 | ±0.065mm | ±0.110mm | Rough machining, drilling | Baseline |
| IT12–IT14 | ±0.10–0.25mm | ±0.17–0.43mm | Sawing, rough operations | Baseline |
Achievable Tolerances by CNC Process
| Process | Standard Tolerance | Precision Tolerance | Limiting Factor |
|---|---|---|---|
| 3-Axis Milling | ±0.02–0.05mm | ±0.005–0.01mm | Tool deflection, thermal expansion |
| 5-Axis Milling | ±0.02–0.05mm | ±0.005–0.01mm | Machine calibration, setup |
| CNC Turning | ±0.01–0.03mm | ±0.003–0.008mm | Tool wear, workpiece rigidity |
| Swiss Turning | ±0.005–0.01mm | ±0.002–0.005mm | Guide bushing wear |
| Cylindrical Grinding | ±0.002–0.005mm | ±0.0005–0.002mm | Wheel dressing, cooling |
| Wire EDM | ±0.005–0.01mm | ±0.002–0.005mm | Wire tension, flushing |
GD&T Symbols Cheat Sheet for CNC Drawings
| Symbol | Name | What It Controls | CNC Note |
|---|---|---|---|
| ⏤ | Flatness | Surface deviation from plane | 0.02–0.05mm typical for milled faces |
| 〇 | Circularity | Roundness of a feature | 0.01–0.03mm typical for turned parts |
| ∥ | Parallelism | Two surfaces parallel to each other | 0.02–0.05mm achievable |
| ⊥ | Perpendicularity | 90° relationship between features | 0.02–0.05mm achievable |
| ◎ | Concentricity | Shared center axis | 0.01–0.03mm for turned parts |
| ⌖ | Position (True Position) | Location of a feature | 0.05–0.1mm typical; tighter is costly |
| ⟂ | Profile of a Surface | 3D surface contour | 0.05–0.1mm typical for 3-axis |
| ○/✕✕ | Runout (Circular/Total) | Wobble during rotation | 0.01–0.03mm for turning |
The Real Cost of Over-Tolerancing
The #1 mistake in CNC drawings is specifying tighter tolerances than needed. A 2025 industry survey found that 40% of CNC drawings have at least one unnecessarily tight tolerance. This costs buyers an average of 25–50% more per part with no functional benefit.
Rule of thumb: Only tighten tolerances on functional interfaces — bearing seats, press fits, sealing surfaces, and alignment features. Use ±0.1mm or the title block default for everything else.
More Quality & Precision Guides
- CNC Machining Surface Finish Guide: Ra Values, Methods, and Cost Impact Compared (2026)
- CNC Machining Quality Control Explained: CMM, FAI, SPC, and Inspection Reports (2026)
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