CNC Tolerance Guide: Understanding IT Grades and GD&T

CNC Tolerance Guide: Understanding IT Grades and GD&T

What do IT grades mean? How does GD&T improve part quality? A practical guide for buyers and engineers.

Tolerances determine how much variation is acceptable in a machined part. Understanding tolerances helps you get parts that fit and function correctly — without paying for unnecessary precision.

Why Tolerances Matter

Tight tolerances require:

• More precise machines
• More setup time
• More inspection effort
• Higher skill operators

All of these increase cost. That is why specifying the correct tolerance for each feature is one of the most important decisions in part design.

IT Grades: ISO 286 Standard

IT (International Tolerance) grades define standard tolerance ranges for different nominal sizes. Common IT grades in CNC machining:

Grade	Tolerance	Typical Use
IT5	±0.005mm	Precision fitting, bearings
IT6	±0.01mm	General precision
IT7	±0.02mm	Standard fits
IT8	±0.03mm	General machining
IT9	±0.05mm	Commercial accuracy
IT10	±0.1mm	Rough fit, non-critical

General Tolerances (ISO 2768)

Instead of specifying every dimension, many buyers use general tolerances:

• m (medium): ±0.1mm for letters, ±0.02mm for numbers
• f (fine): ±0.05mm for letters, ±0.01mm for numbers
• c (coarse): ±0.2mm for letters, ±0.05mm for numbers

ISO 2768-m is the most common standard for CNC machined parts.

GD&T: Geometric Dimensioning and Tolerancing

GD&T is a more precise system that defines not only size tolerance but also geometric relationships between features. It uses symbols to specify:

• Flatness: Surface must be within a specified plane
• Straightness: Axis or surface must be straight
• Circularity: Cross-section must be round
• Perpendicularity: Surface must be 90° to reference
• Parallelism: Surface must be parallel to reference
• Position: Feature position relative to datum
• Profile: Profile of a surface must be within bounds

Common Tolerance Mistakes

• Specifying tighter tolerances than needed: Costs more without benefit
• Ignoring accumulated tolerance: When multiple parts stack, tolerances add up
• No datum reference: Without datums, tolerances are ambiguous
• Treating GD&T as optional: Use it consistently for complex parts

How to Specify Tolerances on Your Drawings

1. Identify critical dimensions — features that affect fit, function, or assembly
2. Apply tight tolerances only to critical dimensions (±0.01–±0.02mm)
3. Use general tolerances for non-critical features (±0.1mm)
4. Consider GD&T for complex geometries and multiple feature relationships
5. Clearly state datum references

Need Help with Tolerances?

If you are unsure what tolerances your part needs, we can review your drawings and suggest appropriate specifications.