The Feature Control Frame

What is a Feature Control Frame?

The Feature Control Frame (FCF) is the rectangular box that contains all GD&T information for a specific tolerance. It's the fundamental building block of GD&T notation.

Every FCF reads left to right and contains up to four compartments:

┌──────────┬──────────┬────────┬────────┬────────┐
│ Symbol   │ Tolerance│ Datum A│ Datum B│ Datum C│
└──────────┴──────────┴────────┴────────┴────────┘

The Four Compartments

1. Geometric Characteristic Symbol

The first compartment contains the symbol indicating WHAT type of control:

2. Tolerance Value

The second compartment specifies HOW MUCH variation is allowed:

┌──────────┬──────────────────┐
│ ⊕        │ ⌀ 0.25 (M)       │
└──────────┴──────────────────┘

This compartment may include:

• ⌀ (diameter symbol) - indicates cylindrical tolerance zone
• Numeric value - the tolerance amount in drawing units
• Material condition modifier - (M), (L), or (S)

3-5. Datum References

The remaining compartments specify the datum reference frame - the features used as reference points:

┌──────────┬──────────┬────────┬────────┬────────┐
│ ⊥        │ 0.1      │ A      │ B      │ C      │
└──────────┴──────────┴────────┴────────┴────────┘

• Primary datum (A) - First point of contact, removes 3 degrees of freedom
• Secondary datum (B) - Second constraint, removes 2 degrees of freedom
• Tertiary datum (C) - Final constraint, removes 1 degree of freedom

Note: Form controls (straightness, flatness, circularity, cylindricity) do NOT reference datums—they control the feature's shape independent of other features.

Reading a Complete FCF

Let's read this example:

┌──────────┬──────────────────┬────────┬────────┐
│ ⊕        │ ⌀ 0.50 (M)       │ A      │ B      │
└──────────┴──────────────────┴────────┴────────┘

Material Condition Modifiers

Three modifiers can appear in the tolerance compartment:

Key Takeaways

• The FCF is a rectangular box with up to 5 compartments
• Read left to right: Symbol → Tolerance → Datum(s)
• Form controls don't use datums
• Material modifiers (M), (L), (S) affect how tolerance is applied
• The ⌀ symbol indicates a cylindrical tolerance zone

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