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:
| Symbol | Name | Category |
|---|---|---|
| ─ | Straightness | Form |
| ▭ | Flatness | Form |
| ○ | Circularity | Form |
| ⌭ | Cylindricity | Form |
| ∠ | Angularity | Orientation |
| ∥ | Parallelism | Orientation |
| ⊥ | Perpendicularity | Orientation |
| ⊕ | Position | Location |
| ⌓ | Profile of Line | Profile |
| ⌔ | Profile of Surface | Profile |
| ↗ | Circular Runout | Runout |
| ↗↗ | Total Runout | Runout |
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:
| Symbol | Name | Meaning |
|---|---|---|
| Ⓜ or (M) | Maximum Material Condition | Bonus tolerance as feature departs from MMC |
| Ⓛ or (L) | Least Material Condition | Bonus tolerance as feature departs from LMC |
| Ⓢ or (S) | Regardless of Feature Size | No bonus tolerance (RFS is default) |
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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Next Lesson: Datums & Datum Reference Frames - understanding the foundation of all GD&T measurements.
