Form Controls
What are Form Controls?
Form controls define the allowable deviation in the shape of a single feature, independent of any other features. They are the only GD&T controls that do not reference datums.There are four form controls:
| Symbol | Name | Controls |
|---|---|---|
| ─ | Straightness | How straight a line or axis is |
| ▭ | Flatness | How flat a surface is |
| ○ | Circularity | How round a cross-section is |
| ⌭ | Cylindricity | Combined roundness + straightness |
1. Straightness (─)
Definition: Controls how straight a line element or axis must be.Surface Straightness
Applied to a surface, each line element must lie within two parallel lines:
FCF: ┌───┬──────┐
│ ─ │ 0.1 │
└───┴──────┘
════════════ 0.1 tolerance zone
~~~~~~~~~~~~ (actual surface)
════════════Axis Straightness
Applied to a feature of size (with ⌀), the axis must lie within a cylindrical zone:
FCF: ┌───┬────────┐
│ ─ │ ⌀ 0.1 │
└───┴────────┘2. Flatness (▭)
Definition: Controls how flat a surface must be.The entire surface must lie between two parallel planes:
FCF: ┌───┬──────┐
│ ▭ │ 0.05 │
└───┴──────┘
═══════════════ 0.05 tolerance zone
~~~~~~~~~~~~~~~ (actual surface)
═══════════════
Use Case: Critical sealing surfaces, mating faces, reference surfaces.
Note: Flatness controls the entire surface at once, while straightness controls individual line elements.
3. Circularity (○)
Definition: Controls how round a circular cross-section must be.Also called "roundness." Each cross-section must lie between two concentric circles:
FCF: ┌───┬──────┐
│ ○ │ 0.02 │
└───┴──────┘
╭───╮
╱ ○ ╲ 0.02 zone between
│ │ two concentric circles
╲ ╱
╰───╯4. Cylindricity (⌭)
Definition: Controls the entire surface of a cylinder—combines circularity, straightness, and taper.The entire surface must lie between two coaxial cylinders:
FCF: ┌───┬──────┐
│ ⌭ │ 0.03 │
└───┴──────┘
┌─────────────┐
│ ┌─────────┐ │ 0.03 zone between
│ │ │ │ two coaxial cylinders
│ │ │ │
│ └─────────┘ │
└─────────────┘- Circularity checks each cross-section independently
- Cylindricity checks the entire surface simultaneously
Form Control Hierarchy
From least to most restrictive:
Straightness (line elements)
↓
Circularity (circular elements)
↓
Flatness (entire plane)
↓
Cylindricity (entire cylinder)Quick Reference Table
| Control | Zone Shape | Applies To | Typical Use |
|---|---|---|---|
| Straightness | 2 parallel lines/planes | Lines, axes | Rails, guides |
| Flatness | 2 parallel planes | Flat surfaces | Mating faces |
| Circularity | 2 concentric circles | Circular sections | Bearings, seals |
| Cylindricity | 2 coaxial cylinders | Cylindrical surfaces | Precision bores |
Key Takeaways
- Form controls do NOT reference datums
- Straightness: line elements or axis must be straight
- Flatness: surface must lie between two parallel planes
- Circularity: each cross-section must be round
- Cylindricity: entire cylinder surface controlled at once
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Next Lesson: Orientation Controls - controlling angular relationships to datums.
