Form Controls

What are Form Controls?

There are four form controls:

1. Straightness (─)

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 (▭)

The entire surface must lie between two parallel planes:

FCF: ┌───┬──────┐
     │ ▭ │ 0.05 │
     └───┴──────┘

    ═══════════════ 0.05 tolerance zone
    ~~~~~~~~~~~~~~~  (actual surface)
    ═══════════════

Note: Flatness controls the entire surface at once, while straightness controls individual line elements.

3. Circularity (○)

Also called "roundness." Each cross-section must lie between two concentric circles:

FCF: ┌───┬──────┐
     │ ○ │ 0.02 │
     └───┴──────┘

         ╭───╮
        ╱  ○  ╲   0.02 zone between
       │       │   two concentric circles
        ╲     ╱
         ╰───╯

4. Cylindricity (⌭)

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

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

---