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:

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
        ╲     ╱
         ╰───╯

Applies to: Cylinders, cones, spheres (any circular cross-section). Use Case: Bearing seats, O-ring grooves, rotating shafts.

4. Cylindricity (⌭)

Definition: Controls the entire surface of a cylinder—combines circularity, straightness, and taper.

The entire surface must lie between two coaxial cylinders:

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.