Straightness

Straightness is a form tolerance in GD&T that controls how much a feature or an element of a surface can deviate from a perfectly straight line. Unlike flatness, which applies to a surface, straightness applies to either individual line elements on a surface or to the axis of a cylindrical feature. Straightness applies in one direction only.

Symbol for Straightness

  • Straightness Symbol: ⏤ (a straight horizontal line)

Types of Straightness Control

  1. Surface Straightness
    • Applies to individual line elements of a surface.
    • Ensures that a single element (measured along a specific direction) does not have excessive waviness or deviation.
    • Commonly used for controlling straight edges or linear guideways.
  2. Axis Straightness (Applied to a Feature of Size)
    • Controls how much a cylindrical feature’s axis can deviate from a perfectly straight line.
    • This is particularly useful for shafts, pins, and holes to ensure they remain functionally straight under size limits.

Tolerance Zone for Straightness

  • Surface Straightness: The tolerance zone is defined by two parallel lines (or planes, in the case of a cylindrical feature) spaced apart by the straightness tolerance value.
  • Axis Straightness: The tolerance zone is a cylindrical boundary surrounding the ideal axis within which the actual axis must remain.

How Straightness is Applied in Drawings

  • Surface straightness is applied directly to a surface and does not require a datum.
  • Axis straightness is applied to a feature of size (e.g., a shaft or hole) and often relies on Maximum Material Condition (MMC) for functional fit control.

Comparison with Other GD&T Tolerances

  • Straightness vs. Flatness:
    • Straightness applies to a single line element at a time, while flatness controls an entire surface.
  • Straightness vs. Cylindricity:
    • Straightness applies to either a surface or an axis, whereas cylindricity controls an entire cylindrical surface’s roundness and straightness simultaneously.
  • Straightness vs. Profile:
    • Profile tolerances define complex shapes, while straightness only ensures linearity.