How do you indicate flatness tolerance on a drawing?

Short Answer:

Flatness tolerance is used in GD&T to control how flat a surface should be. It ensures that a surface does not have any unwanted bumps or waves. This tolerance is applied to a single surface without referring to any other feature or datum. It only controls the form of that surface.

To indicate flatness tolerance on a drawing, a feature control frame is used. The flatness symbol (a parallelogram) is placed in the frame, followed by the tolerance value. This frame is connected directly to the surface that needs to be flat using a leader line or an extension line.

Detailed Explanation:

Indicating flatness tolerance on a drawing

In mechanical drawings, flatness is a form control in Geometric Dimensioning and Tolerancing (GD&T). It is used when a surface must be perfectly even and smooth. Unlike other geometric tolerances, flatness does not depend on any reference feature or datum—it only applies to a single surface.

Flatness helps ensure that parts mate properly, seal correctly, and fit together without wobbling or rocking. This is very important in components like machine bases, flanges, plates, covers, and mounting surfaces.

Let us understand step-by-step how to represent flatness tolerance on a technical drawing.

Symbol and feature control frame

The symbol for flatness in GD&T is a parallelogram (like a flat rectangle).

To apply flatness on a drawing:

  • A feature control frame is used.
  • The frame includes:
    • The flatness symbol (parallelogram),
    • The tolerance value (like 0.05 or 0.1 mm).

Example: ⎡▱⎤⎡ 0.05 ⎤

This means the surface must lie between two parallel planes that are 0.05 mm apart.

Where to place the frame

The feature control frame is attached directly to the surface that requires flatness using:

  • A leader line (with an arrow pointing to the surface), or
  • Placed next to the dimension if the size of the surface is also given.

Important points:

  • Do not include any datum reference in the flatness frame.
  • Flatness only applies to the surface itself and does not relate to other parts.

How flatness is measured

Flatness is measured by checking how much the actual surface deviates from a perfectly flat plane. This is done using:

  • Surface plates and dial indicators, or
  • CMM machines (Coordinate Measuring Machines) in advanced inspection.

If the surface lies entirely between two imaginary planes separated by the given tolerance (e.g., 0.05 mm), then the part passes the flatness check.

When to use flatness tolerance

Flatness tolerance is used when:

  • A surface must contact tightly with another surface.
  • There should be no gaps, rocking, or wobble.
  • It is necessary to improve sealing, reduce vibration, or ensure tight fitting.

Common applications:

  • Engine cylinder heads
  • Flange faces
  • Mounting bases
  • Bearing seats
  • Precision plates

Importance of flatness tolerance

  • Improves performance: Ensures proper contact and load distribution.
  • Enhances quality: Reduces leakage, vibration, and wear.
  • Supports accurate assembly: Prevents gaps and misalignment in mating parts.
  • Helps in inspection: Gives a clear and measurable requirement for surface quality.
Conclusion:

Flatness tolerance is indicated on a drawing using a feature control frame with a parallelogram symbol and a specific tolerance value. It is connected directly to the surface being controlled. Flatness ensures that a surface is even and smooth within a certain limit, without referring to other features. It plays a key role in improving fit, function, and quality of mechanical parts. Proper use of flatness tolerance helps in producing reliable, high-precision components.