# How do you indicate flatness on a drawing?

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## How do you indicate flatness on a drawing?

Flatness is can be measured using a height gauge run across the surface of the part if only the reference feature is held parallel. You are trying making sure that any point along the surface does not go above or below the tolerance zone.

## What is flatness in machining?

Flatness is a measure of a surface’s form compared against itself, indicating that all the points along the surface lie in the same plane. Symbolized in GD by a parallelogram, flatness allows you to limit waviness or variation in a surface without tightening its dimensional tolerance.

How flatness is calculated?

Flatness is can be measured using a height gauge run across the surface of the part if only the reference feature is held parallel. This is a 3D measurement so points must be measured across the length and width of the part to ensure the entire surface is in tolerance.

### How does the flatness callout control uniformity of a surface?

The flatness callout controls the uniformity of a surface or a median plane as needed. It defines two parallel planes on either side of the flat surface as the tolerance zone for the surface. All the points on the specified surface must lie between these two planes for part approval.

How is the flatness callout used in GD & T?

No surface is perfectly flat but using GD, we can develop parts with a surface that is flat enough for our application. The flatness callout controls the uniformity of a surface or a median plane as needed. It defines two parallel planes on either side of the flat surface as the tolerance zone for the surface.

## Is there a flatness callout below the size dimension?

The slot below has a flatness callout below the size dimension. That means that the flatness tolerance applies to the center plane of the slot. The figure below shows the slot at its MMC size with the max allowable flatness error. The derived median plane must fall between two planes that are 0.1 apart.

## When does flatness apply to an outside surface?

Flatness can apply to either an outside surface of a part or a centerplane. When flatness applies to an outside surface, the feature control frame points at that surface or is on an extension line connected to that surface. The surface then, must fall between two parallel planes that in this case are 0.1mm apart.