GD&T - FLATNESS 'CIRCLE 1'

INTRODUCTION

Consider the two surfaces shown. Which surface is perfectly flat? In reality, no surface is perfectly flat. How can we design a surface that is not perfectly flat, but flat enough to function properly? The flatness control tolerance.

FLATNESS CONTROL

The flatness control (c) defines how much a part’s surface may deviate from its perfect flat form.

Flatness Control: Perfect flatness is when all points of a surface lie in the same plane. Flatness is a form control. The flatness control (c) defines how much a surface on a real part may vary from the ideal flat plane.

Tolerance Zone: The flatness tolerance zone is the volume between two parallel planes. The distance between the parallel planes is the stated flatness control tolerance value. The surface being controlled must lie within the volume defined by the tolerance zone.

Feature Control Frame: To control the flatness of a surface, a feature control frame (FCF) is used to apply the tolerance to the desired surface.

To apply a flatness control to a surface, the FCF may point to the surface, or can point to or rest on the extension line that extends from the surface. The FCF is placed in the view where the surface is viewed as a line. The FCF shown below applies a flatness tolerance to the entire surface. This surface must lie between two parallel planes that are spaced 0.2 mm apart.

USES

The flatness control is used to ensure better contact between mating parts, or when a gasket or seal is to be used. It is also used to limit pitting and waviness of a surface.

INSPECTION

The flatness control may be inspected as follows:

1. The part’s surface is placed on a surface plate. This surface plate represents one of the flatness control tolerance zone planes.
2. A dial indicator is placed in a hole in the surface plate and measures the distance from the plate’s surface to the part’s surface.
3. As the part is rotated, the measuring instrument must up or down the part and the profile of the cylinder is captured.
4. The part’s surface is moved over the hole at random. If at any time the FIM (full indicator movement) is larger than the flatness control tolerance, the part is not within specifications.