GD & T Characteristics - A-2 Flatness

A-2: Flatness : 

GD & T Characteriatic - Flatness 

Symbole - 

Flatness Symbole

What is Flatness?

Flatness is the property of a plain surface with an appreciable proportion at its length and width or its diameter. 

The table of a drilling machine, the table of a vertical boring machine, the surfaces of an angle plate, the joining face of the cylinder head with the cylinder block; can be some illustrations where the concerned surfaces are expected to be reasonably flat. 

The symbol for flatness is a small parallelogram (with its height equal to the height of any alphabet or numerical figure used in the feature control frame. length equal to 1.5 times the height of the alphabet). This symbol also is to be put in the first chamber of the feature control frame. 

No datum is required to indicate the flatness of any feature. A typical feature control frame specifying the permissible deviation at the flatness shall appear as shown.

1. Flatness

The surface 'D' under discussion may have horizontal, inclined Or even vertical Under this assumption, it shall mean that the difference between the extreme locations (like A and B) within the given surface must not exceed O. 15 mm, Here again, the surface roughness deviations are to be excluded. 

Measurements and Gauging - 

Different methods can be seen in use judging/measuring the deviation at the flatness of any surface. Applying "Prussian blue" to the surface and tubbing this surface against the surface of a surface plate, and watching for the removal of the blue, can be the oldest method for judging deviation at flatness. However, long straight edges can be applied along with various length locations and various width locations to judge the deviations Of flatness. The use of the level beam comparator or an Autocollimator can detect the amount of the deviation. 

2. Flatness

When any imaginary axial (like A-A) is required to be flat within some specified limits, then the concerned feature control frame is mentioned as shown. However, this practice is not so common. 

The flatness of a surface is a very important property where the surface to surface contact is expected at a maximum possible extent. Contact of an angle plate over a surface plate, resting Of a Component against the face of a post locator or a drilling jig, assembly Of an Oil sump with the bottom face Of a cylinder block, any fixture plate bottom surface in contact with the table Of milling machines can be some of the illustrations where the flatness of the concerned surface plays a very important role. 

Relation to other symbols - 

1. Flatness is the 3D version of surface straightness.

2. The tolerance zone exists between two planes, not between the two lines.

Notes - 

1. Flatness is not the same as parallelism. Parallelism needs datum to control a surface while flatness does not.

2. MMC or LMC Application - Yes

3. Relative to Datum - No

To read ...Straightness