Flatness Tolerance: GD&T Basics

Flatness is a type of foam control. It controls the variations in flat surface, regardless of any datum feature. It ensures required surface is flat enough to perform required function. Flatness value is always less than the dimensional tolerance associated with a part feature. 

You can refer this article to understand the basics of geometric tolerance.

Representation of Flatness

Datum feature is not required to represent flatness tolerance. LMC and MMC modifiers can be used with flatness.

Flatness Tolerance Zone

It makes a three dimensional volume between two parallel planes. 

All points of controlled surface shall lie within those two parallel planes.

Application Of Flatness
Flatness is used to ensure the smooth contact between two or more mating parts. For example flatness is used in rubbing parts to ensure smooth motion.
Flatness Measurement

Dial Gauge can be used to measure the value of flatness tolerance. To measure the Flatness, part is moved in a 2-dimensions over the surface plate and variation in the reading of dial gauge is observed.

Conclusion

To sum up, Flatness tolerance controls the flatness of a surface regardless of any datum feature. It makes a three dimensional tolerance zone.

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