# Flatness and parallelism are never synonymous.

 Author: Clark, Richard Position: Inspection strategies

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"We can't see past the choices that we don't understand."--From The Matrix--Reloaded

A few days ago, I met with a client to discuss his measurement equipment needs for a model part he was planning to manufacture for a relatively new customer. The part was a plate that would be assembled into a motorcycle transmission. As seen in the first image below, the plate had a "bottom" face that was designated as Datum A. The "top" face was toleranced for parallelism (to Datum A) as well as flatness. So far, nothing seemed out of the ordinary.

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As the customer and I reviewed the drawing in more detail, I noticed (in the specific instructions toward the corner of the drawing) what I believe to be a very common misperception: "Flatness will be confirmed with a dial indicator using a surface plate as the datum." Before I went any further with the customer, I felt obligated to clear up some misunderstanding.

If you engage in research about any geometric dimensioning and tolerancing (GD&T) standard, you'll learn very quickly that flatness is not a characteristic referenced to a datum. Most GD&T references define the following:

Flatness--The condition of a surface having all elements in one plane.

Parallelism--The condition of a surface, line, or axis which is equidistant at all from a datum plane or axis.

Flatness is a feature compared to itself, while parallelism requires that a feature be compared to a datum.

To clear up the concept, consider the example of a kitchen table. We purchase a new table from a furniture store and are fairly certain that the flatness of the tabletop to be less than a fraction of an inch. If the table legs are the same lengths within a few fractions of an inch, we can be confident the tabletop is...

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