Flatness and parallelism are never synonymous.

Author:Clark, Richard
Position:Inspection strategies

"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.


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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