Application example of flatness of median plane

[SeasonLee]

Flatness of a median plane has replaced the straightness of a median plane in 2009 standard, I can realize the meaning but can’t image its practical application. Can you give me a real life example please?

Season

 

[fsincox]

Anytime you want to exempt a manufactured part from rule #1. We had ground transfer beams 2" X 6" X 80". and did not really need perfect form at MMC.
Frank

 

[Belanger]

Suppose you're simply trying to slide a block into an opening, or under a "bridge" of some sort, perhaps 100 mm high. You can't let the total envelope of the block exceed the 100 mm clearance of that bridge, but for reasons of weight restriction maybe you can't let the block's "actual local size" exceed 90 mm. Therefore, you can allow flatness on the median plane to be 10 mm; thus it's totally different than surface flatness.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems