Would this valid callout bother you

[sendithard]

If I saw this on a print I'd break out in a rash.......flatness of a median plane???

 

[Belanger]

If you're familiar with straightness tolerance applied to a feature of size -- this is simply an extension of that idea. So are you distressed that it's even in the standard, or is the notion of derived median plane that would cause the rash?


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

 

[Burunduk]

Not really bothered, though I probably never used it, since it's usually simpler not to override rule #1 and just let the size tolerance control the form of the tab (or slot).

Just remember that although the word 'plane' is in "Derived Median Plane" it's not really a plane, but rather an imperfect collection of center points as derived from the as-manufactured feature. It's not the same as a Center Plane, which is a perfect form plane that you get from finding the center of something like precision vise jaws after they collapse on the feature.

 

[3DDave]

They used the word "plane" but they don't mean it. There's literally no other word that matches the likely variation, like "surface," that could have been used, but they wanted the term to rhyme with the zone name.

They tossed the chance to create a median surface control for non-planar features - there's always next decade.

 

[AndrewTT]

We use FDMP with MMC bonus tolerance all the time on "flat" shaped parts stamped from coil stock. It allows for a "pass-through" gage to be employed to check the flatness of the part. It fits our design intent and works great for us.

My rash usually stems from the note "no burrs allowed".

 

[TheTick]

I'd think twice, simply because it will result in some sort of meeting or late night phone call.

 

[3DDave]

Not much different from explaining a straightness tolerance on a piece of round rod. Don't start by mentioning "median plane" when referring to the part variation. Instead, say the median points need to lie at either limit or within a gap of two planes separated by an amount set by the tolerance. Then you can laugh about how the committee got confused and called that potentially lumpy, potentially twisted, potentially compound curved surface fit through the median points a median plane.

 

[jassco]

Hi, sendithard:

No. It does not bother me at all. In fact, I use it quite frequently. Calling it a plane can cause some confusion. But it functions in a similar way as straightness of axis in a cylinder.

Best regards,

Alex