"Flatness area" annotation around a mounting hole - necessary if the plane is already flat

[Rhyder88]

Is it necessary to annotate specific flatness area (say 20mm dia) around a mounting hole(say 8mm dia) of a support bracket,even if the plane(where hole is located ) is already flat?

 

[MikeHalloran]

That sort of annotation is usually added in the revision right after you get a shipload of material that you expected to be flat, but it wasn't, and you had to eat it because you didn't specify local flatness where you needed it.

If you just got there, removing the annotation will probably get you fired.



Mike Halloran
Pembroke Pines, FL, USA

 

[KENAT]

Is flatness of the overal surface/part explicitly specified - or are you relying on 'rule 1'/envelope principle?

Is the local flatness tighter than the overal surface flatness?

If the overal surface flatness is specified, and the local flatness was not a refinement then technically I seen no reason the local call out is required.

However, it is worth trying to understand the intent behind why it was added as Mike implies - just in case there is actually a problem that needs fixing even though the fix applied may not be technically appropriate.

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[Tenkan]

the real question is if you want your company to pay extra inspection costs to inspect the entire surface area that is not required to be as flat?

yes, its necessary to annotate the specific flatness area required for function if the required area is smaller than the entire surface where its located.

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[Belanger]

To the OP... there are two different ideas in your question. You ask if a flatness tolerance is needed. That's a tolerance.
Then you state that the area around the hole is already flat. That's not a tolerance.

It's like saying, "How accurate does the position of the hole have to be if it's already dimensioned as 3 inches from the edge?"

Every surface must have a flatness tolerance of some kind (unless it's a flexible part, or you just plain don't care if the thing is Gawd-awful warped). Most of the time flatness is indirectly controlled by the size or thickness of the part; that's the Rule #1 idea that Kenat mentioned (if you're using ASME).

So the bottom line is no, an explicit flatness tolerance is not required around that hole, but examine the thickness tolerance and read the notes, title block, etc. to assess what is currently controlling the flatness.

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

http://www.gdtseminars.com