Datum Question

[RussL]

We have a part, Datum A is a flat planer surface about 60" X 10"
Datum A has a flattness tolerance of .010.
There are two 3" diameter areas on -A- that are held to a tighter flattness.
The flattness of these areas are .005 with no Datum referenced (not even A)
The customer tells us (verbally) they locate on these two areas in the next higher assembly clamping the part down on them tight.
Opinions on how to locate Datum A???
Note Also: The part is distorted.

[bw_bob]

We have a part, Datum A is a flat planer surface about 60" X 10"
Datum A has a flattness tolerance of .010.
There are two 3" diameter areas on -A- that are held to a tighter flattness.
The flattness of these areas are .005 with no Datum referenced (not even A)
The customer tells us (verbally) they locate on these two areas in the next higher assembly clamping the part down on them tight.
Opinions on how to locate Datum A???
Note Also: The part is distorted.

Technically the 3 areas around the diameters are still part of Datum A. You might want to get something in writing to CYB.

[#2#]

We have a part, Datum A is a flat planer surface about 60" X 10"
Datum A has a flattness tolerance of .010.
There are two 3" diameter areas on -A- that are held to a tighter flattness.
The flattness of these areas are .005 with no Datum referenced (not even A)
The customer tells us (verbally) they locate on these two areas in the next higher assembly clamping the part down on them tight.
Opinions on how to locate Datum A???
Note Also: The part is distorted.

Flatness requires no datum reference. If you have a flatness callout with a datum reference, the callout is wrong.

[Matthew D. Hoedeman]

What I would do is use those 3 areas for the LEVEL in the program, then check the entire surface for the flatness, but I would use ONLY tose 3 areas. Set it up on blocks or whatever so that it is only touching those 3 areas and go to town.

[RussL]

It's only two 3" diameter areas.
I agree flattness should not have a datum reference.
The two areas with the .005 flattness can vary the whole Datum A tolerance as long as they are flat to themslevles withing .005.
When the part is clamped to these surfaces more distortion can be added because the two 3" diameter areas, though part of A may not be parallel to each other at all.
We ended up forcing it down on the two areas and inspecting A afterwards as the customer indicated they want verbally.
But the way I read the print, that is not what they are asking for on paper (model).

[Jan d.]

My opinion:

- measure the whole datum A and use it for leveling. Obviously it needs to be within the flatness spec.
- Then I would measure the 2 areas with a separate set of closely located points (high density) to verify whether those points meet the tighter spec. Just output the flatness there (and flatness does not have any datum references). I also think, but am not sure, that the proper way to call out flatness for different pads is to use surface profile. Typically, if they spec flatness, it MUST be analyzed PER pad. If they call out profile, it must be analyzed as one big place.

I disagree with Mat: the print says the whole suface is datum -A- and however "distorted" it may be, the print calls it A so you have to measure it and DATDEF it as A. However logical it may be to use the 2 small areas for leveling, I strongly advice to NOT do that. If your customer insists that you use the 2 small areas as your datum A, you better get that in writing, because that is NOT what the print says! You may think that your customer told you the whole story, but he may have left out a few details that may be critical for something else. Happens to me all the time...


Jan.