Flatness vs. Profile

[John Kingston]

If you have a FCF with /Profileofasurface/.010/ and the feature is a plane, wouldn't that be the same as /Flat/.010/ ? Just curious. I'm finding some "odd" callouts on this one and wonder what they were thinking.

[bw_bob]

Profile controls surface size & flatness is regardless of surface. The only difference I know of.

[JamesMannes]

Profile controls surface size & flatness is regardless of surface. The only difference I know of.

I think I'm reading Bob right when I say I think he meant profile would control both form and location(depending on the FCF), while flatness would control form only?

[Rob_K]

Profile Could/Not be called toward Datum, while Flatness mustn't. Your example doesn't have any Datum, feature is plane, so /Flat/.010/ would be correct choice.

[John Kingston]

Profile Could/Not be called toward Datum, while Flatness mustn't. Your example doesn't have any Datum, feature is plane, so /Flat/.010/ would be correct choice.

That is what I was thinking. Why use profile with no datum (form only) on a plane?

[Rob_K]

Are there any angels involved ?

[John Kingston]

Nope. The only thing I can think of is there are 5 seperate surfaces that create this one plane. (I just realized that!) My guess would be they are trying to control their location to each other.

[Matthew D. Hoedeman]

That is what I was thinking. Why use profile with no datum (form only) on a plane?

No, I don't think they are the same (prof and flat). Flatness requires the entire plane to construct into a plane that is flat within the tolerance band, but NOT to a position like profile of a surface CAN (and ususally does) do. IF flatness, as calculated by Pcdmis DOES use the MAX to MIN values of each point's deviaiton for the calcualtion, then YES, they CAN be the same, if the profile of a surface is FORM ONLY. If the profile of the surface is FORM & LOCATION, then the profile callout is MUCH more restrictive that a FLAT callout. Your plane could be 45 degree WRONG to CAD data (or print) and still be within the flatness callout, but show WAY BAD if prof-form&loc is used.

Now, engineers will undoubtedly use profile with or without datums in the FCF simply because they will think it ALWAYS means form and location.

Now, if you can check the plane to those tolerances, form and location, NOT form only, then you WILL be in tolerance, even if it is actually form only. No so the other way around. If you are sure your people can continue to make it in tolerance using the more restrictive interpretation, use it.

Also, did the guy who made the print (added the tolerances) KNOW (or could he tell) by looking at a flat, 2-D image, that the surface was in fact a flat plane? If not, (s)he may not have KNOWN it was a flat planar surface and was simply playing a bit of CYA with the print.

[John Kingston]

OK. That makes sense. I wasn't thinking it all the way through.
Thanks guys!

[kbotta]

***Profile (no datum ref) is USUALLY reserved for controling co-planarity between 2 surfaces.
It can also be used in other applications (i.e. "profile/position boundary", composite profile..) without a datum ref.
kb

[cmmguy]

Kevin, isnt profile implied to be parallel unless otherwise specificed were flatness is not?

[kbotta]

Kevin, isnt profile implied to be parallel unless otherwise specificed were flatness is not?

What-a-ya mean
Exp a little more for me if you will, not sure of your intention...

[tking]

Another question for Kev.

Assume 5 pads that are to be coplaner, one actually.5mm above / below the others. This would pass a .5 flatness test but would fail a .5mm profile test (+/- 0.25mm).

No?

TK

[Matthew D. Hoedeman]

Another question for Kev.

Assume 5 pads that are to be coplaner, one actually.5mm above / below the others. This would pass a .5 flatness test but would fail a .5mm profile test (+/- 0.25mm).

No?

TK

If they are 5 distinct surfaces (not connected directly to each other in any way) I would treat each one as an individual plane. Any there are connected edge to edge would get treated as a single surface. And, this is all subject to change, depending on the engineer in charge.

[cmmguy]

What-a-ya mean
Exp a little more for me if you will, not sure of your intention...

In the context of profile-of-a-plane where no datum or basic is called out, maybe just a plus or minus linear dimension to the plane(maybe it is not drawn correctly). Would the profile be implied to be parallel to the opposite surface? If not what would the difference be between that profile and flatness, as John originally asked?