Specific Area Flatness

[dthom0425]

Hi all,

I'm just curious how some of you may call out a specific area on a part with flatness (rather than the whole surface).

I want to control flatness of 5"x5" square, centered in a 8x12 part. I don't believe I need to utilize unit area flatness.

My plan was to draw a phantom box (chain lines), hatch the drawn area, apply a flatness control to the drawn area and apply basic dimensions to the 5 x 5.

Does that sound correct? I didn't see an example in Y14.5-2009.

Thanks,

 

[Belanger]

Yes, what you describe seems to be the best way. Think of it as similar to calling out a datum target area (see Fig. 4-42 in the 2009 standard for an example).
You could also look at Figs. 1-13 and 9-2, which relate somewhat to your situation.
I would then put the feature control frame in the view where you are looking face-on at the surface, attaching the FCF with a dot.

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

 

[CheckerHater]

Area is identified by means of "chain line" which is just like center line, but thicker:

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[dthom0425]

Thanks for the responses.

Just a follow up question since I am now questioning my methods.

I was planning on giving the flatness area/zone basic dimensions. Is it plus/minus tolerancing to locate the zone and basic dims for the 5x5?

 

[Belanger]

No -- use basics for the size of the area, and also for its location.
Some people might say that this would then cry out for datums. Of course flatness can't reference a datum, but since it's not a location tolerance, then datums aren't part of what's being toleranced anyway.
Fig. 9-2 would be an example of a basic dim locating a selected tolerance zone without coming from a datum.

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

 

[Tarator]

dthom0425,

I would recommend making that area raised or recessed (prefer raised over recessed), if it is possible and does not change your design intent, so it will stand out. Manufacturing and Quality will like it. Just a design tip.

 

[chez311]

I would tend to agree with JP. I've discussed this before and to me basic dimensions for size/location of a limited length/area seemed to be the most logical choice - especially since you're not actually determining conformance for the size/location of the limited length/area but the qualities of the area itself.

I can see where someone could have trouble with those basic dimensions not being related and utilized in conjunction with applicable datum features, +/- dimensions would provide some relief of this - with all of the added ambiguity that comes with directly toleranced dimensions, especially with something that is not a tangible feature/boundary like a limited area. I've also seen an elegant solution which worked in a specific case - it was a limited area on a cylindrical feature whose extent from an ID was controlled with a MIN dimension, that way if the entire area conformed it was acceptable, but at a minimum the limited extent must conform. This would be unwieldy to say the least for rectangular limited areas/parts or a pattern of limited areas.

Personally I think something like Y14.5-2018 fig 9-16 and para 7.24.10 would be an "ideal" solution, where either datum features or targets are utilized in conjunction with basic dimensions and a note combined with the limited area flatness tolerance - something along the lines of "DATUM FEATURES/TARGETS A, B, AND C ARE USED ONLY FOR ESTABLISHING ORIENTATION/LOCATION OF DESIGNATED LIMITED AREA". This might be overkill for some applications though. Thoughts?