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Flatness VS Straightness on Surface
3

Flatness VS Straightness on Surface

Flatness VS Straightness on Surface

(OP)
Please ref to the attached figure, which one should be specified on the print? Flatness or Straightness on Surface ? What is the difference between them?

Thanks

SeasonLee
 

RE: Flatness VS Straightness on Surface

Straightness applies to a single linear element, flatness applies to the entire surface.  Only your knowledge of the requirements of the part can tell you which is correct but since you are indicating a surface I would guess flatness is what you want.

RE: Flatness VS Straightness on Surface

I guess it would depend upon the design intent. If you are concerned about the full surface or plane, then I would chose flatness. If you are only concerned about the bend in along the length of the feature, then straightness is applied.

Most often, flatness is used.  

Dave D.
www.qmsi.ca

RE: Flatness VS Straightness on Surface

Agree with dgallup and dingy2. It all depends what you really want.

Actually this is very similar case to circular vs. total runout or profile of a line vs. profile of a surface dilemma.
Tolerance zones for straightness, circular runout and profile of a line are two-dimensional while tolerance zones for flatness, total runout and profile of a surface are in three dimensions. If you are interested in controlling each (linear or circular) segment of a surface independently go for the first group, but if whole surface needs to be controlled second group is much better option.

RE: Flatness VS Straightness on Surface

(OP)
Thanks for the inputs and I agree with all of you.

The feature control frame is directed to the surface because it is a surface control. Straightness on the surface is a single line element control, but it is a series of single line element control on the surface, so we can consider it is a surface control, my question is why not to specify flatness directly? What are the differences between them?

Thanks again

SeasonLee
 

RE: Flatness VS Straightness on Surface

Like I said, straightness controls linear elements of a surface independently. It means that every linear element must be within 0.2 tolerance zone but each of them can float within size tolerance zone (0.6) without any relation to the other. In extreme case one perfectly straight linear element can lie at 30.3 level while the second one at 29.7 level.

Flatness keeps all linear elements within common 0.2 tolerance zone and this zone can float within 0.6 size tolerance. It means the difference between perfectly straight linear elements can only be 0.2 in extreme case. So if one line is at 29.7 level, second one can be at 29.9 maximum.  

RE: Flatness VS Straightness on Surface

Definately flatness.

Straightness only applies in the view where FCF is specified, which means in one direction, allowing for part shown in the picture.

Unless this is the design intent, you are better off with flatness.

If your part has really large length-to-width ratio, specifying different straightness in second view will make sense too.

Also features that are straight, but not flat, like round shafts are obvious choice.

 

RE: Flatness VS Straightness on Surface

FYI ... interesting side note: if straightness of 0.2 were shown in each view, it doesn't equal the same thing as a flatness of 0.2.

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

RE: Flatness VS Straightness on Surface

J-P

No it doesn't equal the same.

From a 3 point set up on the surface, we have a 0 to -.2 all in one direction giving us a straightness of 0.2 in that direction while in the other direction we have a 0 to  +0.2 giving us a straightness of also 0.2 but combine your readings and one achieves a flatness of 0.4.  

Dave D.
www.qmsi.ca

RE: Flatness VS Straightness on Surface

Exactly.   Makes for a good test question, eh?

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

RE: Flatness VS Straightness on Surface

(OP)
One more question : When should we specify the Straightness control on a surface?

Thanks

SeasonLee

RE: Flatness VS Straightness on Surface

From a manufacturing viewpoint, it might be used where there are unavoidable tooling marks that lay in one direction.  Straightness could be controlled tighter in that direction of lay.

From a functional viewpoint, perhaps where there is a sealing surface, and you're worried about fluid leaking through any valleys in a surface.

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

RE: Flatness VS Straightness on Surface

hey checkerhater..
you DEFINITELY need a spell checker. or rely on a checker.

tk369..a checker!

ted kralovic

VisVSA, NX-6, Macbook, iPhone 3GS, among others

RE: Flatness VS Straightness on Surface


To tk:

I would immediately follow your advice, as soon as I find checker who doesn't forget to capitalize beginning of the new sentence.

Not to mention his own name.

Also, I don't understand, are you suggesting flatness or straightness? It is not the best way to start conversation with personal rant not related to the discussion on the forum.

I think you definitely need a moderator.


"Checkers, checkers everywhere" – read in the voice of Cartman from South Park.
 

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