Flatness VS Straightness on Surface 3

[SeasonLee]

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

 

[dgallup]

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.

 

[dingy2]

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.

http://www.qmsi.ca

 

[pmarc]

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.

 

[SeasonLee]

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

 

[pmarc]

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.

 

[CheckerHater]

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.

 

[Belanger]

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

 

[dingy2]

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.

http://www.qmsi.ca

 

[Belanger]

Exactly. Makes for a good test question, eh?

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

 

[SeasonLee]

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

Thanks

SeasonLee

 

[KENAT]

When function requires it, but I'm guessing that's not what you mean?

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Belanger]

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

 

[tk369]

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

 

[CheckerHater]

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.