Implied Flatness Ordinate Dimensions 1

[Oaklandishh]

Does Implied flatness still apply to both surfaces when using ordinate dimensions, or does the side with the 0- denotation sort of become a datum and the flatness only applies to the non zero dimension?

 

[greenimi]

What standard are you using? ASME or ISO?
How you dimension it? with ±?

 

[Oaklandishh]

ASME

it is dimensioned with block tolerances.

 

[greenimi]

Both surfaces.
You can use the origin symbol (per Y14.5) --so called poor man datum -- to show which surface is which.

 

[Oaklandishh]

Thanks,

As a follow-up, where in ASME Y14.5 does it define implied flatness? Is implied flatness the keyword I should search for?

 

[drawoh]

Oaklandishh,

Figure[ ]2.5 in ASME Y14.5-2009 explains the origin symbol. I suggest you look at it.

--
JHG

 

[powerhound]

The flatness is from General rule #1. Flatness is a form characteristic and rule #1 states that as the size departs MMC towards LMC, the amount of departure is equal to the allowable form error. So in this case, if the part were produced at 9mm then there would be 2mm of form error allowed. If it were produced at 10mm the the amount of error allowed would be 1mm and at 11mm no form error would be allowed at all.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

 

[Belanger]

Is implied flatness the keyword I should search for?

No. If using a word search, try this phrase: "limits of size to the extent prescribed." Or simply read paragraphs 2.7 and 2.7.1.

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

 

[drawoh]

powerhound,

Check the figure in the standard. In ASME Y14.5M-1994, it is figure[ ]2.5 as well, but it is not as clear. The origin face works like a datum. Rule[ ]1 does not apply here.

--
JHG

 

[powerhound]

drawoh,

I see what you're saying but there is no dimension origin symbol here. You're probably saying that since one surface is zero then it is, by default, the dimension origin. That might be the case but since no dimension origin symbol is present then it's still open to interpretation, in my opinion.

Besides that, I was really just answering his question on implied flatness. I figured he had heard of rule #1 at some point but just couldn't remember exactly how it was phrased, or the details.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II

 

[Oaklandishh]

So if I am understanding both you guys and figure 2.5 correctly, if I was to use the origin symbol then only the top side would need the specified flatness.

Since there is no origin symbol, both sides are required to have the implied flatness?

 

[Belanger]

Yes, I think that's the consensus (and also per the standard).

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

 

[pmarc]

So if I am understanding both you guys and figure 2.5 correctly, if I was to use the origin symbol then only the top side would need the specified flatness.

Why the top side and not the bottom?

If origin symbol was used, flatness tolerance of the top side would be indirectly defined by +/-1 tolerance, so no additional flatness tolerance would be needed. However, nothing would control flatness of the bottom side in this case.

 

[Oaklandishh]

I wrote that incorrectly.

You are correct I meant the bottom would need the flatness defined as the top would already have it defined.

 

[Belanger]

Good catch pmarc -- I read it as meaning the top surface has an automatic flatness per the size tolerance (not that the top surface needs an additional flatness callout). So we all got to the right place in the end

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