Position and symmetry

[SteelAndSteel]

Hi,

I have two questions on GD&T in ISO, the questions is imbedded
in the pictures below:

http://s3.postimg.org/arf4wvvpf/image.jpg

http://s22.postimg.org/gf9sezlkh/image.jpg

Kindly

Paul

 

[drawoh]

SteelAndSteel,

I am an ASME Y14.5 guy. ISO could have a weird interpretation of someting here.

You are saying "symmetry", but you are showing positional tolerances. According to your drawing, your dimensions are symmetrically distributed across centrelines, but they must be positioned with respect to datums.

In ASME Y14.5, the datums must be real features. All your centrelines show is where your features are centred. Your first example works fine, as long as you select reasonable features for datums.

In ISO, I understand you can use centrelines as datums. I have no idea of how this is done.

In your second example, all you seem to have done is to forget to box the dimensions. I would still refer to the positional tolerance.

--
JHG

 

[CheckerHater]

Symmetry about what? The drawing shows no datums whatsoever.

In ISO, I understand you can use centrelines as datums.

No, in ISO you cannot use centerlines as datums. Even when it was possible, it was only allowed when physical features could be clearly identified on the drawing.

Using positional tolerances without datums like OP did is also long forgoten practice.

 

[dgallup]

In the second one link, case one would be valid if the diameter symbol were removed from the position tolerance. No idea why you would do that but it could be done. It is invalid as it stands.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[SteelAndSteel]

Hi

Thanks for your replies,

Reply to drawoh

For the symmetry tolerance i am referring to the iso standard 2768

http://www.nan-long.com/Metal Steel Technology Summary/ISO 2768-2 Geometrical tolerances.htm

Reply to CheckerHater

[highlight #A40000]Using positional tolerances without datums like OP did is also long forgoten practice.[/highlight]

I would say that is quite commen when it is only the position between the holes that is important.

Reply to dgallup

[highlight #A40000]In the second one link, case one would be valid if the diameter symbol were removed from the position tolerance. No idea why you would do that but it could be done. It is invalid as it stands.[/highlight]

Can you refear to any standard to why it is not valid?

 

[powerhound]

If the holes have to be on the plate then you need datums to show where they need to be. If your answer is, "I don't care where they are." then how about if they're not there at all? Now do you care?

I don't know a lot about ISO but I would think that even ISO would require datums from which to locate features.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[CheckerHater]

@S&S:

I was answering your question about holes being symmetrical about certain plane / line.

From ISO 1101 Para. 18.14 Symmetry tolerance:

“The tolerance zone is limited by two parallel planes a distance apart, symmetrically disposed about the median plane, with respect to the datum.”

So, the definition of symmetry implies datum, no datum, no symmetry.

Now, let’s go back to situation where “it is only the position between the holes that is important.”
Would you accept the part where the holes were punched at proper "position between each other", but, say, at 45 DEG to the surface of the part?