Is this GD&T correct 1

[SiW979]

Hi People!

I'm looking at a drawing and am trying to convince myself that I'm reading it wrong, but I can't. It just doesn't make sense to me and everyone I've shown it to so far agrees, but we're not confident enough with our GD&T knowledge to say it's definitely wrong, as it'll be wrong on many drawings, and dates back nearly a quarter of a century, across a number of suppliers, none of which have said, "no, that's rubbish!".

Is it logical to specify a thickness of 3.9/3.8 (so, ± 50 microns) and have a flatness tolerance band on both sides of 250 microns. I suspect that the 250 microns should be 25 microns, but am not 100% confident. I'm thinking that based on the faxt that flatness is measured between two parallel planes, that the parallel frame is not required and I could just get away with the flatness tolerance? and that the drawing is over constrained? Or am I wrong? Also do I need a datum for a flatness tolerance?

Many thanks

Simon

Best regards

Simon NX7.5.4.4 MP8 - TC 8

http://www.jcb.com

 

[fsincox]

The document I have is a German company standard (SULZER??) it notes: "Corresponding to ISO1101" at the top of the page and is dated from 1976.
The text part is as follows:
"4.4 If two associated features are identical and there is no reason to give preference to one as a datum feature, then the tolereance is indicated as in figure 19."
Figure 19 is a parallelism callout as shown on your document.
I think it is a interesting and valid concept, myself.

 

[CheckerHater]

If it’s from 1976, it may be listed in the back of pmarc’s 1983 as “former practice”. I wonder…

 

[fsincox]

pmarc,
The only problem I have with the concept IS any reference to a datum, in the classic sense. A datum is a reference to the "a concept of perfection". I want "imperfect face" to "imperfect face", I see value in that concept, no more constrained conditions needed, just as it sits, just like the old time shop practice.

 

[pmarc]

There is nothing in the back of '83 edition. The document has got no appendices at all.
Unfortunately I do not have a copy of replaced ISO/R 1101-1:1969 to see earlier practices.

 

[CheckerHater]

Sorry to hear that.

It’s just what I found in the back of 2004:

 

[pmarc]

Yes, this is what I was talking about in my first post in this thread.

 

[fsincox]

I will try to load the page for you too see, I believe it is a German company, I have a Canadian book on ISO that shows it also, We need to see old DIN standards.

 

[CheckerHater]

What I like about your document fsincox, is that using capital letter to identify datum feature is not a requirement but rather an option.
The standard sure went a long way since that.