Is it OK to have drawing with tolerances that are over the ISO standard? 3

[IvanNX]

I have a small question, is it OK to have a note on the drawing that says that all dimensions are in ISO 2768 -c standard, but then some dimensions have tolerances that are larger than what is in the standard. Is that correct to put on drawing or not?

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[aniiben]

Probably instead of saying "it is Ok" you should have asked "it is clear for the reader of this drawing", otherwise stated does the drawing specify clearly and unambiguously the requirements?

 

[IvanNX]

I do not think it is unclear, as tolerances on the drawing are above even very coarse ISO standard (only confusion can be if people who cut based on this have to be blindfolded or just to look away from where they cut to fit into these tolerances), so my question is more if it makes sense or does it looks ridiculous, or is it plain and simple wrong to put them?
BTW my history is in military R&D and airplane design, so for me over coarse is like you said "cut with chainsaw."

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[TheTick]

Maybe not the answer you're looking for...

Why are you unable to establish your own tolerances? If you don't know the capabilities of your manufacturing processes, how can you be comfortable applying any tolerances at all?

 

[ctopher]

If you question it, so will others.
Follow the standard called out, or none at all. Make it simple.

ctopher, CSWP
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[IvanNX]

I tried, they return my drawing and tell me to put tolerances that they are using for years. They say, it get us the product that we ask for and we will not change tolerances now... I answered once that they get correct products because our supplier is ignoring their tolerances and work inside ISO, but older colleagues are just ignoring all my effort to comply with ISO and to increase the quality of drawings. I am sometimes embarrassed that my name is on these drawings.
Sorry that I am whining but I just want to know if I can make a case that this is against ISO, so I can try to fight it once more.

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[TheTick]

Isn't there are hierarchy for tolerances?
[ul]
[li]Title block supersedes ISO standard[/li]
[li]Notes supersede title block[/li]
[li]Explicit tolerances (and GD&T) supersede all.[/li]
[/ul]

 

[CheckerHater]

I just want to know if I can make a case that this is against ISO, so I can try to fight it once more.

No you can't make a case that this is against ISO.

ISO 2768 says exactly this:

There is nothing wrong about specifying tolerance larger than default general tolerance.

"For every expert there is an equal and opposite expert"
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[Tmoose]

Is it possible the note applies to drawing dimensions without tolerances on machined ( not cast) surfaces?

If so, or actually regardless, I believe the note should call out 2768-1 f,m,c or v, and 2768-2 H,K or L.
The usual way to do this is 2768 mK or something similar.

 

[IvanNX]

No you can't make a case that this is against ISO.
ISO 2768 says exactly this:
There is nothing wrong about specifying tolerance larger than default general tolerance.

But they do not have any investigation or even meaningful explanation to larger tolerances. It is just, it is on old drawings so it has to be on new ones also. And they put it on everything, overall dimensions of welded assemblies for example, beams full length, hole positions...
and tolerance chains are nonexisting.

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[MintJulep]

But they do not have any investigation or even meaningful explanation to larger tolerances.

Apparently "they" have years of successful experience to support "their" approach.

Do you understand how the ISO 2768 tolerances were created?

Do you understand that the ISO 2768 "tolerances" are really not tolerances at all? That they are a collective industrial process capability.

 

[IvanNX]

@MintJulep
I am aware that standards are the grammar of language we talk internationally, so we can have mutual starting points of how things are used or what is meaning of certain "words" (items on drawings). In my previous work, tolerances were very important, as choices of CNC machines and processes and tools depended on it. And also this company has no supplier quality control, maybe that is why they have tolerance chains that do not add, as nobody is actually checking...
I do not understand what you wanted me to understand?

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[greenimi]

And also this company has no supplier quality control, maybe that is why they have tolerance chains that do not add, as nobody is actually checking..

Culture, culture and again culture.... "This is the way we are doing the things around here".......I heard this many many times before......
If the things can work like that then why change?
(maybe new workplace)

 

[MintJulep]

I answered once that they get correct products because our supplier is ignoring their tolerances and work inside ISO

There you go.

The ISO 2768 "tolerances" are the codification of a large survey of the process capability of many many fabricators.

The result is that if you specify ISO 2768 "tolerances" you are unlikely to get "out of tolerance" parts.

However it also means that if you don't specify ISO 2768 "tolerances" you will most likely still get parts with exactly the same population variation.

In my previous work, tolerances were very important

And it seems that in your current work they are less important.

 

[IvanNX]

There you go.

I do not think that outcome is only important, the way how we do things is what is more important long term. I can accept this on projects that are in a urgent, or something that makes you to lower the quality of drawings and models for a short period of time, but doing it just because everybody else works inside ISO so you will get good results because of that is not good for the company. They teach young engineers bad behavior.
What if that kind of engineer ends up in production and decides to work on basis of bad drawings, just because he/she does not know better? Would you like that your models are done by an incompetent production engineer?

It seems that this is how things work in 21st century... The quality of everything is dropping, we mass-produce cheap things and we mass-produce undereducated professionals. I can't accept that, so I will complain to my management and try to push this to the correct side.
Thank you all for the conversation (not that it has to end here, just that it was helpful for me so far).

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[Tmoose]

As Mint Julep pointed out "The ISO 2768 "tolerances" are the codification of a large survey of the process capability of many many fabricators."

Even so, not there are sub-categories that may ( must) be specified.
2768 mK or something similar.