What's the default tolerance for 'round' ISO dimensions?

Due to our legacy products and primary vendor preferences, I have been
typically working in inches (sometimes with dual dimensions), using
the ANSI standard as primary. I am interested in gradually making the
conversion to metric (especially for new products) and have been
playing with the ISO dimensioning standard as my primary units. When
using ANSI dimensioning, default tolerances are generally covered by
notes in the title block (i.e. for all .xx two-place decimals the
tolerance is +/-.010", .xxx three-place decimals are +/-.005", etc.).
Can anyone tell me how this is handled with ISO dimensioning??

Example: By default, a diameter of 50.02mm would show up as '50.02' on
an ISO drawing, assuming you had your default place value set to two-
place decimals. However, a 50mm feature shows up on the drawing as
'50' regardless of the place value you set. I understand this is the
ISO standard, but then how do you know what the default allowable
tolerance is for that feature? Surely, you wouldn't need to apply a
specific tolerance to every dimension?

Any advice to a metric newbie would be appreciated!

-JOSH

 

There is no default tolerance in ISO dimensions.
The default tolerance practice in ANSI is wrong & outdated. For
example .XXXX three-place decimals are +/-.005" when it applies for Ø.
50" nominal diameter or Ø15.00" and that is wrong.

 

What do you mean by "wrong"?

 

I think what he means is in ISO standards the bigger the part the bigger
the tolerance is allowed to be. It may be wrong for ISO standards but I
feel that ISO standards are wrong too. Why should you allow for a bigger
tolerance for a larger hole.Even going from a 3mm hole to a 3.1mm hole
the tolerance gets bigger. Now that is wrong

 

Josh,
The standard is ISO 2768 (Untoleranced Dimensions) and it is in fact
based on ranges of size and the sizes using three charts (Linear,
Angular & Broken Edges/Chamfers & Radii)The types of dimensions are
then further refined by designations; Fine, Medium, course & Very
Course, which are based on manufacturing methods. We use "ISO 2768-m
K". Example: A Linear value of 50 falls in the range of 30 to 120mm so
ISO2768-m would interpret it as having a ±0.3mm tolerance.

As a minimum, you'll also need DIN ISO 286 for Fit Tolerancing.

Good Luck

 

Thank you ALL for your input! Does anyone recommend a good
comprehensive reference book or website that covers these standards/
tolerances (aside from the standards themselves)?

As for ANSI being right or wrong, I've never had anyone argue with me
over how to interpret a tolerance on an ANSI drawing, since everything
you need to know is covered on the drawing itself, if not with
dimension specific tolerances, then by the general title block
tolerances. If you want to show a larger tolerance for a larger hole,
then you can do so, but at least if you don't specify a tolerance, the
drawing clearly covers it without the need for a reference book to be
supplied to the machinist with each drawing.

-JOSH

 

Josh,

Just as a side note about ISO tolerancing: it's not nearly as
practical as it pretends itself to be. The ISO 2768 standard even
invalidates itself by suggesting its tolerances should only be applied
when functionality is impacted. (yes, it does say that) I find I
don't even see many European sources using it correctly. I see actual
fits tolerence callouts used more often. I'm sure I'll get flamed for
this comment, but it's true from my experience. There's a recent
interesting discussion about this at eng-tips:
http://www.eng-tips.com/viewthread.cfm?qid=206463&page=1

Also, there is nothing wrong with the Inch decimal method in the ASME
standard. It can be misused just as easily as any other system. It
is more about discipline when detailing a design more so than the
actual standard used.

Either way, I can only say do what is right for your own
operation.

Matt

 

"fcsuper" <> a écrit dans le message de 828e4adb-e539-4943-af1d-
Just as a side note about ISO tolerancing: it's not nearly as
practical as it pretends itself to be. The ISO 2768 standard even
invalidates itself by suggesting its tolerances should only be applied
when functionality is impacted. (yes, it does say that) I find I
don't even see many European sources using it correctly. I see actual
fits tolerence callouts used more often. I'm sure I'll get flamed for
this comment, but it's true from my experience.


I would not comment on what's right or wrong, but, as an European engineer,
I do not recall of a drawing using ISO default tolerances. In fact I did not
know that it even existed before this thread.
Maybe I'm ignorant.
The way I've always seen it (but again I may be short-sighted) and do every
day, is put a general tolerance (ie +/-.1mm) and add more precise tolerance
where needed.

But I surely should put my hands on this chapter and see what I can use.