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Metric tolerances in title block
4

Metric tolerances in title block

Metric tolerances in title block

(OP)
Hi,

I recently started working for a new company, I noticed that the default metric title block we are using only has the specifie tolerance as follows:

TOLERANCE +/- 0.1 (mm)

All current and past drawings are dimensioned with 2 decimal places. So for example right now if we dimensioned the width of a plate as 19.10 mm, with the only reference we have in title block (Tolerance +/- 0.1) this width could be made at 19.00 mm or 19.20 mm. But what would happened if the width is 1900.00 mm and we don't need a tolerance as tight as +/- 0.1.
There is no specification for decimal places whatsoever, the only thing it says is: TOLERANCE +/- 0.1
We design from PCBs to low, medium and line voltage lighting systems and every single aspect of the fixture (housings, electrical boxes, brackets, etc). I really think we need to make some modifications to the title block, as the specification is real vague.

I was thinking something like this:

UNLESS OTHERWISE SPECIFIED:
X +/- 0.5
X.X +/- 0.3
X.XX +/- 0.13

Thank you for your advice.

JJAV 1983

RE: Metric tolerances in title block

Are you ever likely to make a PCB that's 1900 mm in any dimension?

RE: Metric tolerances in title block

(OP)
Certainly no, I meant to say 190 mm, and that was for the box/housing that the PCB is going to installed on.

RE: Metric tolerances in title block

First you need to know (and tell us) what drawing standards you are are using.

Second, every metric drawing standard I am familiar with does not use the number of decimal places to indicate tolerance. As linked above there is an ISO standard that lets you pick a class of tolerances (fine, coarse, etc.) and the default tolerance varies with the nominal size.

Third, many companies (including where I work) use a single default tolerance for a drawing. This can change from drawing to drawing so you can set an appropriate value.

Forth, you can always put an appropriate tolerance on each and every dimension. I encourage this because it involves consciously thinking about the importance of each dimension and the impact of the tolerance.

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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.

RE: Metric tolerances in title block

dgallup,

I agree strongly with your point four.

Thinking about each and every dimension is worthwhile. Also, it does not take long to apply tolerances to each and every dimension, unless you realize that the tolerance does not work. Now, you have some re-engineering to do to make sure everything works.

--
JHG

RE: Metric tolerances in title block

(OP)
dgallup

1. ISO standard.

2.Should I just refer to ISO standard in title block, i.e :
UNLESS OTHERWISE SPECIFIED TOLERANCE: ISO 2768m

3. What do you use as a single default tolerance?. In your third statement ("This can change from drawing to drawing so you can set an appropriate value") did you want to say: This can change from drawing to drawing so you CANNOT set an appropriate value.

4. I totally agree with you on this. We should apply appropriate tolerances on every dimension, although sometimes your drawing can get really messy, using a sheet size A4 don't help also. That's why I wanted to know default tolerances on metric drawings so we can apply this to our format.

RE: Metric tolerances in title block

The official designation of ISO 2768 callout consists of 2 letters.
2768-1 specifies linear and angular dimensions and has 4 letters (f,m,c,v) that specify class.
2768-2 specifies feature tolerances and has 3 letters (H,K,L) that specify class.
When applied to a drawing, you need both; ISO 2768-mK would be a valid callout. Note there is a dash between 2768 and the clasees.

As for a default tolerance, it is not needed when using ISO 2768 as your standard since the standard determines the tolerance based on the size of your part.

Also, note that metric dimensions do NOT use trailing zeros. So your original note the size should be 19.1 with a range of 19 to 19.2, if you use a 0.1 tolerance.

"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

RE: Metric tolerances in title block

JJAV1983,
Are machined or sheet metal parts amongst products offered by your new company?

RE: Metric tolerances in title block

(OP)
pmarc

Yes we have machined and sheet metal parts our line of products.

RE: Metric tolerances in title block

I see you have PCBs. Plastic parts too?

RE: Metric tolerances in title block

(OP)

No plastics parts at all.

RE: Metric tolerances in title block

Quote (JJAV1983 )

3. What do you use as a single default tolerance?. In your third statement ("This can change from drawing to drawing so you can set an appropriate value") did you want to say: This can change from drawing to drawing so you CANNOT set an appropriate value.

No, I meant what I said. If I have a precision machined small part I might choose a +/-0.1 mm default tolerance. If I have a vacuum formed plastic tray to hold my parts I might choose a +/-1 mm default tolerance. In our CAD system the default tolerance is read from the model, our part template has the default tolerance set to +/-0.2 mm but I can change it and the drawing will show that.

There is nothing worse than having excessively small default tolerances and a drawing with all nominal dimensions. It tells me the "designer" gave absolutely no thought to tolerances and how the part functions.

----------------------------------------

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.

RE: Metric tolerances in title block

OK, thanks.

I asked because ISO 2768 is intended for: "[...] dimensions of parts that are produced by metal removal or parts that are formed from sheet metal" and it "[...] may be suitable for use with materials other than metal" and for "[...] features manufactured by other processes", but with special examination required.

Regardless whether you decide to switch your general tolerances to ISO 2768 (poor choice, in my opinion) or not, I thought I should clearly mention this.

RE: Metric tolerances in title block

So, there are enough negative choices for now:

  • You can use number of decimals NOT, because metric dimensions cannot have trailing zeros both in ASME and ISO.
  • You can use one single default tolerance NOT because your parts span over wide variety of sizes.
  • You can invoke international standard ISO 2768.
  • You can invoke American standard ANSI B4.3.
What are the positive choices left?

  • You still can use International Tolerances per ANSI B4.2, creating note like: "UNTOLERANCED DIMENSIONS HOLES H12, SHAFTS h12, OTHER +/-IT12/2"
  • You can actually create a table based on ISO 2768, or ANSI B4.3 and place it in or near your titleblock:

RE: Metric tolerances in title block

CheckerHater

I do not understand why you say:

  • You can use one single default tolerance NOT because your parts span over wide variety of sizes.
The single default tolerance DOES NOT HAVE TO BE THE SAME ON EVERY DRAWING. You just set a reasonable default tolerance for the part on each drawing. What is so difficult about this? Just like ISO 2768 lets you choose a class appropriate fro your part.

----------------------------------------

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.

RE: Metric tolerances in title block

Ok, let me be more specific.

I’ve got impression that OP found the technique unsatisfactory, because one could encounter features of different size within the same part; so he was looking for possible alternatives.

Naturally, you can use different tolerance values for different parts, but you still have to cover features that are too big or too small, for example use direct tolerancing on them.

And, yes, there’s nothing wrong about 2768, it just appears to be unpopular.
This is why I suggested ways of creating your own “class” by other means.

RE: Metric tolerances in title block

I have to say I like 2768 because of the note to not automatically reject parts because they don't meet the tolerance. If they fit, use them. We got a big kick out of this note when we first started making parts to this standard. The inspector brought the parts to engineering and we'd say give me a minute, part fits ok, stamp it accepted. Provided a class of entertainment. That said we are most often fabricating to the tolerance rather than designing based on it.

Regards, Diego

RE: Metric tolerances in title block

(OP)
Thank you for the advice,

I think that CheckerHater suggestion will be the best option. I am just thinking if it would be valid to say:

UNLESS OTHERWISE SPECIFIED:
TOLERANCES: ANSI/ASME B4.3 (in case of using American Standard)

dgallup

When you said: "... single default tolerance DOES NOT HAVE TO BE THE SAME ON EVERY DRAWING. You just set a reasonable default tolerance for the part on each drawing..." You meant for each different size drawing (small part/big part) I would have to go and change the default tolerance?...

As you said it might not be a big deal, I am just thinking maybe one day a designer will forget to adjust that part in the title block and the end result will not suit the design intention. Just trying to avoid that type of problems in the future.

Thanks.


RE: Metric tolerances in title block

JJAV1983,
Let me ask you one question:

There is a statement in appendix to ISO 2768 (and probably in B4.3) that says:
"These advantages [of using ISO 2768] are fully obtained only when there is sufficient reliability that the general tolerantes will not be exceeded, i.e. when the customary Workshop accuracy of the particular Workshop is equal to or finer than the general tolerantes indicated in the drawing. The Workshop should, therefore
- find out by measurements what its customary Workshop accuracy is;
- accept only those drawings having general tolerantes equal to or greater than its customary Workshop accuracy;
- check by sampling that its customary Workshop accuracy does not deteriorate."


Have you had a chance to investigate that these conditions are/can be met in your new company?

--------------

DiegoLGraves,

The newest edition of ISO 8015:2011, "GPS - Fundamentals - Concepts, principles and rules" states in paragraph 4.4 entitled "Workpiece functional level" that: "It is assumed for interpretation that the workpiece functions 100 % within the tolerance limits and 0 % outside the tolerance limits." How would you refer to this sentence?

--------------

All,
Please don't get me wrong. I do not think ISO 2768 (or ANSI B4.3), at least part 1, is pure evil. That is not true. All I am trying to say is that serious and thorough analysis have to be done before such record in drawing general notes appears. Otherwise it can really cause more harm than benefits.

I do not even want to start again a discussion whether the tolerances specified in this standard back in late 80's are realistic in 2010's. I think we all could write a book on this topic based on our experiences or spend countless hours on debating what is right and wrong. The bottom line is - don't do it just like that. It is not that easy as it seems to be.

This is just my opinion.

RE: Metric tolerances in title block

Mr. Graves,
I have been wondering how this works for people who actually use it. That same statement is not very popular with many on this board, myself NOT included.
CH,
Are we sure it is not popular outside of our own little world, here? Really GD&T (ASME or ISO) is not popular outside of here for the most part!
Frank

RE: Metric tolerances in title block

(OP)
pmarc

At the headquarters we just design the fixtures, most of our parts are being made overseas. Whenever the manufacturer send a quotation to us it says TOLERANCE: ISO2768-m, therefore they can meet the requirements no problem.

I really think that wheter referring to ISO2768 or ANSI B4.3 it would be way better than our current tolerance of +/- 0.1 for every single part that we are designing, no matter what size it is. I think if we put this in practice it will force the other designers to think a little more.

RE: Metric tolerances in title block

fsincox,

Your phrase about “outside popularity” gave me idea to look into actual geographical popularity of certain standards. It took some time on the Internet (but not a lot of it though).

Here are the results: General tolerance standards around the world:

Brazil: NBR ISO 2768

England: BS EN 22768:1993 (ISO 2768:1989)

France: NF EN 22768:1993 (ISO 2768:1989)

Germany: DIN ISO 2768

Italy: UNI ISO 2768

Japan JIS B 0405 (ISO 2768)

Russia: GOST 308931-2002 (ISO 2768:1989)

Sweden: SS ISO 2768

Some trivia:

Brazil is known as the most “standard-loving” country. First country in the world to adapt new international standard for electrical outlets. I’ve heard story about sales person who had to run to local hardware store to buy new plug, otherwise he couldn’t do the presentation of his machine.

England - Not so long ago made important decision: abandoned Taylor’s principle A.K.A. “Rule 1” and fully embraced ISO “independence principle” framework.

France and Germany are responsible for most of EU’s GDP. And I mean MOST of it.

Italy - well, 2013 Dodge Dart is 2012 Alfa Romeo Giulietta.

Japan – don’t know where to start smile

Russia – not exactly technology leader, but it spreads over 1/7th of the total landmass - can’t ignore that.

Sweden – the largest European economy outside of Euro-zone.

So, there may be some popularity “outside”.

RE: Metric tolerances in title block

JJAV1983,
If you are really confident that the system will work, use it.

RE: Metric tolerances in title block

JJAV1983, as you've probably gathered the problem with wanting to use your tolerance block is that generally metric dimensions don't have trailing 0's. So you can't really do it unless you come up with your own exception to the standard or equivalent.

Having a default +-0.1 mm is indeed dubious unless all your parts are of a similar size and require similar precision.

Having a single value that you set for each part can work well and is what I did at my employer back in the UK in the early 2000's (never even heard of 2768).

As dgallup says any kind of 'default' tolerance system is open to abuse. Invoking a default without consciously considering what the default is and the implications of it can cause problems.

Typical American block where tolerance varies with number of decimals is also often abused, in which direction tends to depend on what the CAD default # decimals is set at.

Beyond that some can be taken in different ways for instance the way DiegoLGraves applies that clause in 2768 starts the undermine the idea of interchangeability which was the very reason for the original development of GD&T as I understand it.

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