Should the larger tolerance number always be on top??? 2

[Rich944]

Would apprieciate some help here

I am working on a drawing of a hollow cylinder, the outer diameter tolerance states;
189
188
- Larger number on top as I would expect.

But the inner diameter tolerance states;
159
160
- Smaller diameter on top, which is not what I would expect.

I have been told the previous checker explained this by stating that the tolerance that gives the maximum material condition should always be on top.

I've never heard of this, can you tell me if this is correct please?

Cheers,

Rich

 

[ctopher]

I always put the bigger number on top, regardless of tolerance.
Ask the machine shop how they would read it.

Chris
SolidWorks 10 SP4.0
ctopher's home
SolidWorks Legion

 

[powerhound]

I actually had this discussion with a draftsman a couple of years ago. Para. 2.2 in the 1994 standard states how to go about this but it seems to be vague because the blanket statement is "the high limit (maximum value) is placed above the low limit (minimum value)." I think "maximum value" is better stated as MMC because in every case through out the standard it is the MMC value of the FOS that is on top, not the highest.

The discussion also included the statement that Chris made about how the machinist would read it. I really thought it was an isolated case but apparently not. Are we really that dumb that we would get confused if two numbers are out of order on the number line? Two is greater than one, regardless of the order they are presented, and most machinists know that.

I always create my drawings with the MMC value on top because it is more consistent with the examples in the standard. I've built parts to drawings with dimensions that are stacked in both ways and I've always understood the smaller dimension to be smaller than the larger dimension.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[pmarc]

What dimensioning standard are you using?
If ASME or ISO, they both clearly state that in case of limit dimensions bigger value always has to be above the smaller one. For reference see:
- ASME Y14.5-2009 - paragraph 2.2(a)
- ISO 129-1:2004 - paragraph 6.3.1.

 

[Belanger]

As pmarc pointed out, the answer to the subject line of this thread is yes.

FYI -- there was a similar question about 2 months ago:

http://eng-tips.com/viewthread.cfm?qid=291156&page=3

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[powerhound]

I said in the second sentence of my post that the standard I was using was 1994 and the paragraph was 2.2. I did forget to add the (a). I also said that I see where the paragraph says one thing but then the figures in the standard show something different. Does anyone have any idea why this would be? I don't think this disparity could be covered by invoking the "intentionally incomplete" paragraph. Contradictory does not mean incomplete.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[powerhound]

Hmmmm, okay never mind. I stand completely corrected on this issue. I was convinced that I had seen this over and over again in the standard but to my dismay, I must have been dreaming because I can't find it now... Sorry about the bad info...AGAIN.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[TheTick]

MMC helps with manual processes. Helps the machinists ease into the tolerance zone with less reading.

 

[CheckerHater]

ASME Y14.5-2009
...
2.2 DIRECT TOLERANCING METHOD
...
"The high limit (maximum value)is placed above the low limit (minimum value)."

How can you possibly misinterpret that?

And later:

"When expressed in a single line, the low limit precedes the high limit and a dash separates the two values."

This explains the image in the middle of Fig.2-1

"Checkers, checkers everywhere!" - read in the voice of Cartman from South Park

 

[pmarc]

djkatt,

Just some other thoughts to consider:
If the company you work for had defined a dimensioning policy that allows usage of convention we are just talking about, and if this policy is referenced on a drawing and easily available for any user, then I see no reason to arbitralrly throw the convention away due to the fact it seems to be not in line with commonly known standards. Maybe it is really reasonable and useful in certain applications. Maybe it is well recognized practice is some industries.

Imagine extreme hypothetical case when all dimensions and tolerances were expressed in Roman letters. At first glance we would probably say it is crazy, but if there is a reference to any document that permits usage of Roman letters for dimensioning and tolerancing, then from legal point of view nothing wrong really happens.

I worked in a company where I was told to use symmetrical tolerances only. There was a certain reason to do so and everyone was accepting this practice. Theoretically I could go with unsymmetrical tolerances but this would cause extra painful job to a toolshop. Tooling issues were really critical there, so the company decided to adjust dimensioning methods to tooling needs.

And maybe your checker's standpoint was somehow similar.

 

[Rich944]

Thanks everybody for your informative replies!

To be honest I'm not sure what standard we are using as there is none stated on our drawing borders. In the past I have worked to BS8888, I'm in the UK by the way.

Apologies for reposting a similar topic, I did quite a few searches and never found that one!

Rich