Tolerance, Bilateral vs Limit - Page 3

# Thread: Tolerance, Bilateral vs Limit

1. Aluminum
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Originally Posted by litlerob1
You post a link to anything that backs you up. Sorry, but your information is not correct or accurate.

Definition of the word in context of Engineering.

The aeronautical sense of nominal derives from engineering where the nominal value is the specified dimension and the reference point for tolerances. ... (engineering) A permissible deviation from a specified value, expressed in actual values or more often as a percentage of the nominal value.

But even the Web consists of conflict, because the very nature of the word is relative to the environment, the conversation, the background and the teaching chain.

R
You can google nominal pipe size. Wiki has both math and aeronautical sections and neither talk to dimensions. I thought I was agreeing with your post above though, use of nominal here is not really appropriate except to say the widget is nominally 4”. This is what your definition is saying.
Nominal - Wikipedia
Real versus nominal value - Wikipedia

2. Titanium
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Originally Posted by SeymourDumore
If the incoming B/P called for a 4.000 -.001/-.003 dia shaft, when the opsheet is created it will have only a 3.998 +/-.001 on it.
It will not have ANY mention of ANYTHING being 4.00 whatever, because it is completely irrelevant to anything we do on the floor.

So, - in our shop - we aren't changing the definition of the word Nominal, rather redefining the actual Nominal dimension.
IOW, your nominal 4.00 dia becomes 3.998, which is nominal for us.
But you are kind of missing the intent, here. I guess this is a byproduct of CAD models and nc machines but people used to use unilateral dimensions on such things as shaft fits, where going under to an extent was permissible but going over was not. In your example, previously you'd be aiming not for 3.998 but for 3.9999 but not 4.0001. You wouldn't be shooting for a bell-shaped curve centered around the nominal.

So you're sort of changing the way the parts are supposed to fit, but I guess it's inevitable with the tools used now. Unilateral tolerances were supposed to impart information to the guy making the parts but that's not so practical these days ...

3. Aluminum
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Originally Posted by SeymourDumore
Eggzatctly!
It implies nothing more and nothing less than what is intended to imply.

In OUR shop and OUR drawings, NOMINAL means what you're aiming for.
If the word Nominal refers to a dimension, then it is the middle of a symmetrical tolerance band.
If it is a material for say EDM fixturing, then it's nominally 17-4PH stainless first, then use whatever other SS you have laying about with AL as last resort.
If it refers to tooling, then the nominal means the strongest and most appropriate on hand, else use whatever is available.

See, there is no right or wrong, just interpretation of the intention.
I can kind of follow your logic on the dimensions even though that’s not how it’s used normally, but saying aluminum is nominally stainless I do not get at all what you are trying to convey. Going back to the OP, nominal was used as normal in engineering, the shaft being nominally 5”.

4. What Seymour is saying is......."nominal" is whatever he says it is, in his shop, in his environment. Which is accurate, even if it is NOT "nominal" in mine.

R

5. Diamond
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Originally Posted by litlerob1
What Seymour is saying is......."nominal" is whatever he says it is, in his shop, in his environment. Which is accurate, even if it is NOT "nominal" in mine.

R
Yupp, and yet, we would both be correct using the word "Nominal" .

but saying aluminum is nominally stainless I do not get at all what you are trying to convey.
Context! Context is what gives meaning to the word!

If I ask any one of the guys to get, saw or otherwise find something for an explicit Wire EDM fixture, they all know to look in the 17-4 pile
as that is the material we use for Wire EDM tooling/fixturing.
How do they know? Because that is the NOMINAL material we use for Wire EDM tooling and fixturing.
If none is available in 17-4 AND we need to git'r done now, then any SST is the next subject.
Short of any SST that fits, then AL is what the sum'bitch is gonna be made out of.

And for the record, that above process is also the "nominal" process which we use when it comes to all other fixturing, let it be for EDM, milling, turning or welding.

There is also a "nominal" process of handling incoming material, which is: If you don't know what the shit on the dock is for, DON'T TOUCH IT!!!

6. Diamond
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This has gone into the crazy world.
Nominal is a number, it is not the target or where to center your run of parts.
If you do not get that on one sided tolerances you have miles to walk.
Know what a CPK is and the ability control it across small or one piece part making?
That is probably a weird concept to small shops but it would be better if more understood it.
It is sad if we do not teach our employees that will become our future competitors or else in such. (We train our own assassins but at least we trained them well.)
Bob

7. Originally Posted by CarbideBob
This has gone into the crazy world.
Nominal is a number, it is not the target or where to center your run of parts.
If you do not get that on one sided tolerances you have miles to walk.
Know what a CPK is and the ability control it across small or one piece part making?
That is probably a weird concept to small shops but it would be better if more understood it.
It is sad if we do not teach our employees that will become our future competitors or else in such. (We train our own assassins but at least we trained them well.)
Bob
Not trying to be nit-picky. But asking if anyone knows what an acronym is, is like asking to prove that the scalpel is sharp. I don't assume to knowing acronyms, but I might know if I knew what it stood for. ROFLMAO.

R

Because Acronyms are NOT nominal. Hahaha.

8. Diamond
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Is this because CPK is weird? Sorry part of my world for 40 years and I assume most now can get a handle on it.?
I do care about nit picking and that's all good in my would as I do it here much.
If I am speaking strange or maybe to tech side for sure say so.
Bob

9. Titanium
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Originally Posted by CarbideBob
This has gone into the crazy world.
Nominal is a number, it is not the target or where to center your run of parts.
If you do not get that on one sided tolerances you have miles to walk.
Know what a CPK is and the ability control it across small or one piece part making?
That is probably a weird concept to small shops but it would be better if more understood it.
It is sad if we do not teach our employees that will become our future competitors or else in such. (We train our own assassins but at least we trained them well.)
Eek, an SPC lecture. You're mean, Bob

My own fault for not being clear ... if I see a unilateral tolerance, such as 4.0000 +0 -.010 then I assume what the draftsman is trying to say is "What I WANT is 4 inches even but I know you lousy shop guys can't do that so I'ma gonna give you some leeway." So if running an engine lathe I'm going to hang to the high side and try to hold my size tighter. Turning handwheels I guess the Cpk is more variable than with nc

If it's on an NC machine then I'm going to take the accuracy of the machine into account and shoot for a number just under the 4.0000, not a number in the middle of the tolerance band. If the machine can hold a half, then I'll make the programmed target six tenths under the +0 number. It's often bearing bores or seats that get one-sided tolerances, so the fit is more important that the title block suggests, if you want to make a better part.

Maybe this is wrong but looking at a print, it seems common to me. Working off a model I don't see how you can make these judgements. In fact, I don't see how people can work directly off a model, since there are so many ambiguities if the part gets assembled to anything else.

Maybe people should not make these judgements anymore ? Maybe the accuracy of the tools is so much better we don't have to ? But if you get a print from a 1956 KHK to make repro parts on your Pacemaker in the basement, maybe the viewpoint is still useful ?

Kinda niche, hunh ?

10. Aluminum
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Originally Posted by litlerob1
What Seymour is saying is......."nominal" is whatever he says it is, in his shop, in his environment. Which is accurate, even if it is NOT "nominal" in mine.

R
Yeah that much is now clear. He's using nominal instead of normal....as well as the "target" dimension, which the definition you provided somewhat supports. But again it's just the basis for tolerance, not necessarily the middle/mean as was pointed out. Outside of his shop, there is a much more common use of the word, just as used in the original post. Just because it's a somewhat vague word doesn't mean there isn't a commonly accepted use that he's butchering. I asked my GF who's an environmental scientist what nominal was in her field and she said an estimate or rough size. Here's another example, spoiler alert, they use a 2x4 as an example:

What is nominal dimension? definition and meaning - BusinessDictionary.com

11. Originally Posted by SeymourDumore
Mike, you have got to be fucking kiddin' me ( and everyone else ) with this!!!
It is the damned B/P that has to say 2.9933 +/-.0002 ( or whatever ) for the tolerance, but the fucking modeling process shouldn't give one shit or another!
If you can model the fucking hole to be 2.994 for your personal easement, then why can you not model it to be 2.9933, which is exactly what you fucking want the fucking thing to be!!!
No I am not kidding.

How much modeling do you do? Yes, the sotware could care less if I make it 1.9999945 or 1,99, but guess which one is easier for the guy doing the work?? I do it enough to know that for me, YMMV, it is certainly easier to model something to whole numbers than to .0003" (whatever it may be). I get we are all "math geniuses' here because we are machinist and all... but it is still easier and quicker for me (and probably alot of people) to model something to "nominal" (however you want to use the word ) than divide 1.9997 by 2 or 3 to make centerlines,equally spaced holes... and such. Besides, can you not read a print? Ya know, the little thing that is the bible in most shops. So if the dimension (regardless of the model, if there is one) is *1.000 -.0002/-.0008" You don't make it 1" and say "that's what the print says!"

edit: For the record, right or wrong, I see bearing bores called out like that ALOT (with a minus-minus tolerance)

12. Originally Posted by litlerob1
And this is why use of the word "Nominal" is a mistake. It's not as if this is the first time it's come up. Literally the definition is exactly what it is defining, regardless of what it is in fact defining. It could be value associated with Bananas in a bunch, as long as the context is correct, the word used is correct.

R
Here, I was simply pointing out that getting answers and use of a word such as "Nominal" shouldn't go hand in hand. If it's used by different people who use it differently, then there can literally be no solution to the problem. 2+2=4 is irrelevant if one person thinks 2 isn't Two. Doesn't make one wrong or right, it's just bad terminology in the first place. The nature of the word is relative, thus ambiguous. So my suggestion is eliminate Nominal from the vocabulary.

Then Bob wants to start talking Steven Covey, Six Sigma, Voodoo, wackadoo just to compound the insanity.

R

13. Aluminum
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Originally Posted by EmanuelGoldstein
.....if I see a unilateral tolerance, such as 4.0000 +0 -.010............If it's on an NC machine then I'm going to take the accuracy of the machine into account and shoot for a number just under the 4.0000, not a number in the middle of the tolerance band. If the machine can hold a half, then I'll make the programmed target six tenths under the +0 number. It's often bearing bores or seats that get one-sided tolerances, so the fit is more important that the title block suggests, if you want to make a better part.
In this case do you quote as if the part is ±.0006"? If so, how do you win the contracts?

If the same part is toleranced 3.995±.005 but is still shaft for a bearing, does that change how you qoute or program the machine? Should it?

If the print was asking for "a better part" wouldn't it be specified in the tolerance?

14. Stainless
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Originally Posted by EmanuelGoldstein
My own fault for not being clear ... if I see a unilateral tolerance, such as 4.0000 +0 -.010 then I assume what the draftsman is trying to say is "What I WANT is 4 inches even but I know you lousy shop guys can't do that so I'ma gonna give you some leeway." So if running an engine lathe I'm going to hang to the high side and try to hold my size tighter. Turning handwheels I guess the Cpk is more variable than with nc
Tighter tolerances cost more money, whether on manual equipment or NC. If I quote it for a .010" tolerance band, I'm going to use that tolerance band and aim for the middle, which minimizes my chance of making a scrap part. If you want parts between 3.995" and 4.000", or between 3.999 and 4.000", then that's what you should ask for, and I will quote it higher and take the additional care to meet it. If you don't know what tolerances I can meet for how much, ask.

15. Stainless
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Originally Posted by JCByrd24
Yeah that much is now clear. He's using nominal instead of normal....as well as the "target" dimension, which the definition you provided somewhat supports. But again it's just the basis for tolerance, not necessarily the middle/mean as was pointed out. Outside of his shop, there is a much more common use of the word, just as used in the original post. Just because it's a somewhat vague word doesn't mean there isn't a commonly accepted use that he's butchering. I asked my GF who's an environmental scientist what nominal was in her field and she said an estimate or rough size. Here's another example, spoiler alert, they use a 2x4 as an example:

What is nominal dimension? definition and meaning - BusinessDictionary.com
The term nominal should not be used and interpreted for manufacturing purpose.
It just a rough value, meant for reference only.
In day-to-day conversation, it is more convenient to refer to the size as 4, rather than 3.995, the mean value.

16. Diamond
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Originally Posted by JCByrd24
... Here's another example, spoiler alert, they use a 2x4 as an example:
I think a 2x4 is a great example of nominal dimension.
As a kid I actually thought that they should be 2 x 4 inches and the stuff in my hand must be something else altogether.....
Metric size callouts for carbide inserts are another just about the same. If you made most inserts to these numbers they would not fit correctly.
Bob

17. Diamond
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Originally Posted by Mike1974
No I am not kidding.

How much modeling do you do? Yes, the sotware could care less if I make it 1.9999945 or 1,99, but guess which one is easier for the guy doing the work?? I do it enough to know that for me, YMMV, it is certainly easier to model something to whole numbers than to .0003" (whatever it may be). I get we are all "math geniuses' here because we are machinist and all... but it is still easier and quicker for me (and probably alot of people) to model something to "nominal" (however you want to use the word ) than divide 1.9997 by 2 or 3 to make centerlines,equally spaced holes... and such. Besides, can you not read a print? Ya know, the little thing that is the bible in most shops. So if the dimension (regardless of the model, if there is one) is *1.000 -.0002/-.0008" You don't make it 1" and say "that's what the print says!"

edit: For the record, right or wrong, I see bearing bores called out like that ALOT (with a minus-minus tolerance)
All I am saying is that your convenience is going to cost you when I have to modify your model to reflect the median dimension.
If you don't understand the reason behind it, then I'm sorry.

18. Originally Posted by SeymourDumore
All I am saying is that your convenience is going to cost you when I have to modify your model to reflect the median dimension.
If you don't understand the reason behind it, then I'm sorry.
First, hang on a second, I am talking about TENTHS (which you are going to control using tool offsets anyways...).... I'm not saying model something to 4" (per the examples we have seen), but then spec it to 3.989 +/-.001"...*

Second, there is no reason to adjust the model (IMO) unless say you pass it on to your cmm guy for programming, and they aren't smart enough to read the print tolerances. I can easily!! tell my cam software to leave plus or minus stock so the model becomes a reference... but hey, you do you.

Yes, I get it, everything is easier when all your prints/models/software/inspection is/are perfect, but I don't think any of us live in that world (if you do let me know so I can move )....

AND in case I didn't leave this before...

Table of Metric Shaft Tolerances per. ISO 286 Chart Calculator | GD&T Tolerances | Geometric Tolerances | Engineers Edge

ISO tolerances/fits are (almost?) always called out to mean/nominal, however you phrase, it to 10mm H6/h6 and you are expected to know (or look up) that means plus zero/-.xxx or vise versa depending on shaft or hole. You will not likely find a shaft modeled to the 'mean' (10mm h6 =... 9.9955mm) it will be 8-10-12mm and YOU machine it to the correct tolerance.

I do agree with you on some of the stuff, IT IS LAZY to model a part/bore/whatever to 4.000 but then spec it to my example, but I AM TALKING ABOUT TENTHS and such that is easier to model to a whole number, or an even number.

19. Diamond
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Originally Posted by Mike1974

Yes, I get it, everything is easier when all your prints/models/software/inspection is/are perfect, but I don't think any of us live in that world (if you do let me know so I can move )....
Well, being a jobshop, it is my decision to make all drawings and models to be numerically perfect as it makes everyone's life a whole lot easier.

Originally Posted by Mike1974

ISO tolerances/fits are (almost?) always called out to mean/nominal, however you phrase, it to 10mm H6/h6 and you are expected to know (or look up) that means plus zero/-.xxx or vise versa depending on shaft or hole. You will not likely find a shaft modeled to the 'mean' (10mm h6 =... 9.9955mm) it will be 8-10-12mm and YOU machine it to the correct tolerance.
And I'm not even touching THAT idiotic can of worms!

20. Stainless
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Doesn't matter if it's thousandths, tenths, or millionths, modelling to anything other than the median is laziness and can result in an impossible geometry condition. Your laziness may save you a couple seconds, but then there's a good chance it's going to cost the programmer, the operator, and the inspector minutes each at a minimum. When that impossible geometry condition is identified, it will likely cost hours including a bunch of time on the phone for you. This is a form of technical deficit; pay for it now or pay for it plus interest later. I recently had to help fix some H10 driver tip designs where the designer (who's no longer with us) had taken a shortcut and fudged the numbers a couple thou without updating the model. Saved him a few minutes I'm sure. It cost the company weeks of man hours and a couple batches of parts (resulting in a shortage) that could not be made to pass inspection due to an impossible geometry condition. Just take a few seconds or minutes and do it right the first time.