Non GD&T dimensioned features & assumed tolerances 3

[Runz]

We have several drawings that contain partial GD&T positional Tolerance. Most of the diametrical dimensions do not have any GD&T positional tolerance. The titleblock says the drawings are dimensioned and toleranced per ASME Y14.5-1994. There are also tolerances given for 2 and 3 place decimals (.03 & .010) and angular dimensions (+/- 1/2°)
My question is, what are the none GD&T dimensions held to positionally? See pic attached. Are they assumed located to a true center axes? What diameter do is used to find the center axes? Depending on the diameter you use, could it not change your inspection results? Are there assumed positional locations on these diameters or only size tolerances? Just a little confused as to what is assumed when there is no GD&T on a dimension.
Thanks,

John

 

[KENAT]

It sounds like the drawing is incompletely dimensioned/toleranced.

Per ASME Y14.5M-1994 section 2.7.3 just showing diameters aligned by itself does not imply any relationship/invoke any tolerance.

Is there any not about assuming perfect form at MMC?

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[Runz]

Then the diameter dimensions that do not have GD&T controls have no default positional relationship. So as long as they are within size, their positional relationship could technically be infinite?

 

[powerhound]

Runz,

That is correct. You will find a plethora of designers and engineers who insist that some sort of "industry standard" exists that restricts the positional location to the title block tolerance. This is not true. If the drawing invokes Y14.5M-1994 then one of the fundamental rules says that all tolerances must be stated.

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[ewh]

If the drawing invokes Y14.5M-1994 then one of the fundamental rules says that all tolerances must be stated.[/q]

...I agree 100%; however, where in the standard is it stated that the tolerance block is not a valid statement? If it is not, what is its purpose?

I'm not saying that it wouldn't be much better to explicitly position the other diameter - far from it, just that, as poor as of a practice as it may be, the definition is still there and assumptions do not have to be made. I do not understand where "industry standards" enter into the question.

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[pmarc]

ewh,
Reading your response, my question would be: how many decimal places does implied 0 have? Which general tolerance - .03 or .010 controls the relation between cylinders?

 

[powerhound]

I brought up "industry standard" because that has been a common defense when I point out this very issue to drafters and engineers. As much as I've heard about this standard, I have yet to see it. I've never even met anyone who has seen the standard. It's kind of like a "Bigfoot" document. Everyone has heard of it but no one has really seen it.

pmarc pointed out exactly my rebuttal in those cases. The title block tolerances are based on decimal places and they are always "unless otherwise specified". In the case of these coaxial features, there are neither decimal places nor other specification.

Just assuming 2 or 3 decimal places is one assumption, assuming whether each cylinder should be coaxial to a single cylinder or whether each cylinder only needs to be coaxial to the one adjacent to it is another. There are plenty of assumptions to be made here.

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[dgallup]

Re "industry standard". When I worked in France, I asked the head of the drawing department what standards they used since nothing was indicated on their title blocks. The reply was that it didn't matter since "they all went to the same schools and were taught the same way". So I gave them a little test and asked them to tell me what they thought a couple of dimensions and GD&T'S meant. Of course I got a different interpretation from almost every person I asked. So I asked them to produce this common standard that they were all taught. Never did see it in the 2 years I was there.

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[Runz]

It seems the more people that reply to this thread, the "Fussier" it becomes! If you have several coaxial diameters and no GD&T, what is there relationship to one another??? Is there a relationship? I am a bit confused as to what controls there position to one another? Is there an assumed positional location that is derived from the linear dimension tolerance and angular tolerance?
Thanks,

 

[MintJulep]

Always assume that if people need to make assumptions about your drawing that they will assume wrongly and you will assume extra cost, delay and look like an ass to your management.

What's so difficult about concise, complete and unambiguous?

 

[pmarc]

Runz,
There is no clear answer to your questions, because the print you are showing is not clear, means it is ambiguous. If I had to measure a part shown on the print, I would be really confused what to do, except for the feature having positional tolerance applied.

First, I would not know if the coaxiality of other features is controlled relative to datum feature A or in a different way. Like powerhound mentioned, it is not precise whether each cylinder should be coaxial to a single datum feature cylinder or whether each cylinder only needs to be coaxial to the one adjacent to it. Having diameter dimensions on the print only is not clarifying the thing.

Second, even if we assume for a moment that coaxiality of all cylinders has to be controlled relative to datum feature A, it is not clear what controls the coaxiality - 1) general tolerance for linear dimensions (which, like I already described, is muddy enough for 0 dimension) or 2) general tolerance for angular dimension. These two can result in two different interpretations, so that makes things even worse.

I think the key to everything here is a word "assume". There are too many assumptions in this case. Even if I tell you that there is a positional relationship between features defined on your print, I cannot guarantee that my interpretaion would be the same as someone else's.

 

[ewh]

Thanks for the examples; I do stand corrected, as I had not thought of it in that way before. Using only the block tolerances does indeed infer a common origin.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[powerhound]

There is no assumed, implied, or otherwise, positional location between the cylinders. Neither is there a stated one--except for the single positional tolerance callout. The drawing is incomplete and anything created from this print cannot be wrong...nor can it be right.

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[KENAT]

Where do the standard +- block tols even come into it?

The answer is per my initial post and the section of 14.5 I refer to (assuming you're working to that standard).

Runz, yes without some explicit control or an all over note about assuming perfect form at MMC for aligned/coaxial ... features, there is no assumed tolerance for features shown aligned/centered. As such yes they could be infinitely misaligned and technically meat drawing requirements.

Runz, take a look at 2.7.3 of the standard rather than relying on all us talking heads;-).

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[pmarc]

Kenat,
What you are saying with 2.7.3 in Y14.5 is totally correct as long as there is absolutely nothing on the print that implies such relationship. Unfortunately there are those very muddy general tolerances on it and I think this is why Runz wants to know if they are defining mutual relationship or not.

My vote is that depending on who is reading the print the relationship can be interpreted differently. One can say there is a relationship, the other that there is nothing that controls the coaxiality. And this is sufficient to state that the print is ambiguous enough to be rejected.

By the way, if there were two features of size shown at right angle to each other without any geometric control, would you consider the general angular tolerance as a limitation of their mutual perpendicularity?

 

[Runz]

In regards to pmarc's reply, the drawing is ambiguous. And that is the problem. As stated, the only other tolerance notes on the drawings are the tolerance box which states the decimal tolerances and angular tolerance.

That is the point. I personally have never seen a print that has every coaxial feature with GD&T controls. The critical features have it, but the less important features do not. The only other dimensioning notes on these drawings are the default decimal and angular tolerances. Thus the reason for my question.

Thanks to all for their posts!

 

[dgallup]

I think the envelope principle would give an upper limit on eccentricity.

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[powerhound]

I don't know the exact interpretation of the envelope principle per ISO but if it is the exact equivalent of ASME's rule #1 then there is no consideration given to eccentricity. The rule applies to single features only and does not govern any relationship to other features.

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[KENAT]

Sorry Pmarc, your idea of trying to invoke general +- block tols holds no water as I see it. 2.7.3 explicitly says "limits of size do not control the orientation or location relationships between individual features".

Even if you ignore that or think around it and want to invoke the block tols, if they vary with # of decimals how do you know which one to choose?

The OP states "ASME Y14.5M-1994" so I'm not sure why iso came up, or why there's much argument.

As to the 90° issue, this is addressed by 1.4(i) & (j)

Runz, just because it's common for aligned axis etc. not to be controlled, doesn't make it correct per 14.5. I've seen many drawings without such controls.

I believe there was actually a significant legal case relevant to this and I believe it may have been posted in an earlier thread about this topic. Basically a company sent out drawings without explicit controls on the coaxiality of some features. The machine shop was a ways off perfect alignment and the company rejected them. The machine shop took them to court, saying they met the drawing requirements, and the company was forced to pay.

The drawing is incomplete, ask for clarification from the design authority.

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[Belanger]

Paragraphs 1.4(i) and (j) don't answer pmarc's question. They merely state that things that appear as 90º are assumed to be 90º. They say not a word about the tolerance on the 90º.

So for that particular question of two FOS that are shown perpendicular to each other, yes we would go to the general angular tolerance in the title block. That's still not a great way to do perpendicular tolerancing, but I don't think it's ambiguous.

Coaxiality, however, isn't as simple because it's a linear relationship, not an angular relationship. So the number of decimal places comes into play -- we must throw up our hands and say it's an ambiguous drawing since there are no decimal places in the notion of center lines shown aligned with each other.

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