pattern positioning

[dnobie]

I am trying to determine if it is legal within ASME Y14.5M to locate a pattern of holes with conventional dimensioning (no datums) from an outside profile and then use GD&T to control the spacing of the pattern to itself. We are reviewing an old (80's) drawing package in which this was done extensively and are trying to determine if this is/was legal. The holes within the pattern are located with basic dimensions. The feature control frame does not include any datum references. Thank you in advance.

 

[fsincox]

Dave,
I must disagree, I am sorry you say you care but your logic says: "I dont care". The magnatude of the tolerance given should say enough to an educated reader. What you are doing is sticking to the old way "because it is not important enough". While you have the right to do that, I stand by my statement that it is one reason the standard has such trouble being really generally accepted, the other issure was it changed too much for people who don't like change, the worst thing they did in '94 was not make the datum change a recommended option.
Frank

 

[ringman1]

Frank,

Can you elaborate on the 'datum change a recommended option'?

 

[fsincox]

Dave,
I myself would like to see these practices preserved, too. I feel the standard does not adequately provide enough options in handling flexible parts say like sheet metal and plastic. The only one I know of is the restrained condition; they want me to spend time defining the manufacturing condition on a part I can bend back later, anyway.
My criticisms of the standard are too many restrictions (the whole feature of size thing) too much you can’t do this and can’t do that. The parts they show are just starting to be more realistic. I want an option like the one in this thread for parts were I really am saying: “the number from the edge is just that, a number, I don’t really care that much and when we assemble this part we can force it back if we have too. There are a lot of parts out there that are like that. Many people see no justification in using too much GD&T on. I agree with that, but, I do want it understood it means I don’t care enough to worry about spending lots of time on.
.All of my life, I have seen this done on weldments and or castings, not large mass production mind you. Onesy Twosey stuff,” just make something that looks like this” if it is not exactly are you going to throw it away, not likely. When I first started I made the mistake once of placing +/-.06 on a weldment drawing to avoid rounding a dimension to 2 places. The manufacturing people had a fit, I asked: “well what is the tolerance then?” The truth was they don’t know and don’t care what the tolerance was, you get what you get. The ISO is developing standards on this stuff. I don’t care may be the reason GM is the number 2 auto maker in the world the last I heard, I know, Toyota just had a fall, big shots need that every once and a while. It is a shame for the people though.
Frank

 

[fsincox]

Ringman,
I meant, I think, the ISO datum symbol adoption should have been a “recommended” option. From my point of view it made the whole adoption of the '94 standard optional. I am certain it has inhibited its adoption in the companies I have been in due to the resistance to change I have been talking about here. The '94 standard mostly offered clarification on principles, except the feature of size change they snuck in. 2009 is more of the same and thankfully tries to undo some of the damage of feature of size from ’94. They brought back symmetry and concentricity that were never REALLY gone. I feel these standards are mostly consistent and build upon each other, not radical revisions. It is good but not enough to make their adoption mandatory, I can look at a later standard and say restatement of a secondary datum “means this” and it does not contradict the ’82 standard. Composite profile just more composites, etc.
Frank

 

[dingy2]

Gentlemen:

I did not say that a +/- tolerance is not important (every dimension is important) but I did say that it is legal to use and I would recommend the practice if there is no function or relationship of the feature to any other feature on the part or mating part.

Why would one place a positional tolerance on a pattern of holes used to lighten the product? There is no function or relationship of the location of the holes and will give a false impression that the hole location is important to its function. Could we end up making a checking fixture for this pattern? Absolutely! Could we end up confirming the location of the pattern on a regular basis? Absolutely! Does the positional tolerance on on this pattern reflect the design intent? No!

If our primary intent in applying GD&T is to "define the feature in a clear manner", why have a composite feature control frame when the feature is already covered in a "clean manner" with a geometrical symbol? Maybe it is design intent??

As per ASME Y14.5-2009, it states "it is even more important that the design more precisely state the functional requirements". This is a reasonable statement and I totally agree with it.

JP - I did not find anything that stated "Using GD&T is not a statement about the importance of a feature. It's about defining that feature in a clear way."

I do not come from a design background and maybe that is the difference. I do come from the Quality field and I see the struggle people have interpreting drawings with the overuse of GD&T, inappropriate application or just plain wrong. If we could only reflect the design intent with the proper use of GD&T, it would help manufacturing produce a better product at less cost. This just my opinion.



Dave D.

http://www.qmsi.ca

 

[fsincox]

This may in fact be another reason to stick with the '82 standard as I said above it lays the groundwork for most of the good stuff that came later while at the same time contains, in the appendix, references to the style of dimensioning (as former practices) desired in this thread.
It seems that some people here prefer to not remember or are to new to know these practices did exist and were in fact sanctioned by ASME. I would suspect there must be millions of drawings like this out there and to sit here and pretend like they don’t exist, it takes a lot of gall to say these people were wrong, when ASME itself supported it. Really you should say that ASME was wrong.
I believe the ASME can only really recommend standard practices that is why so many have a hard time deciding if this is legal or illegal. The sad fact that so many companies I see are so far behind the current standards shows the great divide between the standard and the potential users. Walt says: (in another forum) “we have to remember the bottom line is to make money”.
The large corporations’ representatives and the GD&T sales people who sit on the committee have apparently lost touch with the small companies and shops that, I suspect, use this stuff the most and find lots of it irrelevant to their daily use. Guys, I am not pleased to have to say these things, but we must have a balance.
I am finding myself in debates with members who claim they care about every single dimension like they are all the same, now I will agree that, “anything can become important when it becomes important” but I can not accept that all things are really equal, this sounds good as a cliché but I believe it is not being honest to the way most of us really work. Do you not have general tolerances on your drawings?? The ISO is brutally honest when it talks about the reason for general tolerances. ASME 14.5 2.1.1 mentions them but never says why most of us, use them. (I would imagine any who don’t use them to be in a small minority).
I have never worked at a large mass production company and always wanted to because I though maybe they would actually appreciate this stuff. I suspect, these companies may make more actual parts in quantity but are not the majority of drawing users out in the field.
I submit it is because most companies/people do not believe all features are equal that title block tolerance usage is so common and that there are features on parts that if they are out you will be willing to throw the part away and many more you would just will live with. This is not the mass production world, but the world I am from and I suspect the majority of the users are too.
I suspect the lawsuit argument is not a big winner in the real world either as I don't see many out here so afraid of them that they hurry to adopt the latest standard either. I suspect many here are not in a hurry to give up their beloved '94 version for the 2009 version, that I definately find superior, not perfect.
Frank

 

[drawoh]

I did not say that a +/- tolerance is not important (every dimension is important) but I did say that it is legal to use and I would recommend the practice if there is no function or relationship of the feature to any other feature on the part or mating part.

Why would one place a positional tolerance on a pattern of holes used to lighten the product? There is no function or relationship of the location of the holes and will give a false impression that the hole location is important to its function...

Let me come at this from the design point of view.

I would carefully apply tolerances to lightening holes. They would be very sloppy tolerances. The relationship of the lightening holes to the datums might not matter as much to me as their position with respect to the sloppily toleranced outline. In that case, there would be sloppy ± tolerances to the edges.

If I told the machinist to somehow keep a hole within a 2mm profile, would you make a fixture to inspect it?

One of my concerns is that, while the tolerance does not matter this time, it might matter next time. Next time, the mass, and the weight distribution of the part may be critical, in which case, I will apply accurate tolerances to the lightening holes. I do not want machinists and inspectors trying to work out whether or not I mean it.

JHG

 

[fsincox]

Dave,
Isn't the fact you don't need me to tell you specifically which is primary, secondary or tertiary show that you don't really care that much?
Frank

 

[dingy2]

Frank & drawoh:

This is a can or worms with no resolution whatsoever but I will quote 1.4D from the ASME Y14.5-2009 standard.

"Dimensions shall be selected and arranged to suit the function and mating relationship of the part and shall not be subject to more than one interpretation."

The standard also states that positional tolerances are recommended (not mandated) on features of size.

1 Would applying positional tolerances at MMC on ALL patterns of holes without regarding their function and relationship of the part comply with 1.4D? Some patterns or single holes with no function or relationship would be shown in the same way so there could be more than 1 interpretation.

2 Would applying positional tolerances at MMC with relatively large tolerances comply with 1.4D? As long as we knew the line of demarcation, it would comply but nothing will ever be shown on the drawing. I think that there could be more than 1 interpretation.

3 Would applying positional tolerances on holes or pattern of holes with no function or relationship in RFS while the holes or pattern of holes with a function and relationship are reflected in MMC? I think that there could be more than 1 interpretation but the chances of this happening are minimal. This may be the best solution????

4 Would applying positional tolerances at MMC on holes or pattern of holes with a function or relationship while holes or patterns without a function or relationship are shown in linear tolerances meet 1.4D? Yes it would.

These were just some final thoughts on this controversial subject. As I stated before, no resolution will be found in this forum but I appreciate the fact that we can discuss it and appreciate other points of view.

Dave D.

http://www.qmsi.ca

 

[KENAT]

You're treading a fine line aren't you though? It says to "suit the function", it doesn't say to "to make the function explicitly clear to every user of the drawing" or similar. If the GD&T is applied properly then there's only one interpretation of what the pass fail criteria for the part is, isn't that what 1.4d is talking about?

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[axym]

Excuse the irony, but there seems to be more than one interpretation of what "shall not be subject to more than one interpretation" means. Dave's statements (and I'm not sure that I understand them correctly) indicate that it means that a given feature could be toleranced in more than one way by the designer.

To me, dimensions and tolerances should not be subject to more than one interpretation when applied to a real (i.e. imperfect) part. So that each dimension, and more importantly, each tolerance, can be defined on a real part in a unique and unambiguous way. Measurement uncertainty will always exist, but that is a separate issue. We're talking about specification uncertainty here, where the specification itself is open to multiple interpretations.

In order to truly obey the statements in Y14.5 1.4D, plus/minus tolerances must be confined to sizes (diameters, widths, etc.) of proper features of size. Applying them to anything else either doesn't suit the function and mating relationship, allows more than one interpretation, or both.

Dave, I agree that we're never going to agree on this. I know that you believe that GD&T should be applied to features that have a function and relationship, and that plus/minus should be used on features that don't. For me, it's just not that simple. I don't even agree with how you decide whether a feature has a function or not! But I agree that it's good that we can discuss various points of view.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[KENAT]

Another thought, people keep mentioning lightening or vent holes etc. as examples of features with little or no real functional requirement, but I'm not sure that's true.

You typically don't want the holes getting too near to each other, or the edges of the part etc, so is the real functional requirement a fairly loose LMC?

The thing I probably have most trouble deciding on appropriate functional tolerances are things like cable lengths. There is typically a functional minimum, however, except in rare cases where impedence or space concerns etc. are very significant, there isn't a very pressing maximum length requirement. You could just give a min dimension but in isolation that would imply an infinitely long cable is OK, which is probably isn't. You can pick a number based on a loose manufacturing capability tolerance, but if a cable came in say 1/8th" over that it would most likely work and you probably wouldn't reject it. So what is the correct answer?

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[Belanger]

The answer: long enough to hold the car if the elevator cable breaks from the top floor! (Just kidding)

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

 

[KENAT]

Fair point, make that "electrical cables" in my last post.

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