ISO Position GD&T and Dimensioning Questions 4

[Jieve]

Hello guys,

I have a number of GD&T and Dimensioning questions for you as I’ve been working on drawings to send to our shop. I am working to ISO.

1)If a hole is located at the center of a rectangular part, is it necessary to dimension the hole from the edges? I thought I read somewhere that where no dimensions are drawn, symmetry is assumed.

2)Modification of question 1: I have a rectangular part with two holes evenly spaced about the midplane. The hole are also equidistant from the edges of the part. Is it ok to simply dimension the distance between the holes, without dimensioning from the part edges? If so, is it necessary to include a centerline (or centerplane) to indicate that this symmetry assumption is correct under ISO?

3)I have a rectangular plate with a hole in the center. I would like to add a position tolerance to keep the hole within 0.1mm from the exact center. I am assuming that the best method be to make the midplanes of the part datums B & C (with the primary A being the surface the part sits on), and then specify that the position tolerance should be relative to A,B,C rather than making the edges of the part the secondary and tertiary datums? Am I correct?

4)I am somewhat confused about the use of basic dimensions. Assume I have a rectangular part with two holes spaced equidistant from the centerplane of the part. The actual distance between the holes is more important than the distance between any edge and any hole. In one scenario, I make the flat surface on which the part sits primary datum A, and each perpendicular midplane datums B & C. I specify a position tolerance of the first hole from datums A, B and C. Then I make the first hole datum D, and specify a position tolerance of the second hole relative to A, D, B. There is still assumed symmetry of the holes about the midplane. Is it required to specify basic dimensions to the edges B & C to the first hole, even though symmetry is assumed?

5)An extension to question 4, if I use a centerplane as a datum, is it necessary to add basic dimensions from the midplane to the hole?

6)I have seen another example where the part in question 4 had only datums A, B & C as described. One hole was dimensioned with basic dimensions from datums B & C, and the second hole was dimensioned from the first hole with a basic dimension as well. Both had a position tolerance with respect to A, B & C. I thought that this essentially means that both hole dimensions need to be inspected with respect to datums A, B & C, and not with regard to each other. When not using GD&T, when the position between two holes is important, the distance between holes should be dimensioned directly to eliminate tolerance stack-up. But what is the relevance of dimensioning between holes in this case when the datums (i.e. measurement locations) are specified as edges? Would it not have made sense for them to have dimensioned both holes with basic dimensions from the datums? This example was in an ASME book.

7)This may seem like a repeat question of one or two of the above, but assume I have a hole in a rectangular part. That hole has a position tolerance from datum B, which is the edge of the part. Can I use a basic dimension from the opposite edge of the part, or does it need to be from the datum edge?

8)I have 4 holes in a rectangular pattern on a rectangular part. Datum A is the flat surface the part rests on, and datums B & C are the other 2 perpendicular part edges. Each hole has position tolerances. One hole has a dimension callout 4x and the diameter. A position control frame is placed underneath this. All holes are the same size, but not all have the same position tolerance. Therefore, I use a position control separately on the other holes with deviating geometric controls. Is the 4x the diameter callout with the position control frame underneath OK even though the geometric tolerances are different on different holes?

9)Extension to question 8, except now datum D is one of the holes. I specify the position tolerance of another hole relative to frame A, D, B, where B is one of the sides. What distance would be measured upon inspection, the distance from datum D or from datum B? B is really only used to eliminate rotation in this case.

I know these are a lot of questions, but all questions I came across today while working on drawings. Your insight and help would really be appreciated, as always!

Thanks!!

 

[pmarc]

Jieve,
Your drawing 2 is clear for me with 2 comments though:
1) You do not actually need CZ modifier in the upper FCF. All positional tolerance zones are tied to each other and to datum reference frame |A|B|C|.
2) If right side face is assigned as datum feature C, I would strongly recommend originating all basic dimensions in horizontal direction from this face and not from the oppposite one - just like you have in vertical direction for B.

CH,
Some thoughts about your latest reply:
- You said: "First, if shop is capable of producing to 2768-mK, it is capable of producing to 2768-m, so there will be no price increase." I agree with you about shop capability, but the mechanism (philosophy) of costs estimation is different in the light of ISO 2768, I am afraid. I think paragraph A.3.c) in addition to already cited d) is clear enough on this: "the drawing readily indicates which feature can be produced by normal process capability, which also assists quality engineering by reducing inspection levels". I have experienced too many product price calculations for which final value depended on the amount of GD&T callouts on ISO prints to start to believe that it is a theoretical problem only.
- You asked: "Do you honestly prefer plus or minus dimension over geometrical tolerance?" Of course I do not and I always encourage to use as much GD&T as needed in order to have part's geometry fully defined, but in this case it is not a matter of my preferences. This is how ISO deals with geometrical product specification. Simple cube with one through hole in the center having only two dimensions on the print - one for block's width/length/height and second for hole's diameter - is fully defined as long as there is a reference to both parts of ISO 2768 in there. This may sound crazy for those who have ever worked to Y14.5 standard, but it is absolutely real.

Frank,
I owe you a reply regarding the meaning of CF, CZ and CT modifiers:
Shortly saying, ISO's CT is something similar to ASME's CF applied to interrupted features of size. ISO's CZ is more for geometrical tolerances. As you know, it is commonly used together with flatness callout on multiple surfaces to control their coplanarity. It is also often applied to orientation and location controls when axes of two or more nominally coaxial features have to stay within the same tolerance zone. There is also idea of using it to invoke simultaneous requirement concept for position of pattern of features if at least one of datum references within positional FCF is modified by (M) or (L), but I am not sure if this has been already standardized.
Generally, if you took a look to the latest editions of ISO 14405:2010 (linear sizes) or ISO 5459:2011 (datums), you would easily notice that the universal communization of ASME and ISO GD&T standards is not something of the highest importancy for those guys. Some concepts have been clarified indeed, but the huge amount of new fancy modifiers rather scares than makes life easier.

 

[fsincox]

pmarc,
You owe me nothing, I am sorry to hear what you say about commonality, but it does appear that way. I was trained before the 1994 version and the big push was the next version was going for communization (like implied across the board RFS). For the life of me I do not know why they made our look like theirs but not be the same, Let's keep the good old American datums then. Then you would know what you had.

Jieve's drawing must have changed, I see no CZ?

I do not think the issue of use of GD&T increasing cost and scaring manufacturing is exclusive to ISO at all. It is practically an everyday battle for me.
Frank

 

[CheckerHater]

Pmarc,
You say: ”Simple cube with one through hole in the center having only two dimensions on the print - one for block's width/length/height and second for hole's diameter - is fully defined as long as there is a reference to both parts of ISO 2768 in there” (sorry for long quotation).
This is entirely not true, plain and simple.
ISO 2768 DOES NOT CONTRON LOCATION OF FEATURES OF SIZE.
Unfortunately, like I mentioned, when it comes to ISO-bashing goals justify the means.

 

[pmarc]

CH,
What about general symmetry tolerance implied by part 2 of ISO 2768 (clause 5.2.4)?

 

[CheckerHater]

Pmarc,
How about Para 2.General saying:
“General geometrical tolerances apply to all geometrical tolerance characteristics, EXCLUDING cylindricity, profile of any line, profile of any surface, angularity, coaxiality, positional tolerances and total run-out”?
When it comes to Clause 5.2.4, please take a look at illustrations given in Annex B5. None of them define position. They only show examples of symmetry about one single datum and rightfully so. ISO 2768 provides no means to identify Primary and Secondary datums, so all characteristics that may possibly require more than one datum are excluded from the very beginning.

 

[pmarc]

CH,
Where did I mention anything about general tolerance for Position in my previous replies?
You said: "ISO 2768 does not control LOCATION of features of size". And I could not agree with this since SYMMETRY is obviously one of location controls. That's all.

My point is that if there is a reference to ISO 2768 part 2 on the print of this simple cube, I can imagine at least 2 separate symmetry callouts being implied:
1) between center hole axis and median plane derived from two sides of cube in (let's say) length direction;
2) between center hole axis and median plane derived from two sides of cube in width direction;
None of them requires more than one datum feature and both can fall under general symmetry tolerance.

 

[CheckerHater]

Pmarc,
If you really want to go into nit-picking, let’s do it.
Yes, I said “position”.
But you didn’t say “location” either.
You said “fully defined”. And when you fully define something you “locate” and “orient” it, and if it doesn’t define “position” than I don’t know what does.
Just like perpendicularity / angle argument this is pure speculation “if we let ISO 2768 control everything…” while it was never intended to control everything.
Let me bring in another quotation, this time from ISO 8015:2011:
“The drawing is definitive. All specifications shall be indicated on the drawing using GPS symbology … Consequently, requirements not specified on the drawing cannot be enforced”
If you want to “define” something, go ahead and define it.

 

[pmarc]

CH,
I do not want to go into nit-picking. I just wanted to know whether you are still thinking that the cube in my example is not fully defined or something has changed. As far as I understand, I haven't been able to convince you. And that is maybe even better, since it shows that one print can be interpreted differently when ISO 2768 general tolerances are invoked, so the whole concept is muddy.

I was perfectly aware when saying "fully defined" and didn't have to say "location", because in the light of ISO 2768 for "simple cube with one through hole IN THE CENTER" it is implied that general symmetry tolerance applies (unless any other callout overrides this), so the location of the hole relative to 2 median planes or location of median planes relative to the hole is controlled. The examples from appendix B.5 you referred to clearly prove that.

As for the quotation from the newest edition of ISO 8015, sorry, but I do not see how it is against what I said. The fact that certain requirements are not directly present on the print, does not mean they are not there. ISO 2768 can make them well existing even though not shown. I really hate it, but this is how ISO approaches to full geometrical product definition.

 

[CheckerHater]

Pmarc,
Maybe we better get out of this discussion. I see your “magic cube” as sort of “intellectual curiosity”. And BTW I admire your intellect; I just think it could be used for better cause.
Sorry, but I just cannot accept your argument. GPS specifications consist of 234 standards (probably more by today). If we replace all of them with ISO 2768, and try really hard to abuse it, we may succeed. What does it prove? That ISO is fundamentally flawed and ASME is infallible?
You say: “This may sound crazy for those who have ever worked to Y14.5 standard, but it is absolutely real”
Do I have to remind you that ASME doesn’t have any general tolerance standard for last 4 or 5 years?
That companies pay lip service to 14.5 and use general tolerances based on number of decimals, even though ASME is forbidding use of trailing zeros in metric?
And even better argument:” I have experienced too many product price calculations for which final value depended on the amount of GD&T callouts on ISO prints to start to believe that it is a theoretical problem only.”
Are you trying to say that this problem is ISO-specific? That incompetent estimators never add 5% for every ASME GD&T callout?
I’ve heard something about noticing speck in one’s eye. Could you remind me how it goes?

 

[pmarc]

CH,
You are right, we should get out of this discussion.

I just would not like to be misunderstood: I do think that neither ISO nor ASME are perfect and it is not that ISO is fundamentally flawed. But there are some things really confusing, like general tolerances concept for instance (at least to me). I am not saying this because I love Y14.5, but mainly due to the fact that I am great fan of unambiguous product definition based on functional requirements, which can be seriously in danger if ISO 2768 is poorly used.

And I also do not think that price calculation based on amount of GD&T callouts on the print is ISO-specific problem. I just tried to visualize that such seemingly wrong approach could be logically explained if keeping-tolerances-within-"customary-workshop-accuracy" was a driving factor in product price estimation.

 

[fsincox]

pmarc,
I agree in general, but, any company standard addendum has the same issues, as far as requirements not specified on the drawing. I am much more concerned about things not on the drawing or implied anywhere like the unspecified coaxial relationship on most of the drawings I see, day to day.
It is really just holding peoples feet to the fire: "if you do not specify, you have signed up for this then". I have seen people mention some ISO provision that goes something like unless specified ISO applies. Is it not?
Frank

 

[pmarc]

Frank,
Are you thinking about invocation principle as stated in clause 5.1 of ISO 8015:2011:
"Once a portion of the ISO GPS system is invoked in a mechanical engineering product documentation, the entire ISO GPS system is invoked, unless otherwise indicated on the documentation, e.g. by reference to a relevant document."?

 

[fsincox]

pmarc,
Yes, thank you, that is it "the invocation principle".
I think most would love to have all information specified on the drawing directly in an ideal world. The question here is: "is it not better to have it covered by something, rather than not covered at all?" I believe, I would also prefer the latter. This is one way of trying to force the issue that has been too long avoided in practice, the goal of complete drawing definition.
I would agree with you, symmetry is a special location tolerance as I understand ISO, like parallelism is a special orientation tolerance for us all.
CH,
Does ISO 2768 actually say that it does not control location of features of size?
Frank

 

[CheckerHater]

I quoted it word-for-word:
"General geometrical tolerances apply to all geometrical tolerance characteristics, EXCLUDING cylindricity, profile of any line, profile of any surface, angularity, coaxiality, positional tolerances and total run-out"
(Except for capitalizing)
So, no position.

 

[pmarc]

CH,
Again, the fact that positional tolerance is not covered by ISO 2768 does not mean that location of features is not covered at all. Let's not equal POSITION with LOCATION.

Frank,
Symmetry, even in Y14.5, is treated as a tolerance of location, so it is not only ISO-specific thing. In ISO, unlike in ASME, symmetry is a special case of position, not location.

 

[CheckerHater]

Pmark,
Yes, in ISO symmetry is a case of position; if they can “position” flat plane, they can position median plane, no problem.
I was under impression that we stopped this argument.
If you want to develop it further, we should figure first, what we are trying to achieve.
We already discussed that I said "position", but you didn't say "location”.
And you didn’t prove that part will be “fully defined” yet.
I also agreed that if we pretend that 243 GPS standards do not exist, and we replace all of them with ISO 2768, then we bend 2768 as much as possible, then we can abuse it and make a bad part.
Now, exactly how bad part?
Let set another thought experiment. Look at the enclosed pictures of your “magic cube”.
Imagine that we send the prints to several manufacturers, allowing them to produce as bad parts as possible, without violating requirements of the prints.
Which print do you think will return better parts?

 

[fsincox]

pmarc,
Sorry, I am well aware of the status of symmetry, IMHO, symmetry is almost verboten in ASME since 1982, it is only paid lip service by ASME because the ISO has it and so they can say: “don’t do this”.
My personal experience is more like you are chastised, particularly amongst the elites, for speaking of it. This attitude does not seem to actually apply on the shop floor at my work, I haven't really ever tried it since it has been basically, forbidden since I was first trained.
Frank

 

[pmarc]

CH,
Why are you saying that I didn't prove the part will be fully defined. I clearly stated that for such cube with the hole in the center at least 2 separate general symmetry callouts could be applied:
1) between center hole axis and median plane derived from two sides of cube in (let's say) length direction;
2) between center hole axis and median plane derived from two sides of cube in width direction.
What else do you need?

And for your particular question, I think lower scheme offers full part's definition in the light of ISO 2768-mK and the upper does not, since Y14.5-2009 is invoked and according to that standard geometric specification is incomplete.

 

[fsincox]

CH,
I had never taken the statement In ISO the same way you do. I take it as just a simple statement, there are no tabled values for those callouts not that general ISO tolerancing does not control FOS. I am curious if anyone else thinks the ISO 2786 general (non-geometric) tolerances are not applicable to control FOS?
Frank

 

[CheckerHater]

Pmarc,
I believe the purpose of 2768 is that at the end of the day someone will say: “we will get better part with ISO drawing than with ASME one”
If you want to continue nit-picking contest, I would say that in my opinion “fully defined” part is the one that is fully constrained to single datum frame, not partially to several datums.
But since the idea of DOFs is universally hated on this forum, I am just too tired to go there.
As a matter of fact, I am happy enough that example of what you considered “bad” ISO actually proved to be somehow “good”.
Frank,
The citation is from Part 2.
Part 1 provides +/- tolerances perfectly suitable for FOS.
But when you try to position those FOS’s, Part 2 is no help. There is no way to establish proper framework of up to 3 datums based on nothing, so you are better off with full-scale GD&T applied