Minor errors in Y14.5?

[Belanger]

Please don't ask why I'm looking through the standard when it's nearly midnight, but... I'm hoping the other GD&T regulars on here can confirm a couple of minor things I've found in the 2009 standard. I'm not trying to nit-pick them (a job well done, I must say!), but here are three things that don't seem right:

1-- Figure 7-45 shows datum feature A with the new "CF" symbol. Is that usage OK? I guess the concept makes sense, but what bugs me is that "CF" is defined in paragraph 2.7.5 as only for features of size.

2-- Check out Figure 4-32. Is it kosher to show the 20 mm basic dimension and just assume that it is centered around datum A? I'm sure this question was in another thread many moons ago. I think most of us said that you should at least have one of the holes dimensioned as 10 mm from datum axis A.

3-- This last one's pretty tedious: Figures 5-2 and 5-3 describe straightness on a FOS this way: "the derived median line of the feature's actual local size must lie within..." The definition of derived median line in paragraph 1.3.31 says that it is formed by segments which are normal to the unrelated actual mating envelope. My question: Is there an internal conflict in those statements? The last sentence of 1.3.31 seems to throw a wrench into the idea of having straightness find the center points for each local cross-section. Heck, if we're checking the straightness based off of the AME, it will always be straight!

John-Paul Belanger
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Geometric Learning Systems

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[pmarc]

1 - Saying briefly, I agree there is a conflict between 7-45 and the definition of continuous feature in 2.7.5 and as far as I recall we discussed this in a thread couple moths ago without any luck of finding a compromise.

2 - I do not see a problem with having basic 20 dimension only. For me the intent is clear. Actually there are more figures in '09 edition using the same dimensioning approach, like 4-33, 7-4, 7-18 for instance. Are all of them kosher?

3 - I do not see a conflict in the area which you are specifically asking - I have some other remarks especially to fig. 5-3, but not related to the definition of derived median line. The definition itself clearly says that derived median line is an imperfect line formed by the center points of all cross sections of the feature. Actual unrelated mating envelope concept is employed here because you have to know the orientation of those "all cross sections". So to get to derived median line of cylindrical feature you would have to...:
- find actual unrelated mating envelope (perfect cylindrical element) of the feature;
- basing on the envelope, find the axis of the feature (perfect line);
- knowing the perfect axis, define the direction normal (perpendicular) to it in which all cross-sections of the feature will lie;
- find the center point of each cross-section - these points would form imperfect derived median line of the element.

The only thing I do not like (which is missing) in definition 1.3.31 is that it does not define a method of finding the center points of each cross-section. I believe this may be defined in math standard or in ANSI B89.3-1972 (R2003) "Measurement of Out-of-Roundness", but 1.3.31 does not indicate that and I do not have copies of those two with me to be 100% sure.

P.S.: Though '09 edition is a milestone improvement of '94, I smell another thread with 100+ posts. I am pretty certain that each of us already faced with more of those "minor errors". Well, as they say, nobody and nothing is perfect.

 

[drstrole]

1 – I agree it makes sense that they should be inline, but there still needs to be a control (profile) to control the extent of inaccuracy. So while it make sense treating them as one feature, it doesn’t for the relationship between each surface. Also, as you indicated 2.7.5 says it is for features of size. It seems like this may be an editing oversight.

2 – I don’t think the 10mm basic is needed. There is a similar precedence in the 94 standard, figure 4-26 on page 71. The company I am at decided to drop the “½” dimension several years ago after discussion with a committee member, and struggling internally with how many decimal places to go out to on the ½ dimension. When the pattern dimension is 1.4999 for instance, it when you make the ½ dimension .74995 people freak out because it is at 5 decimal places, when it is .7499 or .7500 they freak because it is not truly centered. Yes we still use inch units, don’t get me started on that.

Drstrole
GDTP - Senior Level

 

[Belanger]

Thanks, guys. Pmarc, you're right... this could explode into numerous posts. My intent on these 3 specific items was brought to light by some real-world situations that I'm dealing with for a customer.

1-- The "CF" thing seems like it should stay with FOS only, since Rule #1 will handle the alignment in those cases.

2-- For the 20 mm equally spaced, I'm OK with it; I just wondered if there's a slippery slope somewhere in that assumption of equally spaced.

3-- I still disagree on the "median line" for straightness, pmarc. See the attached graphic. Would the median line we wish to check come from the cross-sections of the 1st picture? That seems to be what you're saying when you describe your 4 steps.
But that is contrary to their use of the term "actual local size," which would lead me to the 2nd picture in my graphic.

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

http://www.gdtseminars.com

 

[fsincox]

J-P,
I know you have all heard it before, but, The CF thing is just wrong they should have accepted CZ and moved toward harmonization. It was my understanding, and correct me if it has changed, that the long term goals of changing to RFS implied and of using ISO datums, was harmonization. This just shows how wrong they are; even they thought it was supposed to be CZ.
The dimensioning issue has been used since 1982 so I do not think it is a mistake. Now I know why you guys didn't always see my implied basic ZERO concepts: ANSI Y14.5M-1982, fig 121, page 56. Naturally, I am against imposing rules that are not needed. Since it has been that way for this long why give back, now. There are plenty of fights with checkers I won over that basic principle, just do to those illustrations and others in all the books since.
The AME thing I am still working on but I have a tendency to want to control the actual feature and not some obtuse AME.
Frank

 

[MechNorth]

1) Section 2.7.5 deals with CFs within the context of FOSs. It is poorly written in that it does not spell out that it is within that context that it is being discussed as opposed to an absolute definition of CF. The cited figure provides an alternative use of CF, and therefore is legal also.
Also, "3.3.23 Continuous Feature Symbol
This symbol indicates a group of two or more interrupted
features as a single feature. See Figs. 2-8 through
2-10 and 3-11 and para. 2.7.5." Note that it doesn't say FOSs, just "features".

2) visual symmetry is valid and has its roots in board-drafting. It's a convention used to reduce drafting time. It's an implied condition, which they should add to the standard, however because it's a well established drafting convention, it won't be. When I teach this, I explain to the participants that the detailer isn't allowed to "trick" the end-user of the drawing; if something looks symmetrical, then it is, unless the designer tells you specifically otherwise. Not everyone can visually detect symmetry, so there is no harm in adding a dimension, but there is no requirement for it, nor great value added.

3) I believe the distinction being made is in the "unrelated" vs "related". I don't see a conflict.

3)

Jim Sykes, P.Eng, GDTP-S
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[Belanger]

Good points by all on my first two questions. Thanks.

The 3rd one about straightness is still a thorn. Jim, it has nothing to do with related or unrelated... recall that would only come into play if there were more than one feature on the drawing. For what we're dealing with, there's nothing to "relate" to.


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

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[pmarc]

J-P,
I see your point, though I think my 4 steps interpretation is reflecting definition of derived median line (1.3.31) quite well. The problem is that it seems not to go in pair with actual local size definition, which can be troublesome especially if straightness at MMC is applied.

 

[CheckerHater]

I am all for harmonizing the standards and the more CF looks like CZ the better.
Visual symmetry was discussed before many times and most usually liked Tec-Ease's Nov. 2010 "No Need For Those “Half” Dimensions!" approach.
I have my own grudge for minor 09 discrepancies, but unfortunately not much time right now.

 

[MechNorth]

J-P, "related" means "related to a DRF", not to other features.

Jim Sykes, P.Eng, GDTP-S
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[Belanger]

Right. But I'm not quite sure how you see straightness relating to a DRF.

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

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[MechNorth]

I was going by your original post, rather than checking the actual text, but re-reading it and comparing it to the actual standard, you misquoted; it should be "These cross-sections are normal (perpendicular) to the axis of the unrelated actual mating envelope." <Note "axis" was a critical omission, and "cross-sections" vs "segments".> That statement only indicates where to take the section, so that it isn't skewed significantly. Still don't see any conflict.

Jim Sykes, P.Eng, GDTP-S
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[DeanD3W]

Regarding CF being applied to planar surfaces, Jim correctly pointed out that the wording of 3.3.23 clearly makes this OK... The implication of this is that if a flatness tolerance were applied to the two surfaces that are datum feature A in figure 7-45, then, since those two surfaces are defined as one feature the flatness will impose one tolerance zone and will therefore control the coplanarity of the two surfaces. This is something flatness never had the power to do, just like a size tolerance formerly did not have the power to control coaxiality until CF was added. I can confirm that some of the Y14.5 committee members are not happy about this, since they viewed CF as for features of size only... This doesn't matter though, since 2.3.23 and figure 7-45 clearly sanction CF on planar surfaces. With that modifier flatness and also orientation tolerances will control coplanarity of planar surfaces.

I think the definitions of actual local size, and derived median line should be improved... In addition the word "actual" should be deleted from "local size" unless speaking of something theoretical, and "derived" should be deleted from "median line" since axes and center planes are also derived... We have some extra words to get rid of, if we are able to get such a frivolous thing on a meeting agenda.

Dean

http://www.d3w-engineering.com

 

[Belanger]

Not buying it, Jim. (Well, I buy the part where I mentioned segments ... apparently I glanced at "derived median plane" for a moment.)

But if we all agree on what the unrelated AME is, then being normal to that is obviously the same as being normal to its axis (do a quick sketch). Thus, it's actually a non-critical omission.

So there is a conflict still (in Fig. 5-2, in light of paragraph 1.3.31). Specifically: it shouldn't say "the derived median line of the feature's actual local size" when the very definition of derived median line invokes cross-sections based on the UAME. That was the thorn I tried to illustrate in the graphic.

But for now, I'll resolve my real-world issue with the interpretation of straightness that we all accept.

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

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[MechNorth]

J-P, I still don't see it as a conflict. One tells you to get the centerpoint of a true cross-section; it's still going to be a local center point. The other tells you how to get that cross-section. If it didn't tell you how to get that cross-section, where would you take it? Normal to the local cross-section which is infinitessimally small, and therefore anywhere?

Picture a long pin, bent like an archery bow. The standard says to take the UAME, and take sections along the length of that UAME, normal to the axis of the UAME. For each of those sections, find the individual centerpoint, then go on to the next section. Without that guidance on how to section it, one alternative would be to take sections radially eminating from the center(s) of curvature, which is not reflective of how the pin functions, whereas the UAME is. That's why I don't see a conflict, and even see value in the statement. I am trying to think of why you think it is a conflict, but am hitting a road block.

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[Belanger]

Hmm... now we may be getting somewhere. Just one clarification, Jim: Did you look at my graphic (in post #4 above)? You are saying that the center "dots" would be taken from the first picture? (Not the center across the envelope, but the center of the physical part at each of those vertical slices.)

I get that idea, so then my issue must have had to do with the wording. Figure 5-2 says to find the actual local size -- OK, I can do that -- and then find the derived median line of that... huh?

That was where there seemed to be a conflict (in the wording, not the general idea of straightness). I guess it's one of those things where, no matter how it's worded, some of the definitions will bump into each other if taken strictly.

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

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[MechNorth]

Ah. No, with the attached files being below the signature info, I keep missing them now. Graphic looks good.

Fig. 5-2 says that you find the center point of each local cross section ... and also says, oh by the way, you also still have to have all cross-section sizes within the limits of size. No mention of using the AME as the basis of the median line though.

I do get frustrated at times that some passages of the standard have to be re-read numerous times to get the full understanding.

Jim Sykes, P.Eng, GDTP-S
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[Belanger]

Sorry to keep pushing the point

You and I are now on the same page about the figure's use of "actual local size." Hold that thought...

You then ask: "No mention of using the AME as the basis of the median line though." But the definition of derived median line (1.3.31) says that it's based on the UAME.

Now you see my mild confusion, since the standard uses terms in Fig. 5-2 that within themselves seem to point to conflicting derivations of this notion of cross-sections.
But I'll just chalk it up to a minor wordsmithing task for the next edition.

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

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[MechNorth]

1.3.31 says that the "sections" are normal/perpendicular to the axis of the UAME. It doesn't say find the center point of the UAME. Substantial difference.

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[Belanger]

So we find the cross sections normal to the UAME. And then we have to find the centers of the physical part at those sections.

I get it, visually, but then Fig. 5-2 shouldn't use the term "actual local size"; the term "derived median line" is all that straightness needs in order to be fully understood. (That's why I said earlier that there seem to be contradictory terms thrown together: we are told to use the "actual local size" in a direction based on the UAME to check straightness, but that's not the direction of the "actual local size" that must be checked for size.)

See graphic attached below... notice the blue, then the red. My conclusion is in the green.

Thanks for following me on this, Jim. I can practically hear the eyeballs rolling out there, but as I said this was precipitated by a real question.

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

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