Minor errors in Y14.5?
Minor errors in Y14.5?
(OP)
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!
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
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com





RE: Minor errors in Y14.5?
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.
RE: Minor errors in Y14.5?
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
RE: Minor errors in Y14.5?
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
RE: Minor errors in Y14.5?
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
RE: Minor errors in Y14.5?
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
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Minor errors in Y14.5?
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
http://www.gdtseminars.com
RE: Minor errors in Y14.5?
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.
RE: Minor errors in Y14.5?
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.
RE: Minor errors in Y14.5?
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Minor errors in Y14.5?
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Minor errors in Y14.5?
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Minor errors in Y14.5?
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
www.d3w-engineering.com
RE: Minor errors in Y14.5?
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
http://www.gdtseminars.com
RE: Minor errors in Y14.5?
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.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Minor errors in Y14.5?
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
http://www.gdtseminars.com
RE: Minor errors in Y14.5?
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
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Minor errors in Y14.5?
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
http://www.gdtseminars.com
RE: Minor errors in Y14.5?
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Minor errors in Y14.5?
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
http://www.gdtseminars.com
RE: Minor errors in Y14.5?
Glad we were able to sort it out. Do you have a resolution for your client now?
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Minor errors in Y14.5?
I'm not aware of another approach that would provide the proper orientation of cross-sections, and the LMC sphere concept is already in Y14.5.1. Two point measurements would still model the proper local size for slots or slabs (widths), since they model the size of the sphere that is fit to the feature, but for cylindrical features of size local sizes would then be defined as circles that are an element of a sphere that is fit to the feature. This more clearly defines local size, and also provides a single center point from which a median line can be derived.
Dean
www.d3w-engineering.com
RE: Minor errors in Y14.5?
Dean
www.d3w-engineering.com
RE: Minor errors in Y14.5?
J-P's most recent graphic seems to be in line with this too. Assuming that long dash-short dash center line on blue and red pictures is a derived median line of the banana shape feature, the measurement planes of actual local sizes are indeed always normal to the derived median line.
But this would mean that actual local sizes can be found only if derived median line is known. Isn't it crazy for someone who would like to use caliper to measure diameter of the feature?
RE: Minor errors in Y14.5?
I never noticed the sphere description that Dean mentions. That's a good way to at least verbalize one of the cross sections. (The other variety being cross sections based on the UAME.)
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Minor errors in Y14.5?
I agree that there are several issues with the definitions for actual local size and the derived median line.
Starting with actual local size, there is still ambiguity over what it means. Some statements in Y14.5 indicate that the actual local sizes are based on cross sections, and other statements indicate that they are based on 2-point linear distances. In any case, the actual local sizes do not need to be oriented normal to the actual mating envelope. I agree with pmarc that in Figure 1-1 the actual local sizes appear to be normal to the derived median line, although there is no requirement for this. The derived median line doesn't even belong in that figure.
Now on to the derived median line. The DML Straightness characteristic is one of the deepest swamps in all of Y14.5 :^(. Y14.5-2009 defines the DML in the following way:
derived median line: an imperfect (abstract) line formed by the center points of all cross sections of the feature. These cross sections are normal (perpendicular) to the axis of the unrelated actual mating envelope.
My first problem with this definition is that the center point of an imperfect cross section is not clearly defined anywhere. Not in Y14.5, not in Y14.5.1, not in ANSI B89.3.1. We don't know if it's the center of the maximum inscribed circle, minimum circumscribed, minimum zone, some sort of average, or what. So there are various possible DML's that one could establish.
The second problem (in my opinion) is the requirement for the cross sections to be normal to the AME. This makes sense in certain Straightness applications, but not in others.
Let's start with DML Straightness RFS. It would seem that the cross sections for the DML should be allowed to follow the curvature of the as-produced feature, as the cross sections for Circularity are allowed to. But this would be inconvenient to inspect, and I suspect that the requirement to orient them to the AME was put in to simplify things. I have trouble envisioning applications for this characteristic in the first place. Can anybody else name any applications where DML Straightness RFS makes sense? Perhaps these would guide us as to how the cross section centers would need to be defined.
The requirement for the cross sections to be oriented to the AME makes sense for DML Straightness at MMC, where the purpose of controlling the DML is really to control the surface of the feature so that it doesn't violate a certain boundary.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Minor errors in Y14.5?
I support most of your statements, though still have some comments / questions:
- Just for clarity, derived median line is shown in fig. 1-1, but not in fig. 1-2.
- What makes you think that cross-sections for Circularity are allowed to follow curvature of the as-produeced feature? Per 5.4.3(a) of '09 edition "Circularity is a condition of a surface where for a feature other than a sphere, all points of the surface intersected by any plane perpendicular to an axis or spine (curved line) are equidistant from that axis or a spine". Per 1.3.28 feature's axis is "the axis of the unrelated actual mating envelope of a feature" (unless otherwise stated). So in my opinion, if we are talking about nominally straight cylinder, we are again, like for staightness, forced to use UAME concept to be able to check circularity error properly. Does ANSI B89.3.1 state otherwise? (My apologies if, by asking this question, the discussion will drift away from original path).
RE: Minor errors in Y14.5?
The reference to the spine (curved line) is what allows the Circularity cross sections to follow the curve of the as-produced feature. We can choose any arbitrary spine, to arrive at an optimal Circularity value. We can choose a perfectly straight one for simplicity, but we don't have to.
For DML Straightness RFS, I think it would make sense to allow the cross sections to be normal to an arbitrary spine.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Minor errors in Y14.5?
Peter Truitt
Minnesota
RE: Minor errors in Y14.5?
Yes, this seems to be true for the MMC case. The meaning of Straightness at MMC in terms of the DML is very complicated. The DML is difficult to establish in the first place, and then it must be compared to a tolerance zone that expands differently at each cross section. I don't know of any applications in which the designer really intends to control the DML to a locally expanded tolerance zone. In all the applications I know of, conformance of the surface to the virtual condition boundary is what really matters. Because of this, GD&T books tend to deal with Straightness at MMC exclusively in a gaging context and ignore the DML altogether.
The end result of a Straightness at MMC tolerance is to open up the Rule #1 boundary by a certain amount. That's it, it's a relatively straightforward concept. But because of the way the Y14.5 standard has evolved, the tolerance is encoded in an indirect way that is unnecessarily complicated and difficult to understand.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca