GD&T is it a philosophy
GD&T is it a philosophy
(OP)
I am interested in hearing the different points of view on your philosophy to GD&T.
As new draftsman we had always been told "you can't use bolt circles, only co-ordinates" and "don't dimension from centerlines, only edges" I suspect these are a lot like the caliper guys of today. When I was first trained in Y14.5-1982 in 1987 I found it a very liberating. The philosophy I was told was if it did not violate the basic rules or is not prohibited by the standard it was OK, Notes on drawings were not desired because of language barriers, but in extreme cases, you may need to supplement with a note to explain what you intend. The sense I got was it was a tool kit to be used and the simplistic examples in the text were just that, period. The book certainly did not explore the limits of what can be done it was more of a universal language that would be built upon as languages do. English, for example, has had words like computer and geometric dimensioning and tolerancing added to it (my MS word still thinks tolerancing is not a word or is misspelled). We all know it is a word.
In my first job AGDT (After GD&T Training) I worked with a lot of machines and a lot of dowels patterns that people always wanted located to unimportant edges ("the from the edge guys", always 2 dowels in case some don't know), I said: "fine, we have this new tool called composite position tolerancing that was perfect for that". Eventually, the question was asked: "now, can we put an orientation on the centerline between the dowels to refine for orientation". "Not in the ANSI world", I said, "this is explicitly prohibited", if we were ISO well life would have been easy. I was told the committee was working on just that issue, and, the restatement of secondary datums in a composite position tolerance would do just that. We were also instructed that since the standard did not actually show it we may want to add a flagnote to explain what we meant. This practice is part of the standard now and since Y14.5-1994 an accepted practice, some apparently argued it was implied before in the 82, but, it was not explicitly shown.
Out here in the real world we do not always have time to wait for the politicians to make decisions, the job has a deadline and we need tools to do the job. MMC and LMC (also a new concept at the time) are good valid tools, adding it to profile tolerances to get the job done foe the heavy hitters is great, but why take it away from the poor little radius, Is it really because it is hard to measure? Life isn't always easy.
There's lots I want to get into with guys who are interested in exploring ideas.
Like:
To circle "E" or to circle "I", that is the question?
Why not true position of a surface instead of profile?
Why perpendicular and parallel, not just orientation?
Is a feature defined by a radius really different than the same one defined as diameter?
Doesn't anyone out there use the dreaded ISO and like it?
How can rule #1 not be a violation of all the logic all we are trained in as engineers and assume the worst case, as ISO does, by the way. Must we cling to our calipers in one hand and our concept that we will someday actually produced that perfect feature at MMC in the other? (When I am asked by the shop to accept an oversize shaft is it more perfect, then?)
Anyway thanks, if you bothered to read this far, I guess I will get off my soapbox for now to give someone else a chance.
As new draftsman we had always been told "you can't use bolt circles, only co-ordinates" and "don't dimension from centerlines, only edges" I suspect these are a lot like the caliper guys of today. When I was first trained in Y14.5-1982 in 1987 I found it a very liberating. The philosophy I was told was if it did not violate the basic rules or is not prohibited by the standard it was OK, Notes on drawings were not desired because of language barriers, but in extreme cases, you may need to supplement with a note to explain what you intend. The sense I got was it was a tool kit to be used and the simplistic examples in the text were just that, period. The book certainly did not explore the limits of what can be done it was more of a universal language that would be built upon as languages do. English, for example, has had words like computer and geometric dimensioning and tolerancing added to it (my MS word still thinks tolerancing is not a word or is misspelled). We all know it is a word.
In my first job AGDT (After GD&T Training) I worked with a lot of machines and a lot of dowels patterns that people always wanted located to unimportant edges ("the from the edge guys", always 2 dowels in case some don't know), I said: "fine, we have this new tool called composite position tolerancing that was perfect for that". Eventually, the question was asked: "now, can we put an orientation on the centerline between the dowels to refine for orientation". "Not in the ANSI world", I said, "this is explicitly prohibited", if we were ISO well life would have been easy. I was told the committee was working on just that issue, and, the restatement of secondary datums in a composite position tolerance would do just that. We were also instructed that since the standard did not actually show it we may want to add a flagnote to explain what we meant. This practice is part of the standard now and since Y14.5-1994 an accepted practice, some apparently argued it was implied before in the 82, but, it was not explicitly shown.
Out here in the real world we do not always have time to wait for the politicians to make decisions, the job has a deadline and we need tools to do the job. MMC and LMC (also a new concept at the time) are good valid tools, adding it to profile tolerances to get the job done foe the heavy hitters is great, but why take it away from the poor little radius, Is it really because it is hard to measure? Life isn't always easy.
There's lots I want to get into with guys who are interested in exploring ideas.
Like:
To circle "E" or to circle "I", that is the question?
Why not true position of a surface instead of profile?
Why perpendicular and parallel, not just orientation?
Is a feature defined by a radius really different than the same one defined as diameter?
Doesn't anyone out there use the dreaded ISO and like it?
How can rule #1 not be a violation of all the logic all we are trained in as engineers and assume the worst case, as ISO does, by the way. Must we cling to our calipers in one hand and our concept that we will someday actually produced that perfect feature at MMC in the other? (When I am asked by the shop to accept an oversize shaft is it more perfect, then?)
Anyway thanks, if you bothered to read this far, I guess I will get off my soapbox for now to give someone else a chance.





RE: GD&T is it a philosophy
The business about Rule #1 goes back at least 100 years to Mr. Taylor. Suppose he made a drawing of a cylindrical part and gave it a diameter of 1 inch ± .020. He got tired of his manufacturers sending back 1-inch pins that were slightly bent -- hey dude, this isn't going to fit properly in the machine. So the "Taylor Principle" says that 1 ± .020 means the cross-section AND the envelope size.
We now call this Rule #1. It makes sense to me; in other countries it never caught on, I guess.
As for perpendicularity and parallelism instead of a general orientation symbol, some folks have lobbied that everything could be boiled down to a couple symbols -- maybe the profile symbols, position, and both runouts. But why dumb down the language? How hard is it to remember that perpendicularity is an orientation control of a 90º angle vs. parallelism, which is 0º?
The main point is that a standardized language makes the world work better, and at this point we have ASME and ISO. There are pros and cons to each system. Changes are possible, but both systems suffer from the bureaucracy that us engineers usually hate!
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
I do not really have a problem with rule #1 either; it is the way WE were broughtup. I does seem to me to be an odd violation of engineering logic, though. My issue is with the people that think their way is the only way it can be and berate others who may dare to think differently, the ISO world for example. I am frightened, too, by the seeming complexity of all the implied tolerances of ISO 2768-2(Ihope that is it) but I assume these are rational people and maybe if you are trained in it, it isn't that bad. Besides maybe they have a good idea every once and a while, Like the flatness of a center plane vs. straightness I have been talking about, that to me seems like a win for logic.
My understanding is we created GD&T and we should be proud of that but some of the issues seem more like school yard squabbling, True Position vs. Profile who really cared
Does anyone here know why? I assume position came first, I have my mil-std-8, 1966, 1973 and 1982 at work now so I can't check when profile was first introduced, but,I suspect it was a well this is our's and we don't like it kind of thing and besides we are bigger, na na na.
or maybe it is a well we said you can't use position on surfaces kind of thing an we don't want to change now like we did on implied MMC. I can understand, we all know politics; I just want to know if there is a good reason.
RE: GD&T is it a philosophy
At a very helpful GD&T site they show a condition involving a part with a flanged bolt mounting surface with a large diameter male or female pilot diameter, it doesn't really matter which. The pilot length is very small in height relative the flat mounting face. The claim is made that to control the pilot diameter with a perpendicularity call out at MMC to the face is a waste of time because it cant be checked, not enough length on the pilot to get 2 separate readings, I imagine.
The standard shows these kind of things face primary datum, pilot diameter secondary datum. perpendicularity of the pilot to the bore at MMC so the parts will fit. I thought we were just discrbing the condition of assembly by adding it. I don't really care what they check or don't check or how they check it.
Have things changed? Are we back to the manufacturing tells us what to do days?
RE: GD&T is it a philosophy
I think the reason people do it is an unofficial rule that each datum in a common datum reference frame should be tied back to its preceding datum. So the primary datum face doesn't need any GD&T (except perhaps flatness), and the secondary datum feature should be tied back to the primary, etc. In the scenario you described, the only relationship of secondary back to primary is orientation.
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
As I understand it, the original ANSI GD&T was really the first international standard; it was the amalgamation of the national and defense standards of Canada, UK, USA, and maybe others. ANSI/ASME GD&T was the starting point for ISO's standard(s), and at some point ISO decided to go their own way. There have been / are ongoing efforts to unify the two, but from several sources I've heard that ISO is making it their goal to eliminate ASME GD&T. To that end, there is interest/intent within ISO to add a clause to their standard(s) to the effect that any engineering document which does not specify the governing standard will default to ISO for the standards. That's a substantial shot across the bow. The appropriate course of action should be that the best of both be integrated into a single doc, but I'm not convinced that both parties are actually willing to give & take, or act on the best interests of engineering & manufacturing.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: GD&T is it a philosophy
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: GD&T is it a philosophy
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: GD&T is it a philosophy
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T is it a philosophy
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: GD&T is it a philosophy
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: GD&T is it a philosophy
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T is it a philosophy
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: GD&T is it a philosophy
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
RE: GD&T is it a philosophy
"don't dimension from centerlines, only edges"
Centerlines do not exist without edges. Center of what? Right? ASME does allow you to establish a center plan as a datum. In that requirement, the feature of size edges define the center plane. Just because one person may not "worry" about where the centerline is in relation to the edges, that doesn't mean it isn't needed. Without being tied to a FOS, it would be easy for any manufacturer to declare that the source centerline itself was not defined (no standard, not even ISO creates a default understanding of what a CL is). Some link to a feature of size is necessary or how else would one fully define the product definition?
Also, Regarding ISO trying to eliminate ASME;
ASME is a contributor to ISO these days. There will likely be a unification of the two standards within the next 75 years, but I doubt any contempary move in the US to replace ASME with ISO outright will be very successful, as ISO has many open issues that have been addressed in ASME as a result of experience and hundreds (if not thousands) of lawsuits. Elimination of Rule #1, for example, would be chaos for industry here. As lawsuits start affecting ISO users in Europe, I suspect it will start resembling ASME.
Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
RE: GD&T is it a philosophy
How about position tolerance to a zone of X degrees? Say a wedge shape boss and you want to invoke MMC or LMC. Why not? I don't see anything conceptually wrong with it. Now the directly opposed surfaces are not parallel they radiate from a common center.
Apparently not allowed after 1994. The restriction seems arbitrary to me.
RE: GD&T is it a philosophy
"I think the reason people do it is an unofficial rule".
In the 82 version we had rule number #5, it instructs you to gage datums based on a the virtual condition how do you really calculate it with out a statement of what it actually is. Rule #5 is not an unofficial rule it is still there section 2.11.3 ASME y14.5M-1994. To not have a statement, to me, is no different than showing 2 diameters on center and no tolerance and then saying what is the location tolerance between the diameters, well we don't know.
I get the feeling you agree, though?
RE: GD&T is it a philosophy
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: GD&T is it a philosophy
I agree to a degree.
Paragraph 2.11.3 (1994) which you mentioned only says that a virtual condition exists IF the datum feature is controlled by another geometric tolerance.
I said that there's an "unofficial rule" which tells folks that a secondary datum feature should be tied back to the primary datum, and the tertiary datum feature should be tied back to the primary and secondary datums. Whether that gets into virtual condition depends on their relationships and the tolerance used.
But yes, the whole thing about showing two diameters on the same center without addressing their coaxiality tolerance is ambiguous.
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
Again after a quick read I have only a few cents to through in.
Reducing the number of words has no benefit, none.
Explicit communication provides for more explicit design intent and design intent transfer. And the explicit tools of ASME Y14.5 provide a language to express what is really functionally important. Sometimes the surface (entire surface) is functional and somethimes it is not; often so!
The profile tool can not take advantage of the functional additional available tolerance that position can; simple. Can the standard change the rules and allow mmc for profile? I suppose so but what is the benefit. Can we use a screwdriver to remove paint and pliers to turn bolts? Of course, c- mechanics do it all the time. lol
I am missing something on whatever the issue is with dimensioning to centerlines. What am I missing? Is this a problem? I don't see why if a "standard" is invoked on how to interpret that dimensioning.
I noted that the 2009 standard now allow for angularity to be used in place of parallel and perpendicularity. Chears for someone and it's no skin off my back, but what was the benefit?
I am not sure a get the issue around rule #1. Was the question wondering if a part exceeds the MMC boundary that a supplier jokes and thinks then it has to be "more" perfect? Just say no! It doesn't meet the size requirement and give the work to someone else.
Interesting to be back and sorry if I seem out of synch.
Norm
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: GD&T is it a philosophy
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
RE: GD&T is it a philosophy
Perpendicularity to two datums has always been fine. It means that the surface in question is 90º in two different directions of space. (See Fig. 6-35 of the 1994 standard.)
Angularity is the one that was sometimes used to two datums to show different angular relationships. It was a way streamline things: instead of having a feature control frame of perpendicularity to A and another frame of parallelism to B (same number), angularity could be used once while referencing both A and B.
Technically this wasn't kosher in 1994 (see para. 6.6.2 of 1994) but now it is explicitly allowed (Fig 6-4 of 2009 edition).
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
RE: GD&T is it a philosophy
Your dowel line example brings up a different issue, and goes beyond the "traditional" use of Perpendicularity. It requires referencing 3 datum features in the Perpendicularity FCF, which Y14.5 does not directly deal with. I think I've seen a similar example in a GD&T textbook though.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: GD&T is it a philosophy
Pete- Your example does makes sense to me.
Can I assume, since profile would handle the tabulated part example except for a feature of size where MMC is by design, that this is where it all really came from?
Good discussion!
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
That is correct MMC was the default for ANSI as late as 1973. The story in 1982 was they were trying to harmonize more with the ISO standard. It also seemed to be a step in a more safe/conservitive direction.
Norm,
Composite profile was not in the 82 strandard, most places I see are still avoiding profile (I too, hope that changes).
My own inspectors were poo pooing it untill I told them: "just think of it like true position of a surface".
I thought there was also a "rule of thumb" that you don't use location if all you want is orientation.
RE: GD&T is it a philosophy
Profile seems to confound and intimidate people until they really grasp its essence ... it's a boundary control. I see a lot of work that has a FOS with a position control (with & without modifiers) applied to a purely clearance feature. Walking them thru the actual functionality of the feature and the costs associated with verifying size & position separately often convinces them of the value of profile as an encompassing control. Some, of course, go "Yeah, right, I get it!" then "But we won't do that because nobody else will ever understand what we mean." Alrighty then, my work is done!
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: GD&T is it a philosophy
Jim, I was thinking just the opposite! Profile is NOT a boundary control in the sense that a clearance pin or hole wants. Since profile on a hole or pin automatically controls size and form (think basic dim on the diameter), it would seem to be overconstraining a part to use profile for a mere clearance feature. The neat thing about position MMC is that we don't care very much about size and form, as long as the virtual condition is not violated.
Peter -- your last statement about a "rule of thumb" is interesting. I would say that if all you want is orientation, then you can't use a location control. I often see where folks put a position tolerance on a single hole relative to only one datum, which is the face of the part. If it were two holes (2X) then position is OK, but for one hole they should just be using perpendicularity.
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: GD&T is it a philosophy
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
I submit that Fig 6-4 in the 2009 standard is a terrible example of the angularity do all thing.
First of all, the perp control, although valid, is just really bad practice. The "alternative practice" example that uses angularity, although valid, is just as bad. We know the figures are not intended to be "complete" drawings for simplicity. But, what if we were to finish the drawing of that part in Fig 6-4? The standard still requires that all characteristics of a feature that can be controlled shall be controlled. So, for the considered feature in Fig 6-4, how might we control the location if we "want" to only use the datum feature references for orientation only? Please Please Please don't tell me you can use a +/- linear dimension! The standard "allows" for a +/- linear dimension, but it would be bad practice. And if I am in charge, no you can't use the +/- linear locating dimension.
I know, let's imagine it is a feature of size and thus the +/- linear "size" dimension. I'm ok with that, but then would I really use Datum feature B as shown or would the FOS "realistically" be the functional datum feature?
I really feel for the Y14.5 committee. It seems that sometimes they have to spend all kinds of time solving the rediculous with all the; "well what if we 'want' to?; possabilities.
Fig 6-4 should add the "best practice" to the "alternative practice" and show a profile control.
Jim --- now you know why Mark always has to be with me at Y14.5 meetings; to keep me from rocking the boat.
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
Sorry for the delay, but I couldn't find a ref in my file, so I tracked down one from Don.
http://www.tec-ease.com/tips/july-09.htm
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: GD&T is it a philosophy
Fully agree with your points, particularly the extension into reality.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: GD&T is it a philosophy
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: GD&T is it a philosophy
What I need to do is get involved with Y14.41.
And fix it.
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
You said " Please Please Please don't tell me you can use a +/- linear dimension! The standard "allows" for a +/- linear dimension, but it would be bad practice. And if I am in charge, no you can't use the +/- linear locating dimension."
Bad practice?? Maybe you can explain that a bit further.
I have see quite a few drawings that followed this approach with all surfaces having a default profile of a surface tolerance. In some cases, we had 4 levels of profile of a surface requirements. Possibly the larger tolerances mean that the surface has no functional value?? I really don't know.
I have difficult understanding how this approach clarifies a drawing reflecting its functional needs. The 2009 standard states in the foreword (vi) "that it is more important that the design more precisely state the functional requirements".
Tell me how your approach eliminating +/- tolerances for location "more precisely state the functional requirements".
Dave D.
www.qmsi.ca
RE: GD&T is it a philosophy
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: GD&T is it a philosophy
I have yet to ever find a person in a fundamentals class that can explain a so called "simple" +/- linear dimension intended to locate one feature to another, particulary a surface to another surface. And I teach a lot of classes.
To completely answer you on this forum can be a bit much and so you are welcome to track my information down and call or e-mail me. (see our website for that info)
But the short of it is that very few people understand where and how big the tolerance zoneS are when a +/- linear dimension is used and therefore misunderstand the inherent form control and often invoke rule #1 where rule #1 is not functional in the design of a particular feature.
The fundamental reason that profile more explicitly defines the functional need and inspection process is that it truly establishes the functional relationship to other features as functional datum features. A +/- linear dimension has nothing to do with a datum feature and therefore nothing to do with a functional datum reference frame.
The composite profile control that you seem to be referring to is an excellent method of control but again way too much to discuss here. Perhaps a new thread will be more appropriate and/or track my info down and contact me.
(I'd provide it here but I think it is against the rules.)
As my own humble
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
I come from the Quality field and at one time in my life, I measured automotive parts using CMM equipment. We had to develop the datum sructure in the same manner that Designers today reflect them on the drawing. All dimensions were +/- and the practical use of GD&T certainly would have been of value rather than having all those notes for flatness, straightness, concentricity, etc. on the drawing.
In Quality, Process and Manufacturing management today, there are pre-production meetings where we decide what features are important to its function and relationship so that we might address them from a process and quality perspective. Should we control and confirm surfaces on an regular basis that have no meaning to the part's function? No! Should be make checking fixtures for holes that are only to lighten the product? NO!
The practical use of GD&T should reflect the functional needs of the features on a part as per the 2009 edition and, thus, all the personnel involved in the producing of the part would be on the same page. The 2009 standard does not state that only +/- dimensions must be on features of size only. It does not state that profile of a surface is mandatory on all surfaces.
It doesn't take much thought to figure out the datum structure when some dimensions come from the datum as a basic dimension while another dimension is reflected in a +/-.
I remember one automotive supplier where I was training. They had a drawing where their customer told them that they must have a default profile of a surface of 0.5 mm in notes for all surfaces. I noticed on the drawing a profile of a surface of 0.5 on a particular surface was noted in a FCF. Why I asked since it was also a default tolerance? They said that the particular surface had a function requirement and the default profiles had no functional importance. Is this the intent of the standard??
Maybe a good blend of +/- with GD&T as applicable could best reflect the function needs of the part.
Dave D.
www.qmsi.ca
RE: GD&T is it a philosophy
I agree with Norm that putting a directly toleranced dimension on the width would be a bad practice. To state the obvious, the merit of a design practice shouldn't be based purely on its legality. Of course the standard doesn't make statements about what constitutes a good or bad practice. The standard is a dimensioning and tolerancing tool kit, not an engineering design manual.
A designer could change datum feature A in Fig. 6-4 to that narrow flat on the top of the left leg of the part. The standard doesn't say they can't. Would that feature provide a good stable datum plane for the orientation tolerance? Probably not, and the argument could be made that it's a bad practice. Should the standard somehow forbid the practice then? Should the standard impose rules for a minimum aspect ratio for datum feature extent to tolerance zone extent?
The standard gives designers more than enough rope to hang themselves, and specify bad practices that are perfectly legal.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: GD&T is it a philosophy
Plus/minus should not be used for location or orientation. It's only really good for size, chamfer, and radius. That is, for features of size, and also for chamfers and radii, which aren't features of size :)
Norm, I got confused because you seemed to imply that plus/minus shouldn't be used even to control the width of the part in Fig. 6-4. It's perfectly fine to do that. I see now that the issue is how to tie all the relationships together (since A is primary).
But I agree that the perp control in that figure is off base. The surface in question is certainly not perpendicular relationship to B.
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
RE: figure 6-4 in the standard
I don't know if placing a +/- tolerance on the height is applicable since I don't know the function of the height. If it has no function and is in space, then yes, I would suggest a +/- tolerance.
As far a placing a +/- on the width, there is nothing in the standard that states that I could not do that - nothing! Personally, I don't like any of the angles whether to a surface or a hole. If the feature HAS a functional importance, I would suggest using profile of a surface on a surface or positional in case of a hole rather than angularity, perpendicularity or parallelism.
The point that I was making was not about fig. 6-4 but Norm's contention that it is "bad practice" to place a +/- tolerance on a surface.
Dave D.
www.qmsi.ca
RE: GD&T is it a philosophy
However, there are some clear indications that +/- is not the best choice for tolerance of location. Such as 'dimensions shall not have more than one interpretation' and paragraph 2.6.1 defines the origin symbol to help prevent such incorrect interpretation using +/-, and in the 2009 standard the note in paragraph 2.6 "Tolerance Accumulation" even goes on to make the recommendation that 'locating features using directly toleranced dimensions is not recommended'.
When you say it doesn't take much thought to figure out the datum structure when a +/- dimension comes off of a datum feature that basic dimensions are related to, I have to disagree. Companies and industries can make whatever "policy" for "assumption" they want but the fact of the matter is that a +/- dimension for locating one feature to another has no datum structure and therein lies the problem.
What if the designer does not want a location relationship to be measured from a datum feature simulator? For example a direct thickness requirement. How can that be stated? The general practice is that that is exactly what +/- linear dimensions of location do. It is in many "opinions", including mine, that it is incorrect to measure the location of a feature from a datum feature simulator unless the use of said simulator is explicitly noted on the drawing which is what feature control frames do and +/- linear dimension of location do not do.
And so here we are. Two different interpretations which violates a fundamental rule in both the 1994 and the 2009 editions.
I think it would do the standard well to make the statement that +/- linear dimensions of location shall not be measured from a datum feature simulator.
I too think there can be the right blend of +/- linear dimensions and GD&T, but I just think that is accomplished when the +/- dimensioning is left to limits of size only and I wish the standard would make that statement too!
With regard to the automotive general profile note example, I haven't a clue either what they mean by the default not having any functional importance.
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
The perpendicular control is in fact valid per the standard. I don't like it, but it is valid inb both the 1994 and 2009 versions.
Dave & Evan --- I would not put a +/- dimension on the height again, unless the height acts like a feature of size. That is that it has a mating feature of size. If in fact the "non-functional" feature is only making contact with air, I would definitely not use a +/- dimension and would use profile. The so called height is some sort of requirement even if it only makes contact with air; no mating surface. I would want that requirement measured consistently any where in the world and so how to inspect that feature is clear to everyone. Clear, consice, valid communication. If you use a +/- dimension one can easily argue that it is a feature of size and thus subject to rule #1 requirements. So, I prefer using a tool that clearly states that rule#1 is not in play, is not part of the design intent, and will not be checked for that feature.
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
A size tolerance provides control in terms of requiring that the entire feature conforms to an MMC boundary and that all actual local sizes be within a certain range. This provides inadeqate control when applied to features that have unopposed sections where the actual local size cannot be defined.
Please see the attached diagram of what could happen if the height of the part in Fig. 6-4 was toleranced with a +/- size dimension. Even though the form error of the lower surface is huge, the feature conforms to the MMC boundary and all the local sizes are within the proper range.
If Rule #1 was waived, then the boundary requirement would go away leaving only the local size requirement. Then the lower surface could be almost any shape at all and still pass the size tolerance.
One might argue that other tolerances on the part would prevent my inane scenarios from actually occurring. Maybe, but this doesn't change the fact that a directly toleranced dimension only provides meaningful control of the feature in the areas where there is opposition.
That's why I think it's a bad practice.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: GD&T is it a philosophy
I like your comments, but I need you to clarify a few things. Mainly your statement that "it would do the standard well to make the statement that +/- linear dimensions of location shall not be measured from a datum feature simulator."
I don't even see how it's possible to use a +/- dimension from a datum feature simulator. Datum feature simulators only exist for callouts that invoke one or more datums, and +/- dimensions never invoke a datum, unless a local note specifically says so, per the note at the bottom of p. 52 in the 1994 standard (and even this seems to have been dropped in 2009, unless I am missing it).
So are you just saying that the standard should spell out that what's not possible is not possible? If folks are using Y14.5, this should already be understood. Or am I missing something?
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
I must say, the last few posts, seem to me to provide some possible insight to other reasons the ISO does not use rule #1 as it's default. For all the benefits people seem to think it brings, it seems from some discussion here, like it caused this whole feature of size problem that I have had such an issue with.
When I worked in machine tool we had these 1.5 inch X 6.0 inch X 8.0 FOOT steel transfer beams I had to deal with. Along the beams were various hole patterns that were really only important functionally and practical to relate locally as patterns and somewhat less important over all (I did NOT say NOT important at all). These beams were tied together at the ends with dowels and bolts, forming long chains across a long transfer line say 60 feet.
I knew rule number #1 was really working against me, I knew the beams would not be perfect form and allowed for straightness errors. The datum concept used in the book was impractical, also. I struggled with locally defined datums, individually, and decided on defining all features only in a restrained condition explaining to all that once the part was released from the fixture the holes and the local surface would move together so they could just measure locally.
This is when I first saw the ISO standards. The ISO definition of dimensions meant measured locally not from some datum if there was no one is given, that is what I really wanted all along.
So the little blocks in the book are fine, for what they are, some of us have different parts.
RE: GD&T is it a philosophy
When an angularity (perpendicularity, parallelism or angularity) is the only requirement, the location of the feature cannot be shown as a basic dimension since the angularity of the feature and its location are somewhat independent.
If the feature was a surface and profile of a surface is the requirement, then basic dimensions to the feature must be shown.
Now back to fig. 6-4.
There is no doubt that the dimension from datum B to the surface feature MUST be shown as a +/- tolerance. There just isn't any other way since we cannot use a basic dimension.
Let's say we have a hole and the only geometric requirement (besides rule #1) is angularity. How would one show its location besides a +/- tolerance?? Any takers on this question??
Dave D.
www.qmsi.ca
RE: GD&T is it a philosophy
The problem with using a +/- dimension to locate a hole is this: to locate the hole requires dimensioning in at least two directions of space. Yet with +/- dimensioning, we don't know which edge takes precedence. This requires datums, which mandates GD&T, not a +/- tolerance.
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
You did not answer the question about how one should tolerance the location of a hole that had only angularity (which is GD&T). How would you do that?
Dave D.
www.qmsi.ca
RE: GD&T is it a philosophy
I guess that does bring up an interesting question -- if I say that +/- dimensions can't be used to locate a hole that has angularity on it, what's the point of the angularity? But it's still valid: I would say that a position tolerance should be given (with a larger value than the angularity).
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
I might agree with you about positional rather than angularity but the question was how would a Designer reflect a location of a hole where there was only a angularity requirement. They cannot use a basic dimension.
You stated that +/- dims should not be used for location. What left to use then????
Dave D.
www.qmsi.ca
RE: GD&T is it a philosophy
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
We have a hole that has only a angularity requirement in the FCF. We know that angularity is relatively independent of its location so one cannot use a basic dimension.
How would the Designer show the location of this hole?
Dave D.
www.qmsi.ca
RE: GD&T is it a philosophy
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
You seem to say that angularity tolerance on a hole is not a valid requirement by itself and cannot be used. It needs a positional tolerance where the angularity is a refinement?? Is that correct?
Using your philosophy, perpendicularity, angularity and parallelism should never be utilized without either a positional or profile of a surface since the only way to tolerance the feature location is with a +/-. Is this your understanding?
I certainly didn't read any of this in the standard. I have both the 94 and 2009 standards here and would like to know where to find this information?
Dave D.
www.qmsi.ca
RE: GD&T is it a philosophy
I'm not sure what you're asking here either. Your question is how to tolerance the location of a hole when orientation is the only requirement? This comes across as self-contradictory. Everything John-Paul has said so far sounds perfectly reasonable to me.
Perhaps we're getting confused as to what you mean by "the hole only has an angularity requirement". Do you mean:
a) The only functional requirement for the feature is angularity
b) The only geometric control currently specified for the feature is an angularity FCF, and for some reason the angularity has to remain the only geometric control for the feature
I assumed you meant a) and it appears that John-Paul did as well. Can you clarify?
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: GD&T is it a philosophy
Yes, I am saying the standard should spell it out because as you can read, people do it all the time.
I am still reading the other stuff to comment.
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
If orientation is the ONLY requirement, then why would you need to locate it with a +/- dimension at all?
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
If angularity is the only requirement besides size, why would there be ANY location dimension?
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
A hole that is not a datum feature must be located. And yes, it can be located with a mickey mouse +/- dimension, but that would be giving manufacturing 57% less tolerance area to hit the functional mark than position provides. Why would a designer want to do that? That is why position should be used.
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
Dave, I guess the closest thing to a rule regarding the use of +/- for location would be paragraph 2.1.1.1 of the new standard.
OK -- that's enough GD&T for a Sunday for me...
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
You said "And yes, it can be located with a mickey mouse +/- dimension, but that would be giving manufacturing 57% less tolerance area to hit the functional mark than position provides. Why would a designer want to do that? That is why position should be used."
What this tells me if we have a hole located with a +/- is that the location of the hole is not important to its function where the angle is important. We probably would not include this hole in our Control Plan nor would we make a checking fixture for it. It certainly would be confirmed on sample submission and yearly full layout.
On the other hand, the angularity would be confirm on a regular basis reflected in the Control Plan.
Please note on page 100 of the 2009 standard fig. 6.2 showing a parallelism tolerance of 0.12 mm from datum A. Also from datum A, there is a dimension of 26.6 - 26.9.
Fig. 6-18 on page 107 also has a linear tolerance to a surface.
Looks like we can dimension a linear tolerance to a surface from a datum. If we use any of the angles as a refinement, then the location must be shown in a basic dimension. If not, we must use a linear tolerance for location of the feature.
Dave D.
www.qmsi.ca
RE: GD&T is it a philosophy
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: GD&T is it a philosophy
The figures you cite are totally different from the discussion about the hole with angularity. Figs 6-2 and 6-18 use +/- dims for the feature of size, and the GD&T is merely applied to the surfaces.
You had asked about location control of a hole in conjunction with an orientation control.
So it's fine to use +/- to dimension a height. But when locating a feature of size, +/- is discouraged by para. 2.1.1.1.
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
Yes I do agree that paragraph 2.1.1.1 notes that features of size are "preferable" by using the "positional tolerance method" but it does not state mandatory. It IS still legal to use linear tolerancing on a hole as agreed by Norm (called it mickey mouse) and linear dimensions have their usefulness on a drawing.
If we have a hole with only an angularity geometric requirement in the FCF, we must not use a basic dimension but a linear one. This does comply with the standard.
For the people, especially trainers, who claim that all surfaces must have a profile of surface tolerance and all holes must have a positional tolerances are losing sight that we should be reflecting the functional needs of the feature, if it does indeed have one. It is also in compliance to the newest ASME standard to have dimensions to surfaces with a linear tolerances. It is also in compliance (but not preferable) to have locations of features of size shown with a linear tolerance.
Hopefully, the functional needs of each feature will determine how one selects the tolerancing method.
Dave D.
www.qmsi.ca
RE: GD&T is it a philosophy
The standard cannot (and should not) make warnings about potential misunderstandings of its content and, excuse the term, bad practices. The list would be endless. Should the standard warn against using 3 or 4 CMM points to establish a datum plane, even though the proper simulator and high point plane concepts are clearly explained and illustrated? Or warn against verifying a size tolerance using just a pair of calipers, when Rule #1 and the MMC boundary requirement are there in black and white? People those things all the time too.
Practices like using a simulator for size measurements were developed by inspectors, like Dave at his old job, in a well-intentioned effort to get repeatable measurements for plus/minus tolerances that were ambiguously defined. Examples could include the height or width of the part in Fig. 6-4 of '09, or the location of a hole. The meaning of the tolerance is open to interpretation, so you pick one and go with it.
In many cases (like the largely unopposed height dimension in Fig. 6-4), using the simulator results in the size tolerance being inspected as though it were Surface Profile. Which is probably what should have been specified in the first place, even on a "non-functional" feature!
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: GD&T is it a philosophy
Agreed!
My point was that if you choose to forego the recommendations of the standard, you need to hope and pray that the inspector sets up the implied datums in the correct sequence (the +/- dims still have to originate from two or more surfaces). Wouldn't it be better to avoid ambiguity and simply give a very generous position FCF with the datums clearly spelled out?
But yes, until the standard makes 2.1.1.1 more forceful than a recommendation, I'll just take a page from Norm and suggest that ASME use stronger language next time.
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
There could be an example of plus/minus tolerances used to locate a hole. Then a series of figures:
This: Means this, and this, and this, and this, and maybe this ...
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: GD&T is it a philosophy
I am not saying it is ideal but it does seem to me to avoid the issues you are bringing up here.
My point with angularity was that there is defiantly a case to be made for saying we really want things to be good simultaneously. But there are a lot of +/- drawing out there they need to have some definition.
RE: GD&T is it a philosophy
When locating a feature of size, either in ISO or ASME, the world of +/- leaves some ambiguity.
I think you're just talking about a feature of size dimension, not location. So +/- is not a problem in either ISO or ASME. The only difference, of course, is that ASME imposes Rule #1 for form control, and ISO does not (unless the E symbol...)
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
RE: GD&T is it a philosophy
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: GD&T is it a philosophy
The physical reality of locating holes with position tolerance has been proven for many years; just do the math.
There is no reason a position tolerance has to provide a "generous" tolerance to allow for an angularity refinement. The position tolerance should be calculated; simple as that. If an orientation refinement is needed then we have both multi-segment FCFS and composite tolerancing.
Working world wide and wanting to be class A, I as a long time designer does not want someone "picking" a way they personally feel like inspecting my parts.
The figures Dave mentions, as already stated, are dimensioning features of size. The +/- size tolerances do NOT originate from the datum feature. If they did, the origination symbol should be used to state that interpretation. The orientation refinement of one surface of the FOS is only that and again, I will say that it is a requirement that I would question in a valued design review. yes, there can be times thatone side of a FOS has an addtional requirement and thus Y14.5 tools provide for that flexibility. But the size tolerance should be checked for "local" size throught the feature of size. Making a guess, in fig 6-2, that the block may be placed on a datum feature simulator and tehn measure from the simulator to the top surface to inspect the 26.6-26.9 dimension would be incorrect and NOT checking for size. Form errors could easily exist on the surface designated as datum feature A for other relationship requirements that could easily cause the FOS requirement to fail while passing a profile inspection set-up much like someone my incorrectly guess at doing.
Let's try this, what is the allowable form error on the feature in fig6-2 identified as a datum featue A as shown? And would setting that feature up on a simulator check for that form error? Would form error be an independent check or root cause analysis?
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
The form error on a datum feature is often overlooked. I'll throw another variation into your questions: If Fig. 6-2 used profile of a surface across the top, and also showed the height as a basic dimension, then a flatness tolerance must be included along the bottom, because as you've pointed out, simply calling something a datum feature means squat when it comes to form error.
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
Watch out now though. Although I would certainly use flatness, it is not a MUST. One could use profile there too without a datum reference.
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
So what is the allowable form error of the feature in Fig. 6-2 identified as datum A as shown? I don't think this is as obvious as it seems.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: GD&T is it a philosophy
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
A hint: Size controls form.
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
Does the feature conform to the size tolerance?
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: GD&T is it a philosophy
This is always a good point that you are making Evan. The size tolerance of .3 controls form for the feature FOS, but that FOS ends when there are no longer opposing elelments. So the question remains; What is the form error tolerance on the rest of the feature? And thus an ambiguity.
It is my contention however, but not "clear" in the standard that I know of, that the feature of size is made up of two related features and the form error continues to extend throughout a feature "unless otherwise specified". So, IMHO, a form error of anything greater than .3 is to be rejected. But then again, how often would this detail be inspected? My guess is not much.
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
Rule #1, form error, is rarely checked at all so hanging one's hat that every feature of size will be checked for form may not happen.
Review any layout report since all dimensions are numbered. Is the shape of the feature of size numbered separately or included in the local size? Is it really checked at all???
I don't believe that many shop floor people know rule #1 involving the shape. They all understand about the local sizes must meet the requirement though. Even after a seminar, just ask the participants what rule #1 means and you may see some blank stares. They may explain flatness or circular runout but rule #1 - not many answer that one correctly.
Dave D.
www.qmsi.ca
RE: GD&T is it a philosophy
You said that "the FOS ends when there are no longer opposing elements" and wondered "what is the form error tolerance on the rest of the feature". I had never thought if splitting the feature up that way. But what is the "form error tolerance" for the FOS part in the first place?
The way that Section 2.7 of Y14.5 is worded, it would be easy to conclude that there are implied form tolerances that go along with a size tolerance. For example, if I have a size tolerance of 0.3 on a width feature, then there would be an implied flatness tolerance of 0.3 on both surfaces. This is wrong! That's not how it works!
The control of form variation isn't an implied condition, it's an indirect consequence of conformance to the size tolerance with Rule #1 applied. If a feature conforms to both the Rule #1 boundary and the actual local size requirement, then there is a limit to how much form error the feature could have. This is illustrated in Fig. 2-6, laying out worst-case scenarios for taper and bending of a cylinder. Note that the feature is "well-behaved", in that there are no unopposed elements.
Many GD&T textbooks use similar figures and develop the rule of thumb that a size tolerance of .XXX controls all of the feature's applicable form characteristics within .XXX. So a size tolerance of .XXX on a cylindrical pin gives me cylindricity, circularity, and straightness within .XXX as well. Hooray for Rule #1! Take that, ISO! You know the story.
This rule of thumb is very useful, but what is often lost is that it only works where the feature's elements are opposed. If we have a size tolerance on a feature that has unopposed elements, then all bets are off! The form of the unopposed section is not controlled. This is why we run into so much trouble when size tolerances are applied to partly-opposed width features (and partial cylinders, as well).
I agree that the form of the feature is rarely checked when there is only a size tolerance on the feature. That's because the "form tolerance" is not a requirement, and it's not supposed to be checked! They're supposed to check the Rule #1 boundary requirement and the local sizes, that's it. If these both pass and the form is still bad, it's not the inspector's fault.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: GD&T is it a philosophy
"Rule #1, form error, is rarely checked at all..."
I agree with that from my observations over 30 years. This is exactly why I do not find the ISO's position on not assuming rule #1 to be heresy, beside the fact; it seems inconsistent with other engineering philosophy/logic (assume the worst case).
RE: GD&T is it a philosophy
I agree with the ISO position and I tell all the Designers that attend my GD&T seminars to place the requirement in a FCF if there is a need otherwise do not expect the Rule #1 relating to form will be checked.
Dave D.
www.qmsi.ca
RE: GD&T is it a philosophy
How would the Rule #1 requirement be placed in a FCF? There might just be a size tolerance on its own. Or would you have a note invoking the envelope principle, even though it is the default? Or do you mean to specify a form tolerance (such as cylindricity) for the feature?
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: GD&T is it a philosophy
Why have so little faith in Rule #1? It's a rule, and if the actual local size and the actual mating envelope are not being checked, then the inspector is not doing his job per the ASME standard!
What you guys are saying seems to mandate that we put a straightness of zero at MMC on every feature of size that is a diameter (this is how Rule #1 is invoked if you don't think it's implied).
It would also seem to mandate a flatness tolerance of zero at MMC (if using 2009 standard) on every plate thickness, etc. That's being redundant with the whole notion of Rule #1.
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
RE: GD&T is it a philosophy
Yes, I would invoke straightness of zero beyond MMC on certain features of size if there is a functional requirement. I would not put this on all features though. In this way, the FCF would end up with a dimensional number and it will be confirmed - absolutely!! A checking fixture of the MMC size must either go into the hole or over the pin.
Do you both feel that form is automatically checked on all features of size on the shop floor?? How about you Evan? Did you check all forms using the CMM when confirming a feature of size assuming that you probably were using a CMM before you were accredited in GD&T?
What about programmes on a CMM. Do the programmes force one to check the form in addition to the average size?
Dave D.
www.qmsi.ca
RE: GD&T is it a philosophy
If we started modifying the GD&T language because some folks aren't fluent in it, the drawings would be a mess.
To start putting straightness of zero at MMC on everything is redundant with what a simple diameter tolerance is already communicating. Plus, it might get people thinking that diameters without this FCF do not require an envelope check. That assumption would be in violation of Y14.5.
Fsincox mentioned the "I" symbol. Good point: that should be put on any diameter or thickness that really doesn't need to be held to an envelope (assuming that it's not already exempt from Rule #1, such as free state or stock size).
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
From some of the most recent posts (I quickly scanned, so hopefully I got it right), it seems that there is a common belief that just because a feature has a tolerance or control on it, it is implicitly expected to be inspected. WHAT? So, if you don't think Rule #1 is used, you put a form control on the feature equal to what you'd have per Rule #1 anyway, and THAT means they have to inspect it??? Please, anybody, enlighten me as to where in the standard that is indicated? Personally, I favor a separate inspection protocol, but will go along with putting an inspection symbol for specific features as needed.
If I'm mistaken about the posts, sorry ... I'm really tired tonight.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: GD&T is it a philosophy
But when a feature truly acts like a feature of size the rule should most certainly be checked unless an exception is written. And if an exception is written, including the knew "I" symbol, other form controls need to be in place.
The fundamental rule of all characteristics of a feature must be controlled and so that there is not more than one interpretation. What is so difficult about this?
All the work arounds and persoanl practices or exeptions to the standard is what makes GD&T seem so confusing to those who do not take proper training and depend on folklore.
After having this same discussion a million times, I get to my common question; "Are we looking for solutions or just in love with our problems?"
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
Holes usually have their form control using positional and some feel that every hole should have positional whether or not the respective hole has a functional purpose. Holes may not be an issue.
Let's use a hypothetical example of a shaft with a length, diameter and with the "complies with ASME Y14.5M-94" in notes. Is the diameter form checked automatically? I don't think so. If we added a straightness with a 0 beyond MMC, it would be covered although theorists would state it is redundant. This is the safest way of covering the form.
Let's say that we do not have the additional FCF and, somehow the form is checked. How would one know whether the form is the problem or the local diameter??
I have an example of a OD that is 10 +/- 0.25 mm. The local size measured with a micrometer is 9.8 - 9.9. The form has a diameter of 10.35 since the feature is not straight. Should it be separated into diameter and form or just a stated actual diameter up to 10.35 - non conforming. If we report only the 10.35 mm, the information will lead to making the OD smaller in tooling.
How should one report form with respect to Rule #1 right now on the shop floor?
Dave D.
www.qmsi.ca
RE: GD&T is it a philosophy
Yes, the unrelated actual mating envelope of the shaft should be checked.
I don't know what 0 beyond MMC means or how it is stated.
I assume the example is intended to be derive median line straightness and not surface straightness.
Both form and size needs to be checked and you would report on both.
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
Also, regarding the simple shaft example, you asked "Is the diameter form checked automatically?"
Yes -- that's exactly what Rule #1 says! If an inspector merely uses a micrometer to check the diameter, he has not done his job correctly. He must also use a go gage over the full length of the shaft to verify the envelope (or a CMM can do it of course). Only then can he put a check mark in the column for diameter being met satisfactorily.
Your example of a form error of 10.35 (due to a slight bow) causes the part to immediately be rejected because 10.35 > 10.25, which is the maximum diameter allowed. You can't think of the word "diameter" as just cross-sectional diameter.
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
I, myself, am interested in learning more about the ISO's differences and why they are, where the issues are not just political.
From my observations ISO implied tolerances are generally greater, they grow as parts grow. Envelope is not assumed unless needed. They have a statement as to general tolerances that explains it as a logical approach. The implied geometric tolerances probably take a while to get used to and maybe they get ignored like our envelope does.
I have stated in this forum before that I am not a big fan of +/-.010 and +/-.03 at all. For all but small parts and as a general tolerancing principle, I believe, it is not realistic and I do not like to round .625 to .62 just because I want a larger tolerance. In our NC/CAD driven world I must actually redraw it to .62, it is just stupid a complete waste of time.
When I was first trained in the standard in 1987 I was elated. No more of this, we will use profile, well thirty years later the standard committee is there in their 2009 version's statement that implied tolerances are mostly only suitable for features of size. Most of you in this forum seem to be almost there, but the engineering world, I see and work in, is FAR behind.
We are in a global competition and clinging to our old methods is killing us. The rest of the world takes our ideas and sometimes improves on them. We need to work smarter so we don't have to work harder. GD&T is a tool to do just that.
Happy Thanksgiving, by the way.
RE: GD&T is it a philosophy
I don't think that the boundary requirement is "automatically" checked on the shop floor, or even checked most of the time. But that doesn't mean that a redundant requirement should be added to the drawing to make it explicit. I agree with John-Paul on this one.
I'll admit that I didn't check the boundary requirement in my early days on the CMM, before I studied GD&T. I went with the default "average" size, just like they taught me in the CMM training class. But if the drawing had to have warnings against every GD&T inspection blunder I've ever committed with a CMM, it would go on forever. "Make sure to inspect the entire axis of the hole, not just one center point". "Be sure to probe more than 3 points when inspecting flatness". "Don't inspect size tolerances using a datum feature simulator ;^)".
Norm,
When you say that a feature functions as a feature of size, or does not, what does that mean exactly? What are the criteria for functioning as a feature of size? I have had a similar idea before but I wondered what it meant to you.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: GD&T is it a philosophy
I am now going to add to my GD&T symbol flip charts with one that will only state "Rule #1". What does it mean? How is it checked and how should it be reported? That is going to be a group of tough questions for GD&T trainees.
I am now out-a-here. Have a good holiday.
Dave D.
www.qmsi.ca
RE: GD&T is it a philosophy
Time for some turkey. Peace and blessings, everyone...
John-Paul Belanger
Certified Sr. GD&T Professional
http://www.gdtseminars.com
RE: GD&T is it a philosophy
And of course, Happy Thanksgiving to all!
Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com
RE: GD&T is it a philosophy
I am not abandoning you, I do believe the envelope principle should go the way of implied MMC. I want only to make clear I can not advocate ignoring the standard. I would say lets change it if all it is is an adherance to 100 year old concepts that are obsolete.
Perfect, what is perfect, perfection is a concept. Is perfection relative? Perfection is not relative in my mind. Aren't we are making it so? I would argue assuming perfection everywhere forces us to make it relative. The whole feature of size issue goes away it seems from our discussions here. Some of you will say how close to zero do I have to get, true, but at least we have to state it where needed and it is not a blanket that applies everywhere.
The ISO concept of general tolerances, the general tolerancing principle, seems consistant with general engineering philosophy assume the worst tempered with statistical process control (experiance) and state what you need.