Pmarc,
I do not expect to change your mind, either, I just want to post my responce to your OP.
You really pushed me into many other areas I have been thinking about for a while. First I want to restate my belief that the ASME standard is a good work, I spend so much time here tearing it down that I suspect people might wonder. I doubt I could do any better, but, it is clearly not perfect.
Your objections to my example seem to be based on the concept of how do you gage it. “What is my tolerance to make a gage for it?”
I have been suspecting that the discussion of the ASME standard has become too reliant on gaging to express principles. We all know parts are often not checked that way and some have even stated, here, that the use of gages is declining. Small machine shops, like mine, rarely rely on gaging and the idea that you have to gage to check part tolerances does not come from the text of the standard, people only infer this from the “pictures in the book”.
No one can be expected to stop production until they have a gage to check a part. It is ridiculous to say that all parts must be gaged . Gages do have their beneficial qualities like they actually check all points at once so it is definitely a useful tool to have in your box, but, it ain’t the only game in town, nor should it be. Most companies, just by judging by the voting of their dollars, don’t seem to be willing to invest them there.
I also think the over reliance on gaging in the standard starts to make it irrelevant to the majority of the people who actually use it, while, it certainly makes it easier for those who want to teach it. The ISO must feel that way, too, as they seem to be heading down the path of making it more relevant to CMM measurement. This is also almost certainly why the ASME is interested in the mathematical definition of the standard so it is relevant outside the gaging world.
The standard, as I was taught, basically has always thrown away the tolerance you are looking for. How perfect is a flat surface plate, how coaxial are the 2 “A-B” chucks, what is the tolerance of a basic dimension on a gage? How perfectly perpendicular are the three mutually perpendicular planes of measurement? The answers have always been something like: “we do the best we can do to simulate this perfection, do we achieve it, no? Do you achieve it, no? Is yours better that mine, maybe? The concept of perfection is implied all over the standard.
Your OP example relies on our ability to establish the primary datum, mine is an establishment of a secondary datum. To me there is a difference. The primary datum establishes the orientation of the axes of measurement of the part to the whole world. The secondary and tertiary datums refine this, but, do not stop us from getting started.
I can find the axis of my shaft. Yours we can put in chucks/collets no matter how eccentric they are. Do I know how perfectly coaxial those chucks/collets will be, no but, we treat them as perfect. I am just not sure both of your examples parts are “identical”. I did not say, either of the examples can’t be done.
When those of us who do not use gages check our parts we check them to the best of our ability: establish the center of the shaft, roll the keyway to center on to center as perfect as we can, how perfect is it? There is the tolerance you are looking for. We know the keyway is to be located on zero, the standard supports it in the shown examples, if not directly in the text. The fact you don’t have a tolerance to build your gage has never been part of this standard, you would have to build it the best you can like all the other measurements made to the standard.
When Mr. “doesn’t recommend bothering to tolerance the orientation of short secondary datum pilot diameters” builds his gage it doesn’t bother him either it has to be perfect, how perfect? It was not an issue to him, either. Do you object to his use too? I do feel it is very bad practice for him to teach this and I really did not recommend using the method I discussed either. I just said it was not illegal.
My point is your issue is really an ASME “rules of the game” issue not my “rules of the game” issue. The issue of perfection permeates the ASME standard to say it can’t be done is arguing with them not me. Remember, I am an Independency supporter for just this reason. Zero position tolerances at MMC, why are they not used more often and resisted buy the shop, in horror? Why do they ignore the envelope principle in the shop making ASME’s insistence on it a paradox for real day to day engineers (as opposed to people who teach)? The ISO do not really by it completely, either, they allow it to be used because the biggest gorilla in the room insists on clinging to it.
In summary, 14.5 is not a gaging standard and it makes no provision for it, which is why you have a standard specifically for that. The reliance of the standard on the concept of perfection does threaten to make it irrelevant but those are their rules of the game not mine. You can’t let some play the game one way and not let all do it and be taken serious with me. The part you show is not the same condition as mine and yet it to could be checked in theory, I just don’t know if it the same (the actual question). Isn’t the fact that they had to add a statement like 4.5.2 to the 2009 standard, proof enough that they had to finally admit what was obvious to some of us long ago, already?
Frank
