Powerhound,
Principle: what you think you know for sure.
Extension of principle: what you make up when you don’t know the answer.
If you have better definition please educate me: is it somewhere in ASME standard?
Belanger,
There is no need to prove negative – it’s logical fallacy.
MechNorth,
“Per unit” by itself is extension of principle – it is not supported anywhere in ASME Y14.5.1M-1994. I am waiting for new edition.
What I mean (and I stick to it) that there is absolutely nothing in ASME Y14.5-2009 to support your point of view. Please take a look:
Section 5 Tolerances of Form:
Para. 5.4.1.3: “Straightness may be applied on a unit basis as a means of limiting an abrupt surface variation within a relatively short length of the feature.”
Para. 5.4.2.2: “Flatness may be applied on a unit basis as a means of limiting an abrupt surface variation within a relatively small area of the feature.”
In both cases method and its limitations are clearly described in writing, with appropriate illustrations. No other geometrical control has “unit basis” attached to it, not even any other Form control.
Section 6 Tolerances of Orientation:
Para. 6.5: “Where it is desired to control a tangent plane established by the contacting points of a surface, the tangent plane symbol is added in the feature control frame after the stated tolerance.”
And later (in small print):
“NOTE: The tangent plane symbol is illustrated with orientation tolerances; however, it may also have applications using other geometric characteristic symbols where the feature is related to a datum(s)”
This is what you call “an extension of principle”: tangent requirement may be used with other controls, even if it is not described in detail or illustrated in the standard.
So far standard is well thought-off and balanced: Unit basis is applied to controls without datums (and not even all of them). Tangency is applied to controls “where the feature is related to a datum”.
Nowhere in the standard there is a mention of mixing two together.
Now short “Executive summary” for those not reading long posts:
1. Straightness MAY be applied on a unit basis
2. Flatness MAY be applied on a unit basis
3. Tangency MAY be applied to parallelism and other orientation tolerances.
4. Tangency MAY be extended to other controls where the feature is related to a datum(s)
5. Anything else is figment of your imagination.
I have no much interest in arguing with people in denial.
Now I am very curious about the eye-opening experience, the epiphany that made pmarc change his opinion to exactly opposite.
So far his logic is a bit flawed: “ISO 1101, 1983 edition, actually showed parallelism per unit callout as an example of callout containing a refinement in lower segment of FCF. In next editions of the standard (2004, 2012) it was replaced by straightness callout, however in my opinion that does not mean it is forbidden”
Let say, if I get rid of my Ford and buy a Chevrolet; it doesn’t mean anything; I am still a Ford owner. Convincing.
So pmarc, your current opinion is exact opposite of what you expressed in this forum:
What happened? Apparently you‘ve learned something I have no idea about. Please share.