Combining +/- tolerance & basic dimension to locate a hole. 2

[Madhu454]

Hi All,

One more basic question. Please see the attached drawing.

Only one face of the block is mates with its counter part (datum feature B). the other side is open and does not mates with any surafces. The hole needs to be located with closer tolerances with respect to datum faeture B, the location of the hole from the other free face which can have a wider tolerance. In such case can we combine the +/- tolerance and basic dimensions to locate the hole.

Is it a legal specification in ASME? I have seen few drawings of this kind.

 

[TheTick]

I disagree w/ CheckerHater's solution. Having only two datums does not imply "directionality" to the FCF. In this case, the second datum only contributes orientation, which is moot, since there is only a singular round hole and not a slot or pattern.

The solution I previously posted is similar to the scheme which was widely accepted by automotive customers. It was usually applied to slot dimensions which could be allowed more tolerance in the length directin than the width direction. The key is that applying the FCF to a hole size dimension that has a directional inference.

 

[Belanger]

Tick is right -- both of the proposed pictures are incorrect. The first one has only one datum, which is perpendicular to the hole. Therefore, you can't use the position symbol, which is primariliy a location control, and orientation (perpendicularity) comes along for the ride only if the primary purpose of location is met.

The second picture does not automatically imply what is desired. Though most people would know what you're getting at, the standard is clear on how to display position tolerances that are only meant for a specific direction.

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

 

[Belanger]

pmarc ... just a quick thing about your first post on this thread:
You stated that plus/minus tolerances for location are "illegal because Y14.5 clearly says that basic dimensions shall be used to define true position."
I don't quite agree with that statement. Maybe plus/minus is illegal for locating a hole (I might argue that illegal is too strong of a word), but it wouldn't be for that reason. Yes, basic dimensions define a true position, but the term "true position" goes hand-in-hand with the GD&T position symbol. IOW, that rule doesn't extend to the general desire of locating a hole on a part.
I think we all neeed to be careful of the implications of the two words "position" and "location."
If the standard said that basic dimensions shall be used to define a feature's location, then plus/minus tol on a location would be clearly forbidden.

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

 

[CheckerHater]

Guys, allow me to disagree respectfully.

Using Position to single datum to indicate perpendicularity is perfectly fine in composite frames, so everybody knows what it means; see for example Fig. 7-38(c) in Y14.5-2009.

And TheTick, how could possibly second frame contribute "orientation" but not "direction"? Could you have one without another? Almost like "position" and "location".

 

[Belanger]

There was a thread about this a while back, but I'll summarize my statements from there... Position's main job in life is to control location. If location is not being controlled, then position is the wrong symbol to use. It's akin to profile: profile's main job is to control form. If you want it to additionally control orientation, location, etc., that's fine. But you can't have a profile tolerance be separated from its desire to control form.

The lower frame of a composite zone does more than perpendicularity; it controls the hole-to-hole location (even though you don't see any datums explicitly referenced for that).

I agree that most people would get the idea, but rules is rules

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

 

[Belanger]

And don't anybody bring up the tangent plane modifier on profile. I guess there are exceptions to every rule!

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

 

[fsincox]

CH,
I would propose a revision to your picture the first position callout to ABC with the larger gross location (to "C" so to speak) and a second separate position call out to AB (to "B" so to speak) wirh the tighter number. That I believe is also legal and does what you want.
Frank

 

[fsincox]

CH,
I will disagree with the other's and say I do not think what you have shown is a technical violation in any way.
Tick,
While not implying "directionality" the extent of framework control only exists to the extent the stated framework can actually extend control, the effect is the same.

 

[Belanger]

The ASME standard does not allow the position symbol to be used for only perpendicularity because of paragraph 7.2: A position tolerance defines "a zone within which the center, axis, or center plane is permitted to vary from true position." And the same paragraph defines position as "the location of one or more features of size relative to one another or to one or more datums. (My emphasis added in this last quote.)

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

 

[fsincox]

J-P,
I would certainly agree with you on the issue of not using Position to experess simple Perpendiculariy control. Just by its implication it makes someone expect more or think that they are missing someting that was intended to be implied when they would not be. The result is it tends to adds to confusion and not to clarity, violating the old KISS principle.
Frank

 

[pmarc]

J-P,
I am not sure that I understand your remarks regarding one of my posts correctly, but that is the end of a hard week, so please forgive me if my answer makes no sense.

pmarc ... just a quick thing about your first post on this thread:
You stated that plus/minus tolerances for location are "illegal because Y14.5 clearly says that basic dimensions shall be used to define true position."

I do not think I ever used term "location" in any of my previous posts in this thread.

I think we all neeed to be careful of the implications of the two words "position" and "location."
If the standard said that basic dimensions shall be used to define a feature's location, then plus/minus tol on a location would be clearly forbidden.

For me location is either position, symmetry or concentricity - at least this is how I understand fig. 3-1 in Y14.5-2009. Someone can say that runout and profile are indirect location controls and that is fine for me. But in this particular case - with this particular part geometry - I do not see anything applicable besides position. If you are trying to say that op's print is controlling hole's position in vertical direction and hole's location in horizontal direction, then I am asking what kind of location is he trying to control? Is there any specific standardized name for this geometrical characteristic?

 

[fsincox]

J-P
Whoops, Sorry, I guess I also agree with your position on the first of CH's examples. I do not agree on the second, particularly now that you are supporting the mixing of +/- and position as in the OP, we do not really know the design intent so can it be done, by the book, I have to say, yes.
Frank

 

[Belanger]

Sorry for any confusion, pmarc! I was basing my comments on your statement that "+/- dimension for controlling position is ... illegal because Y14.5 clearly says that basic dimensions shall be used to define true position."

The OP's example never used ± dimensions in conjunction with a position tolerance, so I had to read into your question a little bit, and apparently went beyond what you really meant.
But I was saying that the rule you give doesn't apply to the OP because his position tolerance was held only to datums A and B, which already do have basic dimensions. So I presumed your statement was focusing on the location in the east/west direction. As it is, the OP's example doesn't violate any specific GD&T rule (other than a change needed for where the FCF is placed on the drawing).

Then you ask:
"If you are trying to say that op's print is controlling hole's position in vertical direction and hole's location in horizontal direction, then I am asking what kind of location is he trying to control? Is there any specific standardized name for this geometrical characteristic?"

My reply: Yes, I am saying that the print controls the position in vertical, and location in horizontal. There is no name for the geometric characteristic in the horizontal, simply because it's outside the realm of GD&T. So we must use the more generic term of location.

That said, I agree with everyone that ± should not be used for location! It's really for size, chamfer, and radius only. But we were getting down to what's "legal," not what's "wise."

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

 

[pmarc]

I said it in other thread about position tolerance in a past - unexpectedly ISO GD&T is a little bit ahead in terms of potential applicability of plus/minus dimensions for position of FOS, because ISO 5458:1998 Annex B clearly says that such practice is no longer recommended due to the fact that the meaning of plus/minus dimensions is not standardized in such applications.

 

[CheckerHater]

Is any of you guys getting confused when approaching an airport?

Somehow you reminded me one of my favorite movie quotes:

Eddie: Okay, we gotta pick a road. Arrivals or departures? We're arriving, but then we're departing. Which one, Snake?
Snake: What do you think?

Big Trouble (2002)

Read the standard: Location is the common term for position, coaxiality, concentricity, symmetry.
Then Standard explains that location between FOS, or between FOS and other feature you pick as a datum, is called POSITION, OK?

You cannot have location along the X and position along the Y, like you cannot be arriving and departing at the same time.

Many times I asked myself to keep quiet on Mondays :-(

 

[Belanger]

Why is this so confusing? Position is always location, but location is not always position.

So if I choose to use ± to locate a hole (which is fraught with issues, I realize, but we're just dealing with the concept right now), I would discourage you from calling it a position tolerance. Anyone else agree?

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

 

[CheckerHater]

You cannot call it "True Position", but it's still a Position.

Vote?

 

[MechNorth]

In the standard ('00) "1.3.64 True Position: true position is the theoretically exact location of a feature of size, as established by basic dimensions." The geometric control is just "position".

Again, there is nothing in the standard indicating how to find the center of a hole when a +/- location tolerance is used. Perhaps "illegal" is too strong a word, but "indefensible" surely applies here. That ISO makes this somewhat clearer (though still not outlawing it) is a step. In '03, Y14.41 indicated the appropriate uses for +/- tolerances, and location of a FOS was not one of them. It is unfortunate that the list was not carried forward into Y14.5'09. One of the issues with a consensus standard is that good and seemingly-obvious items are sometimes diluted or dropped completely to achieve at least temporary closure.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[CheckerHater]

true position is the theoretically exact location of a feature of size, as established by basic dimensions

So, no basic dimension, no true position. Did I say differently?

 

[MechNorth]

CH, still not a position either because there's not a geometric positional tolerance. It may be a location though.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com