Redundant or not? Please help

[greenimi]

Imagine a shaft --datum A-- Ø.365/.360-- with 2 thru holes 2x Ø.195/190--positioned with ± .185/.165 tolerance from each end of the shaft.
A slot is added to this shaft in the middle (equal distance from both ends). The slot is .100/.090 and is called datum B
The GD&T is as follow:
For the holes: multiple single segment (not composite )
2x
position of Ø.002 at MMC to A at MMC--first segment
and
position of Ø .004 at MMC to B at MMC--second segment--

For the slot (datum B): position of Ø.002 at MMC with A at MMC

The question for you is: the second segment of the multiple segment callout, isn't it redundant callout?

Simultaneous requirement is implied, since the slot and the holes have the same datum reference frame (positional at MMC to A at MMC). Please help.
Again, I am thinking that position of Ø .004 at MMC to B at MMC--second segment--is redundant, but I am not sure.

 

[CheckerHater]

Thank you greenimi,

I started my post saying that it was more about "legal" then "redundant".

One of my concerns was that if the shape of tolerance zones are significantly different, requirements may be less redundant, but it is not the case here.

Also there is another fine line here. As datum [A] symbol is not aligned with 365/360 dimension, the datum [A] is NOT axis of the shaft, but rather something else, it creates different reference frame, and your requirements are NOT redundant.

There is no need to be defensive. I am just exploring possibilities.

 

[fsincox]

CH,
I believe in the case of a cylinder, sphere or cone (without other limiting indications) it is treated as the same thing.
Frank

 

[CheckerHater]

Maybe in some places standard is more vague, but if you compare, say, Fig.3-4 and Fig.4-45 (Y14-2009) it's kinda specific.

 

[fsincox]

Fig 4-45 is exactly the kind of thing I was thinking of.
Frank

 

[Belanger]

I think a datum feature symbol doesn't have to be aligned with dimension arrows when dealing with a cylindrical part as the datum feature. This is because there's only one way to interpret a datum on a cylinder: an axis. See Figs. 6-14 and 15 of the standard for an example.

The case of a planar feature of size is different, though, because there the datum symbol not aligned with the FOS would mean only a surface as the datum feature.

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

 

[fsincox]

Thanks, J-P, for the help.
This is the way it was explaind to me also, if I put a finish mark on the surface it is automatically applied all way around on a cylinder, sphere, or cone. so the standards are consistant in that sense.
Frank

 

[CheckerHater]

Frank, JP,

Where exactly in the standard datum derived from cylinder is shown like in OP picture?

 

[MechNorth]

There would be an infinite number of tangent planes / lines that could be derived from the feature given the datum callout (A) in the OP figure. If one of those were the intention, it would be referenced as a tangent plane (T) in the DRF and would need some specific basic dimensions/angles to locate it wrt other datums; it would not be an acceptable primary datum then.
As there is no other reasonable interpretation, the interpretation leads you to the center axis; this is supported by the attachment of the datum callout to the cylindrical surface in an end view (see Fig.4-44 in'09, datum A). Note that a similar situation arises when attaching the datum callout to the side-profile of a cone, as in Fig. 4-45 ('09).

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

CH, it's paragraph 3.3.2(c) of the standard: The datum feature symbol may be "placed on the outline of a cylindrical feature surface or an extension line of the feature outline, separated from the size dimension, when the datum is an axis." (That statement doesn't say that it has to be in a view where the feature looks like a circle; it just says that it may be placed on the surface of a cylindrical feature.)


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

 

[Belanger]

Jim, we agree that datum A in the OP picture is the axis of the cylinder, not planes or lines formed from the top arc of the cylinder. But if we really wanted lines or planes taken from the top, wouldn't a datum target be used? I've never seen the "T" modifier next to a datum reference.

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

 

[CheckerHater]

JP,

Paragraph 3.3.2(c)states "see Fig.3-4, illustrations (e) and (g)" plain and simple.

I see no reason to continue this argument.

 

[Belanger]

Didn't see it as an argument ... so you believe the datum callout in the OP drawing to be illegal? Paragraph 3.3.2 doesn't verbalize every possibility; we've already seen that cones can be labeled as a datum feature in two ways (Fig. 4-3(e) and Fig. 4-45), neither of which correspond to the letter-of-the-law in para. 3.3.2.

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

 

[MechNorth]

J-P, targets would be much easier, but is there anything illegal / prohibited about using the (T) modifier on a secondary or tertiary datum? My thought is that it would not establish a specific line in space which may or may not reside on the feature, but rather one which is guaranteed to be on the feature (at least at one point or line). Just a thought for consideration.

btw, don't recall noticing (c) on Fig. 3-4 ('09) previously. Have taught (h) & (f), though I've had people argue with me about their validity.

Also, I had mentioned the callout for cones for the very reason that J-P mentions; it sets precedence for cylinders, whether or not explicitly laid out in the text or other graphics.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

The thing about "T" on datum references never occurred to me, because the very definition of establishing a datum is already based on tangent contact. Think of a traditional flat surface datum... the simulator only feels the high points anyway (tangent plane). You're talking about the top of a cylinder, but I would rather use targets than try to impose "T" in the datum reference portion of a feature control frame. Shrug.

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

 

[MechNorth]

Ah, but there is a distinct difference between a plane based on 3 points of contact and one based on a tangent plane mutually perpendicular to higher-precedent datums. Anyway, it was just another item for thought.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com