cylindrical feature datums shown in a cross section or side view 5

[Tenkan]

Questions about cylindrical feature datums shown in a cross section or side view

1) Is it correct to place a datum symbol on a profile line or leader line but not lined up with a dimension line? If correct, how is this different from placing the datum symbol in line with the dimension line of the cylindrical feature?

2) When a cylindrical FOS is used as a datum doesn’t it always mean the axis is the datum?


For clarity, the question applies to either an ID or OD.



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[SeasonLee]

For your cylindrical feature posted, there are few ways to specify a datum axis:

1. The datum feature symbol can be touching the surface of a cylinder, but this will be on the end view I/O on the side view as you posted.
2. The datum feature symbol can be touching the bend of a leader line of a FOS.
3. The datum feature symbol can be touching a feature control frame that is applied to a FOS.
4. The datum feature symbol can be touching in line with a dimension line and touching the extension line of a FOS.

Attached is a modified sketch for your reference, you may ref to Fig.3-4 of 2009 standard or Fig.3-4 of 1994 standard for more detailed information.

Datum feature symbol place in line with the dimension line means it applied to the axis of the feature, but it will occurred on the cylindrical feature, no matter it’s an internal or external feature (or FOS). However, for a planar FOS, there is a difference in meaning between in line with and not in line with the dimension line, if datum feature symbol placed in line with the dimension line, that means to establish center plane of the width (or height) of the FOS, if datum feature symbol placed on the extension line (off the dimension line), it means to establish a plane from the high points on that surface.

Season

 

[pmarc]

Koda94,
1. Per para. 3.3.2(c) of ASME Y14.5-2009 (and the very same paragraph in Y14.5M-1994) such placement of datum feature symbol indicates that the datum is an axis. If we are talking solely about cylindrical datum features I do not see any difference in meaning between this method and datum feature symbol placed on an extension of the dimension line.

2. If the entire surface of a cylinder is used to derive a datum, then I would say the datum is always an axis.

 

[Tenkan]

Yes, I am regarding solely about cylindrical datum features placed in a side/cross section view.

Pmarc, in your response #1… I was always taught that the datum had to be placed in line with the dimension indicating the axis (which is why in Solidworks the datum will snap to the dimension…). It was argued that if not (placed in line), then it could mean the datum is actually the line element running the length of the cylinder, as if one had to balance the cylinder there somehow… otherwise why the requirement to place the datum in line with the dimension in side/cross section views?


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[Belanger]

Koda -- this topic comes up from time to time, and its' true that the standard isn't very clear about it. But the consensus seems to be that if you really wanted only the tangent line on the side of the cylinder then you'd have to use a datum target symbol.

This is because a tangent line element of a cylinder doesn't really qualify as a "feature," as in the term "datum feature identification symbol." Thus, we take the datum feature to be the full cylinder. If you have a copy of the current Y14.5 standard, check out paragraphs 1.3.16 and 1.3.27 in light of the paragraph cited by pmarc.

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

http://www.gdtseminars.com

 

[Tenkan]

Excellent, that is a clear answer, which concludes the datum can be placed anywhere on the object line or leader line and does not need to be in-line with the dimension line.

I cant help but wonder what the purpose of having the datum snap to the dimension in Solidworks is for… when it does not matter where you place it in the view!


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[CheckerHater]

I cant help but wonder what the purpose of having the datum snap to the dimension in Solidworks

ASME Y14.5-2009 Fig. 3-4

 

[SeasonLee]

Koda

Here is an example of line element application, sorry for I can’t remember where I get this, but it’s not an ASME standard, it’s from ISO standard for sure.

Season

 

[CheckerHater]

Correct.

ISO is far stricter about where to put datum symbol.

But when you align datum symbol with the dimension it will mean the same in both ISO and ASME.

 

[Tenkan]

Interesting, So from the replies I read that ASME does not define the line element as a datum but ISO does and it thus more critical on placement of the symbol to distinguish the difference from the axis!

In other words, if I’m following ASME then it does not matter where the symbol is placed it means the axis.


(bear with me everyone as my company does not provide me with Y14 or ISO standards but we need to use GDT all the time... )

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[CheckerHater]

In ASME you still have to align datum symbol with the dimension if the datum is the middle plane.

 

[Tenkan]

In other words, if I’m following ASME then it does not matter where the symbol is placed it means the axis.... of a cylinder. If a centerplane, then it must be in line with the dimension

thus the snap to in Solidworks...




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[CheckerHater]

Well, since you don’t have a book…

 

[Tenkan]

Checkerhater, I owe you a beer

So I don’t see anything in figure 3-4(a)(b)(c) that indicates it allowable to place a datum symbol on a cylindrical FOS on a line element.


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[CheckerHater]

You are not alone
Unfortunately, like Belanger mentioned, the wording in ASME is more open to different interpretations, so it doesn’t look like the way is strictly forbidden either.

 

[Tenkan]

Agree, it’s not totally clear. However if the drawing does not specify the standard used then its best not to be confused with the ISO standard that allows the line element to be used, if you’re really going by ASME.

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[SeasonLee]

The earlier post is from ISO #5460 5.3.1

Season

 

[Tenkan]

to follow up with this I dug up an old textbook from a colleague circa 1985 where it shows datum symbol placement on the leader line (again, regarding side/section view...), then separately it explicitly says if your intending to use the axis to place it in line with the dimension. This might explain why I still see so many drawings this way, especially since ASME does not explicitly say you cant even though it has not shown it placed aside from the dimension line since...

SeasonLee, thanks for your input with ISO, I think the use of the cylindrical line element as a datum is why its important to not place the symbol in this manner on an ASME drawing even though its not explicitly forbidden.

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