cylindrical feature datums shown in a cross section or side view
cylindrical feature datums shown in a cross section or side view
(OP)
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.
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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RE: cylindrical feature datums shown in a cross section or side view
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
RE: cylindrical feature datums shown in a cross section or side view
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.
RE: cylindrical feature datums shown in a cross section or side 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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RE: cylindrical feature datums shown in a cross section or side view
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
RE: cylindrical feature datums shown in a cross section or side view
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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RE: cylindrical feature datums shown in a cross section or side view
ASME Y14.5-2009 Fig. 3-4
RE: cylindrical feature datums shown in a cross section or side view
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
RE: cylindrical feature datums shown in a cross section or side view
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.
RE: cylindrical feature datums shown in a cross section or side view
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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RE: cylindrical feature datums shown in a cross section or side view
RE: cylindrical feature datums shown in a cross section or side view
thus the snap to in Solidworks...
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RE: cylindrical feature datums shown in a cross section or side view
RE: cylindrical feature datums shown in a cross section or side view
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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RE: cylindrical feature datums shown in a cross section or side view
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.
RE: cylindrical feature datums shown in a cross section or side view
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RE: cylindrical feature datums shown in a cross section or side view
Season
RE: cylindrical feature datums shown in a cross section or side view
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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