Is Datum B understood to be a centerline?
Is Datum B understood to be a centerline?
(OP)
I'm trying to reference Datum B which is the centerline of a cylindrical surface originating on a previous sheet. I see examples similar to this where the Datum is actually the 33.57 dimension (see example at bottom).







RE: Is Datum B understood to be a centerline?
Important to remember: the triangle symbol is NOT a datum symbol (it doesn't get attached to the datum). Rather, it is a datum feature symbol. So you want to attach that symbol to the actual feature, not an imaginary line.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Is Datum B understood to be a centerline?
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
RE: Is Datum B understood to be a centerline?
I recommend referring to Y14.5-2009 figure 4.3, page 50 and Para 4.8.2. page 57 to expand your understanding.
Certified Sr. GD&T Professional
RE: Is Datum B understood to be a centerline?
RE: Is Datum B understood to be a centerline?
If I understand you correctly, you are trying to "repeat" a Datum on another sheet of multi-sheet drawing other than the one showing the datum feature itself (33.57 in your example) to prevent the reader for searching thru all of the sheets. If this is correct: per Y14.5 you can never attach a datum feature symbol to anything but the physical datum feature itself (never attached to theoretical construction lines like an axis line). Given this all I can recommend: place a note under the FCF frame that references a datum on another sheet that states "see sheet X of X for datum X"? This will direct the reader to the correct sheet. We use the "Datum X (sheet-row-column)"
Certified Sr. GD&T Professional
RE: Is Datum B understood to be a centerline?
"Know the rules well, so you can break them effectively."
-Dalai Lama XIV
RE: Is Datum B understood to be a centerline?
Correct me if I misinterpreted your post. But..assuming that the datum feature is shown on another sheet of the drawing and not on the sheet with your note, the reader can not determine which feature establishes the datum axis unless they search for it on another sheet - the dilemma the OP is trying to avoided.
Certified Sr. GD&T Professional
RE: Is Datum B understood to be a centerline?
Is it a center line (axis) or a physical surface? It can't be both.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Is Datum B understood to be a centerline?
RE: Is Datum B understood to be a centerline?
It is the axis of Datum B, a cylindrical surface.
I ended up removing the Datum callout frame and handled it with a note as suggested.
RE: Is Datum B understood to be a centerline?
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
RE: Is Datum B understood to be a centerline?
RE: Is Datum B understood to be a centerline?
- on sheet 1 identify datum cylinder B with datum feature symbol using one of the methods defined in the standard PLUS use a note "DATUM AXIS B" with a leader pointing to the centerline;
- wherever needed on subsequent sheets, use the note "DATUM AXIS B" with a leader pointing to the centerline.
RE: Is Datum B understood to be a centerline?
RE: Is Datum B understood to be a centerline?
You're correct.. the spec does not address this issue. There have been several viable options presented in earlier posts. Pick one or use your imagination.
Certified Sr. GD&T Professional
RE: Is Datum B understood to be a centerline?
RE: Is Datum B understood to be a centerline?
You can reference a previously defined datum/datum feature, you just have to do this properly. As it was already said, you shall not attach datum feature symbol (the triangle and the square block connected by a line) to the centerline, because the symbol is for datum feature not for datum (that is why all the pictures you showed in this thread are incorrect). But you can simply use a note DATUM AXIS B with a leader pointing to the centerline on subsequent drawing sheets.
As AndrewTT mentioned, this practice has been shown in the standard for cast, forged and molded parts (Y14.8), but is also allowed by Y14.5-2009 (see para. 4.24.13). Even though the paragraph talks about datums established from complex features or datum targets, there is nothing wrong with applying this technique in your case, especially that it does not seem to introduce any ambiguity to the drawing.
RE: Is Datum B understood to be a centerline?
Thx
RE: Is Datum B understood to be a centerline?
"Know the rules well, so you can break them effectively."
-Dalai Lama XIV
RE: Is Datum B understood to be a centerline?
Certified Sr. GD&T Professional