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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?

The example from the bottom is from the Y14.5 standard, so of course that is the correct way to do it.

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?

It's hard to say if what you've done is correct because I can't see where that extension line leads to BUT if the datum feature symbol is on the centerline of the bore then it is incorrect.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

RE: Is Datum B understood to be a centerline?

Barnon:

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?

(OP)
Yep I've studied that page. My issue is I've got a 20 sheet drawing with Datums defined on previous sheets that I need to reference. In the case below the red center line represents a very large cylindrical surface that can't be seen on the enlarged detail view. From my understanding the example below, left side shows Datum B as the red centerline and right side would show Datum B as the center of the dimension 33.57. If not, how else can this be done?





RE: Is Datum B understood to be a centerline?

Barnon:

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?

I have in the past used a note "CENTERLINE (DATUM)" with the leader pointing to the centerline. This avoids illegally attaching the datum symbol to the centerline but gets the point across.

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

RE: Is Datum B understood to be a centerline?

ewh:

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?

Barnon -- I'm confused by the terminology. In your most recent post you wrote that "the red center line represents a very large cylindrical surface."

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?

I would section the part in a way to show that the circles represent the features so that both features have a clear relationship in the same view. This would eliminate the ambiguity. You could also add a reference dimension for the diameter datum B is to be based on.

RE: Is Datum B understood to be a centerline?

(OP)
Belanger:

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?

In that case, you cannot attach a datum feature symbol to it and expect it to be a legal callout. I don't know how ewh thinks his note makes it legal since it's still doing exactly what the standard is trying to prevent.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

RE: Is Datum B understood to be a centerline?

What about a note under the 33.57 dimension that says "MEASURED FROM DATUM B"? If needed that same note could reference the sheet number to view datum feature B.

RE: Is Datum B understood to be a centerline?

My suggestion would be following:
- 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?

(OP)
Datum B is called out properly on a previous sheet which does establish an existing theoretical axis of a cylindrical surface. I'm surprised there's no "Legal" way for me to reference it afterwards when it's not visible. I couldn't find any verbiage in the spec that said I couldn't reference a previously defined datum. Can someone quote the spec?

RE: Is Datum B understood to be a centerline?

Barnon:

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?

The casting standard has figures (4-3, 4-4) that use a leader to describe a datum center plane by name (e.g. - DATUM CENTER PLANE B). This is done to clarify what a collection of datum targets actually yield. Even if there is no wording in Y14.5 clearly allowing such a note (I couldn't find wording that disallowed such a note) the spirit of Y14.5 is for a single interpretation of the drawing so I don't see why you could not use a similar note on your drawing (leader to axis with wording DATUM AXIS B).

RE: Is Datum B understood to be a centerline?

Quote (Barnon)

Datum B is called out properly on a previous sheet which does establish an existing theoretical axis of a cylindrical surface. I'm surprised there's no "Legal" way for me to reference it afterwards when it's not visible. I couldn't find any verbiage in the spec that said I couldn't reference a previously defined datum. Can someone quote the spec?

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?

(OP)
A note it shall be.

Thx

RE: Is Datum B understood to be a centerline?

Thanks for partial vindication, pmarc!

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

RE: Is Datum B understood to be a centerline?

In my opinion placing the "DATUM AXIS B" note on the datum axis on another sheet of the drawing is meaningless. The reader still does not know which feature establishes the datum. There could be numerous coaxial features that could be selected as the datum feature. If the note is applied it must direct the reader to the sheet where the datum feature is identified (with the symbol) so there is no doubt which specific feature is the datum feature.

Certified Sr. GD&T Professional

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