Clarify correct usage of the Dimension Origin symbol

[Dawg2012]

I have a situation where I am being asking to add a dimension origin symbol to a dimension specifying the height of a dowel pin protruding from a surface, and that surface is a datum.

I don't think this is right, as dimensioning from a datum should set that surface as the origin for that dimension, however I can not find any direct statement in 14.5 that says this.

Can someone either correct me if I'm wrong or provide some type of evidence (that I can use) clarifying that the origin symbol is not required in this situation.

As always thanks for the expertise!

 

[JNieman]

Without seeing the drawing, how clear is it that the dowel projection is related to the Datum surface? A linear dimension doesn't tie itself to a Datum surface unless it's made clear that it does so. I would say that the origin symbol would make good sense.

1.9 LOCATION OF FEATURES
Rectangular coordinate or polar coordinate dimensions
locate features with respect to one another, and as
a group or individually, from a datum or an origin. The
features that establish this datum or origin must be identified.

That last line seems to be the one worth examining.

_________________________________________
NX8.0, Solidworks 2014, AutoCAD LT, Autocad Plant 3D 2013, Enovia DMUv5

 

[Dawg2012]

It's pretty clearly tied to the datum. See attachment (if it worked, haven't done this before).

It sounds like it's not wrong though, even if it is clearly tied to the datum? If that's the case I'll likely use it for clarity (and to keep the peace).

 

[KENAT]

What version of 14.5 do you work to?

http://www.draftingzone.com/shoppingzone/6-1.pdf

section 2.5 for what it's worth.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Dawg2012]

We currently work to 14.5-2009.

 

[CheckerHater]

Datums are intended to serve as origing for geometrical tolerances.

As dimension on your dowel is not basic and the end of it is not controlled by FCF, there is nothing wrong about specifying origin of measurement if necessary.

 

[axym]

Dawg2012,

The dimension origin symbol is necessary.

The fact that one of the surfaces is labeled as a datum feature does not make that surface the origin for directly toleranced dimensions. I don't think you'll find anything in the standard that states otherwise.



Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Dawg2012]

Sounds good! Thank you all for your replies and your patience.

 

[Keith1029]

This may be a foolish question but I have been unable to find an explanation so far that is perfectly clear to me. What would be the functional change in the callout from adding the origin symbol?

 

[pmarc]

I would just like to remind that the Y14.5 does not provide clear interpretation of directly toleranced dimension applied to a non-feature of size, thus this technique of defining height of a dowel pin is simply ambiguous = leads to different possible interpretations (see attachment). Or putting it differently, directly toleranced dimensions should be "reserved primarly for the size dimensions for features of size" (from the Foreword to Y14.5-2009).

Use of dimension origin symbol, as it is shown in fig. 2-5 (2009), eliminates the confusion (assuming that the side defined as the dimension origin is additionally controlled for its form). It is now clear how the part must be mounted for inspection, what the direction of measurement is, and how the tolerance zone for the opposite side looks like.

http://files.engineering.com/getfil...4-b918-a2446d07fa75&file=directly_tol_dim.png