Selecting a Basic dimension as a datum 1

[jflongwell]

I am working with a client that has placed a datum symbol on a basic dimension that has opposing elements. Is this standard compliant?

 

[pmarc]

Yes, this is standard compliant. See fig. 4-33 in Y14.5-2009.

 

[jflongwell]

Thanks. I have another client using reference dimensions on a ID as a datum (FOS). Is this standard compliment?

 

[KENAT]

Potentially, is the 'datum diameter' something that comes in as part of a stock part to be modified or similar?

Posting guidelines faq731-376

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

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

 

[CheckerHater]

Reference dimension is a different kind of animal, because it doesn’t have tolerance associated with it.
Your regular FOS is usually directly toleranced, and basic size has some geometrical tolerance attached to it. Either way some sort of LMB / MMB condition exists.
When it comes to reference dimension, I guess only RMB datum may be safely specified.

 

[greenimi]

1.3.24 Dimension, Reference
dimension, reference: a dimension, usually without
a tolerance, that is used for informational purposes only.



NOTE: A reference dimension is a repeat of a dimension or is
derived from other values shown on the drawing or on related
drawings. It is considered auxiliary information and does not govern
production or inspection operations. See Figs. 1-19 and 1-20.
Where a basic dimension is repeated on a drawing, it need not be
identified as reference. For information on how to indicate a reference
dimension, see para 1.7.6.

1.4 FUNDAMENTAL RULES
(c) Each necessary dimension of an end product shall
be shown. No more dimensions than those necessary for
complete definition shall be given. The use of reference
dimensions on a drawing should be minimized.

 

[fsincox]

A basic dimension is a dimension without tolerance, people always think it is a "dimension with a box", that is just the way it is commonly shown on drawings with implied tolerances.
Frank

 

[Tunalover]

I don't see why you can't assign a datum to a reference diameter. What if you purchased, say, a bushing from a manufacturer but need to modify it in some way. The as-manufactured ID or OD of the bushing can establish a datum for modifications. It can be attached to a reference dimension and still serve as a valid datum.

Tunalover

 

[fsincox]

The problems come in primarily because of the material modifier portion of a datum requirement, if RFB not so much, but if MMB or LMB then what are they without a tolerance specified?
Frank

 

[dgallup]

I'm with Tunalover. I have used reference dimensions as datums. Primarily diameters that were created previously in the production process. You can't use a datum material modifier (MMC/LMC). Frequently we buy a forged blank and use some of the forged features and datums for the machined features.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[pmarc]

Frank,
Maybe that is why the definition says: "a dimension, usually without a tolerance, that is used for informational purposes only."?

Of course one may imagine following situation:
So I enclosed a toleranced dimension within parentheses. This solves the problem of datum feature referencing at MMB or LMB for primary datum features. But what about secondary and/or tertiary datum features referenced at MMB/LMB? These MMBs/LMBs are usually generated not only by size tolerances, but also by geometric tolerances applied to these datum features (secondary DF is geometrically controlled to primary DF, tertiary to primary and secondary). Should I also make these tolerances reference in order to be able to calculate MMB/LMB of datum features properly?