How to Deal with Multiple Datum Reference Frames

[JasonNicholson]

Is this the correct usage of multiple Datum Reference Frames? I don't really want to use multiple Datum Reference Frames but I don't have a choice in the matter. I have to make the best of this. How can I best indicate the toleranced distance between the two datums? A PDF is attached if that is helful. Any information on this issue is greatly appreciated.

-Jason Nicholson

 

[fsincox]

Some thoughts and questions,
ASME or ISO?
I assume you will be using those datums later, somehow?
Can you make an "INDIVIDUALLY" referenced datum work for you?

Datum on centerline not allowed in ANSI, we sometimes say CL datum "A" or some such verbage, I am assuming this will be much more complicated drawing than is shown, or I would just drop it, altogether.
Position tolerances should be on the feature, not the dimension as you show.
Frank

 

[fsincox]

I am curious on others thoughts on this. Multiple reference frames are, IMHO, inevitable in complicated real world parts if the standard is taken seriously and all available tolerance is to be shown, we avoid it because we do not want to do the whole job (as a shortcut) and at our own peril, again, my opinion.
Frank

 

[drawoh]

fsincox,

I am trained in the ANSI standard. I don't know the ISO standard.

The basic function of datums is to show how the part is to be constrained in fabrication and inspection. I will add datums over and above this requirement when there are features that must be controlled to a non-datum feature more accurately than to the datums.

Confronted with this problem, my first option would be to re-think my datums. Failing that, it is perfectly reasonable to add datums. The OP does not show what is being located to datums[ ]E and[ ]D.

We actually do not know what the OP's problem is. The part looks like an extrusion. Perhaps they are having trouble achieving the tolerances. If they made the part asymmetric, and used one side of the part as datum[ ]A, they would improve that bit of fixturing.

JHG

 

[fsincox]

drawoh,
I realize sometimes rethinking the datums can help, however, I differ from you in that I believe the datums are also to be used to describe the functional relationships of the part not just the manufacturing processes. The text of the standard has supported this position for some time, I also realize it is not the way it is often used in practice.
I actually thought you should not have datums on the drawing that are not referenced in a DRF.
Frank

 

[KENAT]

Per fsincox questions about what standard, and how you intend invoking datums D & E.

Posting guidelines faq731-376

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

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

 

[drawoh]

...
I actually thought you should not have datums on the drawing that are not referenced in a DRF.

I pointed this omission out in my post. It is very likely that there is[p/i] a FCF. The OP has chosen to not show it to us.

It is logical that your mounting system for final use is the same as your mounting system for fabrication and inspection, but it is not absolutely necessary. In any case, final use is relevant only to the designer, and the final user. Fabricators and inspectors are only interested in fixturing.

Take the case that I design a machined casting. I design datum targets into the casting so that the foundry and the machine shop all jig to the same points. The machine shop machines a flat surface on the bottom, then it drills mounting holes. These machined faces are used for mounting.

JHG

 

[fsincox]

drawoh,
I was just checking to see if I made it up in my mind, I do that sometimes, I guess.
Frank