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Multiple Datums on Single Feature (Or What is a Feature?)

Multiple Datums on Single Feature (Or What is a Feature?)

Multiple Datums on Single Feature (Or What is a Feature?)

(OP)
Have a question for folks - given a single surface on a part, I'm attempting to define two datums. One at one end of the part, and one at the other. Normally in a scenario like this, I would define a datum on one side of the part (or the whole part) and provide a straightness call for the rest of the part.

The reason I have two datums (one on each end) is that each end has a feature that needs to be located relative to the surface; and the surface within that region must be of a tighter tolerance for mating. I'm fine with the surface in question here varying in straightness by a significant amount - but the feature on each end MUST be located centrally to the end on which it sits, and that surface must be controlled more tightly.

See the image I'm attaching. You'll see the intent here, I hope. This is a dramatically simplified example for the true part in question here (which is proprietary), but it is of the same principle.

I'm not as concerned about the method I'm using to do this (which is a conventional datum indicator combined with a basic dim and some notes), as I could also use datum target regions to do this - I'm also not as concerned about the method used to control the straightness of this part (I'm using coaxial between the two datums). What I'm wondering about - can I put two datums on the same feature per the 2009 standard? There's no examples in the standard like this, but it seems like a perfectly functional and applicable way to do this. The second question - what truly defines a feature? Is the outside surface I'm using here a single feature, or is it broken into 3 features due to the different tolerancing scheme?

All of this would be a moot point if I put a step (change in diameter) between the two ends - but I'm curious about the scenario shown in the image - is it per standard?



RE: Multiple Datums on Single Feature (Or What is a Feature?)

Yes, I think this is a valid way to call this out, as long as it's clear that the 10mm basic is indicating the length of the datum region from the end. That is what that is indicating isn't it?

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: Multiple Datums on Single Feature (Or What is a Feature?)

glangston,
The portions selected as datum features, if not specified as datum targets, should be indicated by chain lines drawn parallel to the surface (see para. 4.12.5 and fig. 4-27 in Y14.5-2009).

Additional thing - what about a requirement for mutual orientational relationship (parallelism) between two 0.50 grooves? Is there such? With current datum structure the grooves can freely rotate relative to each other ending up for example with the situation as shown in fig. 4-41 of Y14.5-2009.

RE: Multiple Datums on Single Feature (Or What is a Feature?)

(OP)
The groove are allowed to freely rotate - that rotation is not important for this functional result.

It's somewhat difficult giving an exact example of what I'm working with without giving the actual part.

Imagine a connecting cable - two ends that must be positioned relative to the end on which they reside - but in between, the thing can bend and move around as needed. However, it's all one surface at heart.

RE: Multiple Datums on Single Feature (Or What is a Feature?)

pmarc's point is valid. Do the slots have to be clocked to each other? As it is, orientation of the slots to each other is uncontrolled.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: Multiple Datums on Single Feature (Or What is a Feature?)

Just use datum targets and remove all doubt.

RE: Multiple Datums on Single Feature (Or What is a Feature?)

(OP)
Appreciate the info everyone. The main battle we're fighting over here is that we don't have a lot of other staff that understand GD&T well. I need to be able to communicate design intent effectively without confusing the heck out of people. Datum targets would be even MORE confusing to most of our folks than what I've got above.

The answer there is, of course, training.

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