Position control with only one datum

[SeasonLee]

A simple question but need your comments.

Please ref to the attached sketch, it’s a zinc alloy die casting part, can we use position control to locate the tab with only one primary datum? The designer intends is to control the centerplane parallel to the datum A or just its location?

Thanks for the assists

SeasonLee

 

[fsincox]

Sure, but the way you have it laid out cries out profile. Is there a reason for position? I would dimension to the center if I wanted position.
Frank

 

[pmarc]

Answering to your first question - it is absolutely OK to use only one datum in positional control of the tab. I would just apply basic dimension to the centerplane of the tab, not to the bottom face.

As for second question, based on what is shown his intent is to control both - loaction (position) and orientation (parallelism) of centerplane relative to A. Simply because position indirectly controls orientation (but not opposite).

 

[SeasonLee]

pmarc
You caught the point, the problem is on the basic dim.

SeasonLee

 

[SeasonLee]

Frank, pmarc

Sorry, I should say both of you caught the point.
Thanks

SeasonLee

 

[pmarc]

Well, if you are specifically asking about legality of basic 1.70 dimension in conjuction with positional tolerance, I would say it at least creates ambiguity and for that reason I would not recommend it. As a support of what I just said, I would refer to para. 1.9 of Y14.5M-1994 clearly stating that the dimension locating the feature shall be applied to its center.

 

[ak762]

Just wondering, if depiction of this position symbol potentially may confuse someone with design intend and additional comments used for clarification then why do not exchange control at more unambiguous?
Can it be exchange to symmetry or parallelism for both tab's surfaces?

 

[fsincox]

I assume pmarc might agree that if the size dimension was also basic, for some reason, then a basic chain would be available to establish the location with basic dimensions and it would be OK.
pmarc?
Frank

 

[pmarc]

If the size dimension was basic 6.00, I would not apply position callout to this basic dimension, but use two profile of surface callouts applied to bottom and top surface of the tab.

 

[fsincox]

OK, I say it is not illegal. I can't think of a good example/reason someone might do it, off the top of my head, so I will leave it at that.
Frank

 

[fsincox]

pmarc,
I guess I am not suprised we disagree. I really do hope you are not M. Foster from yahoo GD&T group.
Frank

 

[pmarc]

No I am not, Frank.

Look to para. 7.2 of Y14.5-2009. It clearly states that: "Position is a location of one or more features of size relative to one or more datums".
Now, 1.3.32.1 says: regular feature of size is "... a set of two opposed parallel elements or parallel surfaces, each of which is associated with a directly toleranced dimension".

That being cited, basic 6.00 would not be directly tolerance dimension, therefore would not meet feature of size definition, therefore could not be controlled by position callout. Does that make sense?

 

[fsincox]

Yes, It does.
Thanks,
Frank