Two datums adequate? 1

[TWJR]

Hello,

I know typically we use 3 datums, primary, secondary, and tertiary to restrict freedom of movement of features. However, in this case, I was wondering, since there are no internal features (holes, etc), this would be adequately defined using only two datums? I'm thinking that a 3rd datum wouldn't really do anything here...

 

[JNieman]

What would locate the profile along the extrusion axis, (if you think of that as an extruded angle shape)?

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NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[TWJR]

Well, this is the crux of my question. If this isn't adequate, I need a suggestion as to what more is needed...

 

[PaulJackson]

This tolerances the remainder.
Paul

 

[TWJR]

I can see how that works, but what if I don't want as close on the thicknesses?

 

[JNieman]

It doesn't apply to the thicknesses, that's not how 'all over' works, I believe. Reference 8-8 in Y14.5-2009.

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NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[Belanger]

If we use "all over" then there's probably no need to reference datums. And it would control thickness.
But to your original question, since there are two profile tolerances that reference the same datums, they are gaged simultaneously. Thus, your original inkling is correct; no other datum is needed. The profile tolerances themselves control the degree of freedom that is not covered by datums A and B.



John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[TWJR]

Thanks, Belanger. That's what I was thinking, but I wasn't sure.

 

[CheckerHater]

Beat me to it, Belanger!
Unfortunately Y14.5-2009 says very little about new “all over” requirement.
The way it’s shown on Fig. 8-8 and explained in Para. 8.3.1.6, Profile All Over is
1. Three-dimensional
2. Requires the entire part to be dimensioned Basic
3. Datumless, because the entire part is referenced to itself
4. If all of the above is true, you’ll have to drop datums, and dimension thickness Basic, so thickness will be controlled indeed.
I have to admit I’ve never seen All Over actually used and even less often seen sufficient explanation of what All Over can and cannot do.
Any committee members here? What were you guys thinking?

 

[TWJR]

Well, for whatever reason, we're using 1994 anyway, so the 2009 standard would not apply to us.

 

[pmarc]

For your information, ALL OVER is mentioned in 1994 too, see para. 6.5.2(a), page 167.

But coming back to the original drawing and question, it is true that the tertiary datum reference is not needed here.

Secondly, there are some issues on the drawing:
1. Some of the surfaces (side faces) controlled by profile between C and D are already controlled by profile between A and B.
2. Because horizontal thickness of the part (.250) is not basic, basic dimension .750 shouldn't start at the top face of the part, but rather at the datum feature A (thus the dimension should be basic .500).

 

[JNieman]

Apologies for the confusion on thickness. I did not see some of the detail of the tolerance zone in Fig 8-8 that led to my error.

_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[TWJR]

I agree with pmarc on item #2. Is #1 truly an issue? If so, how to get around it?

 

[pmarc]

By moving points C and D from the upper corners down to the points where radii R.060 start.

 

[TWJR]

OK. I see your point. Didn't think it was an issue since the two profile tolerances don't conflict.

 

[CheckerHater]

No, it’s not an issue.
You can specify more than one requirement for the same feature.
As long as your part satisfies both, you are OK

 

[pmarc]

Para. 1.4(c): "Each necessary dimension of an end product shall be shown. No more dimensions than those necessary for complete definition shall be given. [...]"

On the OP drawing profile applied between points C and D adds no value to the definition of side faces. It is just a repetition of the requirement defined by profile between A and B, thus there is no reason for it to be there.

 

[fsincox]

pmarc,
How about shifting C & D down to the points of tangency of the two opposing fillets, though?
Frank

 

[pmarc]

By moving points C and D from the upper corners down to the points where radii R.060 start.

Is this what you mean, Frank?

 

[fsincox]

pmarc,
Right, sorry, I should have slowed down, thank you.
Frank