Two datums adequate?
Two datums adequate?
(OP)
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...
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...





RE: Two datums adequate?
_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5
RE: Two datums adequate?
RE: Two datums adequate?
Paul
RE: Two datums adequate?
RE: Two datums adequate?
_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5
RE: Two datums adequate?
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
RE: Two datums adequate?
RE: Two datums adequate?
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?
RE: Two datums adequate?
RE: Two datums adequate?
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).
RE: Two datums adequate?
_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5
RE: Two datums adequate?
RE: Two datums adequate?
RE: Two datums adequate?
RE: Two datums adequate?
You can specify more than one requirement for the same feature.
As long as your part satisfies both, you are OK
RE: Two datums adequate?
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.
RE: Two datums adequate?
How about shifting C & D down to the points of tangency of the two opposing fillets, though?
Frank
RE: Two datums adequate?
RE: Two datums adequate?
Right, sorry, I should have slowed down, thank you.
Frank
RE: Two datums adequate?
If the flatness and perpendicularity were extracted leaving just the profile all over... max flatness on surface A would increase to .004 and max orientation of surface B to A could exceed .004.
I should have boxed the thicknesses... my mistake.
Paul