Implied Symmetry to Datum Features of Size 4

[farmerandy]

In the attached drawing there are 5 thru holes in a rectangular block. Four of the holes are located with implied symmetry to datum features of size B and C. The 5th hole is not symmetric to datum features of size B and C. Do I need to declare new datums that are just the edges of the block from whence the 5th hole is basically located?

 

[3DDave]

Yes - it is not located from any midplane which is what Datum Feature Symbols B and C currently define.

 

[pmarc]

From design perspective, does the 5th hole need to be located from the edges (surfaces actually)?

If yes, then the edges should be defined as additional datum features and called out in the positional feature control frame for the 5th hole. If, however, the hole needs to be controlled relative to datums A, B and C, then vertical basic .629 dimension and horizontal basic 1.429 dimension from the upper left hole of the 4-hole pattern will work just fine.

 

[farmerandy]

Thanks 3DDave and pmarc. The common message from your responses are "it depends." What I hear is:
[ol 1]
[li] If your design intent is that 5th hole's location is indeed from the edges of the block then new datums are required.[/li]
[li]If however, the design intent is that the 5th hole's location is to be located from the center planes defined by the datum features of size B and C, the basic dimensions and FCF are acceptable as shown. In this case would it be better to have basic dimensions from the center planes themselves? [/li]
[/ol]

 

[pmarc]

2. If however, the design intent is that the 5th hole's location is to be located from the center planes defined by the datum features of size B and C, the basic dimensions and FCF are acceptable as shown. In this case would it be better to have basic dimensions from the center planes themselves?

If the 5th hole is to be located from the datum center planes B and C there has to be a basic relationship/connection between the center planes and the true position of the hole. As shown now, such relationship is not defined because basic 1.629 and 2.429 dimensions are from the edges.

Yes, I think it would be the best (the most straightforward) way to show the center planes (even though the part is not symmetrical) and dimension the true position of the 5th hole from them.

Side note: If the drawing is in inches, I don't think you want to show leading zeros.

 

[farmerandy]

Thanks again pmarc.

Leading zeros; some esoteric setting in NX that I failed to check.

 

[drawoh]

farmerandy,

Is it a good idea to define symmetry about sloppy FOS datums? If this were my drawing, the four-hole pattern would be my datum[ ]B feature. The big hole and the outline would be dimensioned from the holes and positioned and profiled from datums[ ]A and[ ]B.

--
JHG

 

[farmerandy]

Hi drawoh,

I see your point and were this a real part I would consider that. But since I referenced B and C RMB, haven't I taken the slop out at least for the 5th hole?

farmerandy

 

[chez311]

To be fair, we don't know what the tolerance is on B and C, though assuming it is held to a general print tolerance it is likely equal to or larger than the .01 the 4x holes are held to.

farmerandy,

Actually I imagine you'll have more issues referencing sloppy FOS datums at RMB than at MMB/LMB since you are allowing variation within a wider band for the datum feature then fixing it during inspection to whatever the as-produced part comes in at to control a feature (or pattern of features) to a tighter tolerance. At least with MMB/LMB you get the advantage of datum shift to allow the as-produced datum feature to shift relative to its larger tolerance zone - however this is still not recommended. Your datum features should always be more accurate than the features it controls.

 

[farmerandy]

Thanks to all who responded to this thread. Quite a lively and dignified discussion; something we need more of today.