Parallelism, Flatness and Plus/Minus

[rhmeng]

I have a surface on a part that I am dimensioning w.r.t to a surface Datum A. If I put a dimension X +/- .001, is there no point in putting a // to Datum A of .002/.003? Because the plane has to lie within that tolerance from the +/- .001 anyways correct? Also for the same surface, if I callout // of .004 w.r.t Datum A, that is sort of like saying the flatness of the surface is also .004 right? Seems like when you get to tight tolerances flatness, +/- and //ism go hand in hand and sort of say the same thing.. Thanks in advance

 

[KENAT]

Rule 1/Taylor principle does indeed apply.

This has been discussed before, I believe it was recently touched on here thread1103-383476 & earlier thread1103-342370

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[CheckerHater]

Yes, in general flatness is a refinement of parallelism and parallelism may be refinement of size if envelope requirement applies.

Note that parallelism controls surface in question WRT the surface marked as A, but it doesn't control surface A itself.

Rule 1 tells you even less, ass both surfaces are allowed to vary as they please.

If you want both surfaces to be flat and parallel, it makes sense to use flatness on your A surface.

Also, as you imply Rule 1, I imply you operate to ASME standards, but if it's not the case, the situation in ISO is slightly different.

All together can be summarized in the enclosed picture - note that surface A has the worse condition than surface being controlled.

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[Belanger]

As a side note, I've always found it interesting that if you read Rule #1, it only mentions form. So a purist might say that Rule #1 itself says nothing about parallelism (which is not form, but orientation). Obviously parallelism is a by-product of Rule #1, so I'm not saying anything's wrong -- I certainly agree with the other comments here. I'm just voicing a thought that I've noticed.

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