Compound datum question, can I use two planes (offset)as primary datum 3

[sheevaraam]

I have this part which has two planes offset from each other.

1. can I call one plane A, and another Plane B, and call Primary as A-B
2. If i do the above How do I tolerance those two planes relative to each other? Should i call out a profile tolerance relative to A-B?
3. Where would the basic dimensions originate. will it originate from either of the plane or from center of two planes?

Thanks for all answers in advance. I took a GD&T course a year or so ago and am getting good at it. But I dont have the ASME Y14.5 Book, I guess I need to get it cos there are lot of tricky things that get me stuck. Thank you all! Appreciate your help.

 

[SeasonLee]

Using offset planar surfaces and create a common datum plane is an usual practice in application, when offset surfaces are specified as datum features, basic dimension should be used between the surfaces.

Normally, profile tolerance will be used to specify coplanarity when a surface is entirely interrupted, and this principle may also be extended to offset surfaces on this case, you may find this typical application example on page 14-8 from “Advanced Concepts of GD&T” by Alex Krulikowski, please note that the profile tolerance applied on both surface and this will simplify the drawing.

SeasonLee

 

[dingy2]

SeasonLee:

Having noncoplanar surfaces shown with a profile of a surface stating "2 places" does not exactly following the 94 standard or the 2009. The example on page 172 of the 94 standard fig. 6-20 reflects that they are now assumed as 1 plane. We cannot assume 1 plane when the surfaces are separated by a basic dimension.

Kenat's drawing makes more sense having a flatness requirement on datum A and a profile of a surface requirement on datum B that is shown with a basic dimension. The primary datum is now A-B.

Dave D.

http://www.qmsi.ca

 

[powerhound]

Dave,

I think the intent of the "2 SURFACES" concept is to communicate that the 2 surfaces should be used to establish a datum plane based on how the 2 surfaces are specified. I don't believe they necessarily have to be planar. I personally don't have a problem with SeasonLee's example from Krulikowski's book. I think it is a valid extension of principle.

Powerhound, GDTP T-0419
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[SeasonLee]

The primary datum A-B is a created common datum plane, when offset surfaces are specified as datum features, a basic dimension should be shown between the surfaces.

I have no objections on Kenat’s drawing makes more sense, but saying this will simplify the drawing.

SeasonLee

 

[dingy2]

Powerhound:

Just look at the example that I quoted in the 94 standard. I am not saying that the Krulikowski's book is incorrect but it does mean, according to the standard, that we must set up on both surfaces and assume one plane with a basic dimension between them. Sure, it can be checked but Kenat's pic is much more useful and is used quite a bit.

Dave D.

http://www.qmsi.ca