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





RE: Compound datum question, can I use two planes (offset)as primary datum
One could also qualify datum B from datum A using profile of a surface and this would be better than not placing a geometrical requirement. Please make sure that they is a basic dimension shown between the planes, probably from datum A.
Hope this helps.
Dave D.
www.qmsi.ca
RE: Compound datum question, can I use two planes (offset)as primary datum
RE: Compound datum question, can I use two planes (offset)as primary datum
What exactly is your resultant 'primary' plane meant to be?
If the mid point of the 2 surfaces then you may be better making that distance between them the datum feature and then your effectively datum will be the mid-point. Like you would if using the width of a part as the datum feature. (Not sure I explained that well but hopefully you understand)
Or else the datum targets SDETERS mentions may be an approch.
I'm having trouble understanding how you'd use 2 off-set faces as an "A-B" primary datum.
KENAT,
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RE: Compound datum question, can I use two planes (offset)as primary datum
If the part mounts on 2 planes simultaneously, then we MUST have both planes as the primary datum. That replicates the mounting condition. Naming one plane as datum A and the other as datum B just makes sense and I mentioned early that I would suggest qualifying datum B from datum A using profile of a surface.
I have seen drawings where the Designer placed and A1 and A2 on the datum planes but that is wrong. It might be fine for datum targets but we have 2 different features here that must be noted differently.
Datum targets on both planes - One could have datum targets A1 & A2 and datum A and then a B1 on the other plane. We could also reverse it if desired. We still need a 3 point setup and we must have the distance between the planes shown as a basic dimension.
If the part has 2 planes and only 1 is used during assembly for mounting, then we only have datum A. It all depends upon part assembly.
Dave D.
www.qmsi.ca
RE: Compound datum question, can I use two planes (offset)as primary datum
Sdeters and Kenat, I think I am going to go with Dave on this one. Thanks for all replies!
RE: Compound datum question, can I use two planes (offset)as primary datum
RE: Compound datum question, can I use two planes (offset)as primary datum
RE: Compound datum question, can I use two planes (offset)as primary datum
RE: Compound datum question, can I use two planes (offset)as primary datum
RE: Compound datum question, can I use two planes (offset)as primary datum
Want to check before I do this
thanks
RE: Compound datum question, can I use two planes (offset)as primary datum
RE: Compound datum question, can I use two planes (offset)as primary datum
RE: Compound datum question, can I use two planes (offset)as primary datum
Dingy, I can see how in reality you'd use the 2 stepped surfaces as datum features, I can even imagine inspecting it by having a gage block the height of the off set. I'm just not sure that the standard supports that symbology/definition.
KENAT,
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RE: Compound datum question, can I use two planes (offset)as primary datum
h
I'm not sure quite how well it would work practically because of the tolerance on the distance between A & B. Datum targets allows for this.
KENAT,
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What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Compound datum question, can I use two planes (offset)as primary datum
Work backward, and find one example of a callout that uses such a double datum for a stepped datum feature.
This is one of those things that seems right because it is not hard to interpret, but is just not part of the GD&T language.
RE: Compound datum question, can I use two planes (offset)as primary datum
If it contacts on 1 surface, it would be called the primary datum. If that surface is too small for a good set up (3 point set up), we could use another surface for a 3rd datum target such as shown in the 94 standard on page 75 fig. 4-33. The 3rd datum target is on another surface but the datum itself is still 1 plane.
If the part contacts on 2 surfaces or planes simultaneously as shown in fig. 4-20 on page 67, we call 1 surface datum A and the other datum B. The example here does reflect that they are on the same plane but are different surfaces. Here is where I have performed a bit of extrapolation. If the surface are different planes, how would this be shown? - by using a basic dimension to show the theoretical distance between the planes.
The 2009 standard reflects this condition very clearly on page 67 figure 4-22 where we have a datum A surface and a datum B surface with a 20 mm basic dimension.
Hope this helps.
Dave D.
www.qmsi.ca
RE: Compound datum question, can I use two planes (offset)as primary datum
I suppose I must me imagining it though if it's explicitly detailed in the 09 version.
KENAT,
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What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Compound datum question, can I use two planes (offset)as primary datum
Your example shown is exactly what I mean.
Dave D.
www.qmsi.ca
RE: Compound datum question, can I use two planes (offset)as primary datum
But I still don't like it.
RE: Compound datum question, can I use two planes (offset)as primary datum
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
RE: Compound datum question, can I use two planes (offset)as primary datum
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.
www.qmsi.ca
RE: Compound datum question, can I use two planes (offset)as primary datum
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.
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RE: Compound datum question, can I use two planes (offset)as primary datum
I have no objections on Kenat's drawing makes more sense, but saying this will simplify the drawing.
SeasonLee
RE: Compound datum question, can I use two planes (offset)as primary datum
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.
www.qmsi.ca