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

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.

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

Yes you can use 2 planes as a datum and you are correct that they will be referenced as A-B.

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

Could you use stepped or offset datums targets and have two targets on one surface and a third target on the other surface.  So you just have one datum datum as "A"?  Per 4.6.3.1  

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

Yes I'd say you need the ASME Y14.5 std.

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

Using 2 planes as a primary datum - Let's look at how the part mounts on its mating part. In most cases, it mounts on 1 surface and it will become the primary datum and it might be called datum A.

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

(OP)
Thanks Dave, I agree, that was very helpful.

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

I simply can't buy into the A-B datum thing.  Doesn't quite pass the sniff test.  Datum targets to establish a single datum seems most kosher.

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

(OP)
Yes my part mounts on both these planes simultaneously (although they are offset) that why I have to use both as my primary

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

What is the difference between using two Datums as A-B and a stepped datum?  Do you not get the same results?  Please explain in more detail in having A-B versus having A1 A2 A3 datums on these surfaces?  You are doing a stepped datum but just calling one surface B and one surface A instead of having a single Letter designating the datum.  I am confused and please reference in the standard where they explain your method Dave.  I would like to read more about this.

 

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

(OP)
Sdeters, how would we control the offset between the two planes if we use a single datum A? If it is A & B, we can control it using a basic dimension and a profile callout as dave mentions. Is there an alternate way of controlling it if it were a single datum A?

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

Is it allowable to post the pics from the Standard?

Want to check before I do this

thanks

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

Sorry to answer your question in the standard they use a +/- dimension to define the difference between the two planes. plus a basic dimension.   

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

(OP)
Interesting thanks!

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

Maybe I'm mistaken but I'm imagining the OP's case is something like Figure 4-33 in ASME Y14.5M-1994 "STEP DATUM FEATURE", it's explained in 4.6.3.1 of the standard.  This uses datum targets.

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,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
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

Dingy, is this what you mean?

http://files.engineering.com/getfile.aspx?folder=94862671-4912-4fc1-ba26-52bc2fef7685&file=stepped-part.tif

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,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
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

re: "A-B" 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

Datums depend upon what is called "true geometrical counterpart". In other words, how does the part contact the mating part?

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 still have a niggling doubt that the tolerance between the two surfaces poses an issue.

I suppose I must me imagining it though if it's explicitly detailed in the 09 version.

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
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

Kenat:

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

I'll concede.
But I still don't like it.

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

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
 

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

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.
www.qmsi.ca

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

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
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

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

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
 

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

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.
www.qmsi.ca

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