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.

 

[dingy2]

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.

http://www.qmsi.ca

 

[SDETERS]

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

 

[KENAT]

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,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[dingy2]

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.

http://www.qmsi.ca

 

[sheevaraam]

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!

 

[TheTick]

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.

 

[sheevaraam]

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

 

[SDETERS]

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.

 

[sheevaraam]

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?

 

[SDETERS]

Is it allowable to post the pics from the Standard?

Want to check before I do this

thanks

 

[SDETERS]

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

 

[sheevaraam]

Interesting thanks!

 

[KENAT]

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 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[KENAT]

Dingy, is this what you mean?

http://files.engineering.com/getfil...-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 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[TheTick]

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.

 

[dingy2]

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.

http://www.qmsi.ca

 

[KENAT]

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 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[dingy2]

Kenat:

Your example shown is exactly what I mean.

Dave D.

http://www.qmsi.ca

 

[TheTick]

I'll concede.
_{But I still don't like it.}