Contoured, irregular surface as a datum

[Jimmerz]

Is it correct to use the entire surface of a contoured part to define a datum? The dwg calls this out as datum A, without the use of datum targets...

I believe that you must define at least 3 datum targets to define a plane. I also believe that a datum must be a plane and cannot be the entire surface, as this would lock all six degrees of freedom.

I also believe that a datum reference frame is made up of 3 mutually perpendicular planes.

Am I way off base here? My co-worker seems to think so.

 

[pmarc]

Per the newest revision of ASME Y14.5 published in 2009, it is absolutely legal to use complex contoured surface as a primary datum and constrain all six degrees of freedom at once. Refer to fig. 4-3(g) in the standard.

 

[powerhound]

Yes a contoured surface can be used as a datum BUT if it can't be reproduced for inspection then it does no good. Datum targets make more sense to me.

It would be beneficial to know what standard your drawing is being interpreted to because as pmarc said, the entire surface can be called a datum per the 2009 standard but not the 1994.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
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[Jimmerz]

I guess I owe my co-worker a coffee...

I will have to look at the dwg first thing Monday to confirm which standard is involved.

I assume since no datum targets are defined, the cad dataset defines the origin of the DRF? Does datum B define the orientation of A since they must be mutually perpendicular? How can Datums A and B be mutually perpendicular if Datum A is not a plane? My head is spinning!

I can't get my head around the idea of distance to datum A since measurements to a lumpy surface yields different results depending on where you measure to...

Contoured surface is datum A
2 holes define B1, B2
All other holes positional to A|B

Thx for the help.

 

[SeasonLee]

We can’t find an example of using contoured surface as primary datum in ASME Y14.5M 1994 standard, but the standard allows mathematically-defined curved surfaces (section 4.5.10.1 page 68) to be used as datum features, that’s why we can see a lot of examples of using contoured surface as primary datum before new standard released in 2009.

Figure 4-28 in 2009 is an excellent example of using contoured surface as primary datum, please watch how they defined the primary datum, it might be a good guide on you case.

SeasonLee

 

[Belanger]

It will also help to be clear on the exact terminology: A "datum" is ALWAYS a plane, axis, or point, and yes, datums mentioned in the same feature control frame make up a "datum reference frame," and those datums within the DRF are exactly perpendicular to one another.

However, the "datum feature" is the term for the actual surface of the part that is used to establish the datum. So when looking at the figures in the standard -- such as Fig. 4-3(g) in the 2009 standard, which is the closest example to what you ask -- notice that the actual surface is complex, but the real datum is actually a complete DRF itself, consisting of a plane, axis, and point all in one shot.

I suspect many cups of coffee have been won/lost because people casually say "datums" when they are really thinking of "datum features."

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

 

[dingy2]

J-P:

Please look at figure 4-28 in 2009 reflecting contoured features as a datum feature.

Dave D.

http://www.qmsi.ca

 

[DeanD3W]

Figure 4-3 in Y14.5-2009 provides great help in clarifying the difference between datum features and datums, but I do wish some different terms were used... For 4-3g the datum really should be identified as "line, point, & plane", rather than "axis, point, and center plane". Some complex features (compound curved surfaces) may have a center plane, but many, such as the one shown in figure 4-28 have a reference plane that cannot be called a "center plane". Using the same reasoning, the datum includes a reference line, but not an "axis".

So, for a compound curved datum feature, or for a "complex" datum feature as shown in figure 4-3g the datum is a point on a line on a plane... In other words, the datum feature simulator can be defined with basic dimensions from a point on a line on a plane (if "BSC" is included as in figure 4-28 then those basic dimensions are the same as those used to define the datum feature, but if (M) or (L) are referenced then the datum feature simulator will be constructed at MMB or LMB respectively... Then we get to talk about how the datum feature can fit with its simulator which is not necessarily a simple discussion).

Jimmerz - Make sure you push all thoughts that those three mutually perpendicular planes are "datum plane A", "datum plane B", and "datum plane C" out of your mind... In my opinion it is much better to think of a datum reference frame as a right hand rectangular coordinate system which only has planes "XY, "YZ", and "ZX". There are many cases, such as any time a cylindrical datum feature is used, for which the "planes of the datum reference frame" include no "datum planes". What do we have if three spherical features are identified as primary, secondary, and tertiary datum features? We have three datum center points and no datum planes, then we constrain a coordinate system to those points. Now with Y14.5-2009 available, we can add X, Y, and Z axes to a drawing which will tend to make more clear that the only planes for that case are XY, YZ, and ZX.

Dean

http://www.d3w-engineering.com

 

[Belanger]

Dave ... I'm not sure what you're getting at. I think I accurately described the difference between a datum and a datum feature. The figure you reference is another good example of a contoured "datum feature." But the "datum" is still always a perfect plane, axis/line, or point.

Also, I don't think this figure helps the OP much, because he said that the example he's dealing with doesn't use datum targets (Fig. 4-28 does).

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

 

[ewh]

SeasonLee,
While it may be seen as more common, per the '94 standard you could not use a contoured surface as a datum unless that surface was related back to the three primary datums.
Good to see that they have found a better way.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[dingy2]

J-P:

I agree with you on the definition of a datum (1.2.13 in 2009 edition)and I now understand that your post was more of the definitions of datums and datum features and their differences.

The initial question was about the use of a contoured surface as a datum but I am going to assume that Jimmerz really meant datum feature. Yes, contoured surfaces can be used as datum features if the 2009 edition is utilized.

Dave D.

http://www.qmsi.ca

 

[ptruitt]

I had a similar question awhile back. Forum members provided some excellent guidance to help me refine the example I posted. (Thanks, again!)

http://www.eng-tips.com/viewthread.cfm?qid=288484&page=1

You might be more inclined to use a similar approach if you and the supply chain uses software (SmartProfile is one of them) that can fit the inspection point cloud to the CAD design. But a nest can be built to simulate the feature, as others are saying. In my opinion, this is a reasonable approach in many situations, but it drives the design to specify something that is not necessarily depicting the function of the part, which is sub-optimal.

Peter Truitt
Minnesota

 

[SeasonLee]

There is no doubt that the datum planes are mutually perpendicular to one another, but the OP questioned : “Is it correct to use the entire surface of a contoured part to define a datum? The dwg calls this out as datum A, without the use of datum targets...”

I am trying to say we can use the contoured surface as a datum feature to define the datum per 94 standard, I would like to quote page 93 and 94 from James D. Meadows book GD&T Applications and Techniques for Use in Design, Manufacturing, and Inspections to explain why we can use mathematically defined surface to define the primary datum, the examples on page 2 / 3 are also snapshot from James D. Meadows workbook.

Jimmerz
Would you mind to post your sketch, it will help us to have a better understanding on your case.

SeasonLee

 

[ewh]

I appreciate what you are saying, SeasonLee... I just always get hung up on ¶4.6.7.
Maybe I'll get my head around it one of these days.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]