DOF constrained by datum feature reference in FCF 2

[AM Engineer]

Hello all.

I have a question regarding degree of freedom constrained.

Does the datum feature referencing in Feature Control Frame constrains Degrees of Freedom of a part or tolerance zone with respect to datum reference frame?

Any help shall be highly appreciated.

Best Regards

Waqas

 

[Sem_D220]

I agree with the above explanations but to add just a little bit comprehensibility that may be helpful, I would say that the datum reference frame is established from the datums, which are derived from datum features by utilizing datum feature simulators. For example, for a primary or secondary cylindrical datum feature, the datum feature simulator is used to derive a datum axis, on which 2 perpendicular planes of the datum reference frame intersect. The tolerance zones are located or just oriented relative to the planes of the DRF.

 

[AM Engineer]

Thankyou all for your valuable inputs.

Best Regards

 

[axym]

Waqas,

I think you're right that DOF constraint could be described in terms of the datum feature simulators. This is where the constraint actually happens - the part's datum features are required to contact the datum feature simulators in some way. Y14.5 adds the concepts of datums and a datum reference frame (coordinate system), in order to aid understanding. The idea is that the part is eventually constrained relative to a coordinate system that becomes the "origin of measurement". There are several steps involved in establishing the constraint, and trying to understand the process by reading the text descriptions is very difficult for most people.

I would suggest having a look at figures 4-15 and 4-18 in Y14.5-2009. The "means this" figures show the actual part geometry along with the datum feature simulators and datums, which I find makes it much easier to understand how the constraint actually works.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

Evan,

On page 53/ Y14.5-2009 there is a note (should I say disclaimer) --below fig 4-8--- reading:


NOTE: The sequence of establishing a datum reference frame from datum features and datum feature simulators is described in the preceding paragraph. In subsequent text, for brevity, this process will be described as establishing a datum reference frame from datum features.

So, everything before paragraph 4.5 is described in one way and then everything after 4.5 is described in a “summarized” way.
Is my understanding correct?


Also, here is what you wrote few weeks ago in another discussion about this subject (or loosely related)

“This example (my note fig 4-48) also shows, however, how confusing and arbitrary Y14.5's concept of datum planes can be. It was built from simple datum feature combinations, where a three-plane XYZ coordinate system can be placed in a reasonably obvious way. In more complicated examples such as in Figure 4-48, part of that arbitrary nature is acknowledged (for stepped surfaces, the datum plane should contain at least one of the datum targets). In even more complicated examples such as Fig. 4-26 with the hole pattern datum feature, the origin of the DRF can be put at the center of the pattern or at any other location defined by basic dimensions. In other words, the placement of the "datum planes" and DRF origin are completely arbitrary. The only thing that really matters is the mutual relationship between the datum feature simulators.”

 

[greenimi]

Here is another paragraph I found extremely valuable (at least for my own understanding)

"As far as what type of basic dimensions can be used, I would still maintain that the essential thing is that the relationship between the toleranced feature(s) and the datum feature(s) is fully defined. Many different arrangements of basic dimensions could accomplish this. But I would also agree that certain arrangements would be more direct and clear to the typical reader, and other arrangments would be less direct, unclear, and convoluted.

I agree with observations on the Y14.5 figures. They show that basic dimensions can be applied to any feature (even features that are not the considered feature or one of the datum features for that FCF) or even to things that are not features (imaginary intersection points out in space as in Fig. 4-7). This does not cause any issue because the basic dimensions are not applied directly to the actual part geometry.

I agree that the intersection point of an axis and a surface is not a feature. But it doesn't matter in this case - the basic dimensions don't need to apply to features (see above). So I would maintain that the Before dimensioning scheme, and the After scheme is not necessarily better. Another advantage of basic dimensions is that if you don't like the ones that were annotated on the drawing, then you can calculate others and use those instead. As long as the relationship between the considered feature(s) and the datum feature(s) is fully defined (which I don't think it is in either the Before or After scheme), then you can use a different scheme and it doesn't change anything (except possibly rounding-related inaccuracy.... "

Thank you again Evan for your posting.

It is from this discussion:

https://www.eng-tips.com/viewthread.cfm?qid=438628