Customized DRF 1

[powerhound]

On figure 4-45 of the 2009 standard: if B were made the primary datum, would that eliminate the need for the customized DRF? I know the inspection result will be different than having A as primary so there's no need to go there.

Thanks,

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[pmarc]

I don't think the need for the customized DRF would be eliminated.
I am picturing an application where the primary datum surface B sits on a flat counterface (2 rotational DOFs constrained), yet this counterface is not fixed in translation in assembly (something like a spring loaded platform). So the customized DRF becomes |B[u,v]|A[x,y,z]|.

Would that make sense?

 

[powerhound]

Well, the intent would be for B to constrain 3 degrees of freedom ([z,u,v]) and A to constrain 2 ([x,y]) but since both can constrain translation in Z then would the fact that B is primary mean that it is the one that constrains translation along Z or would it make no sense either way? We're discussing this here and have some differences of opinion so I'm posting it here to get some feedback.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[pmarc]

Okay, so I did not get your answer correctly.

The standard, in the very first sentence of paragraph 4.22 states that the intent of the customized DRF concept is: "To override the degrees of freedom constrained by datum features referenced in an order of precedence." To me this means that there is no point to apply the concept if it doesn't change anything in comparison to the traditional method of DRF definition. And the traditional method is based on the idea that translation in Z will be constrained by that feature which is of higher order of predence in a FCF - in this case by B.

As another example, consider simple round plate where bottom flat surface is primary datum feature and a hole in the center (nominally perpendicular to A) is secondary datum feature referenced RMB. While the secondary datum feature could also (at least in theory) constrain the very same 2 rotational DOFs as the primary datum feature, it is never considered that it actually does so by default, because the 2 rotational DOFs have been constrained by the primary surface in the first place.

 

[powerhound]

Aha, excellent example.

Thanks pmarc.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[CheckerHater]

Didn't know if it is worth starting another thread.

I have a question for pmarc:

As customized DRF allows us to explicily specify both coordinate axes/planes and degrees of freedom we want to be constrained; does customized DRF eliminate the need in translation modifier?

Is there a restrained condition that can be desribed using TM but cannot be described using CDRF?

Just your opinion?

 

[axym]

Hi All,

I agree with pmarc's explanation. The intent of customized DRF's is to override the default constraint rule, in which datum features constrain all of the open DOF's that they are capable of constraining.

Regarding CH's question on customized DRF's versus the translation modifier, that's an interesting thought. I would hope that the translation modifier provides some kind of unique control that a customized DRF cannot, but I never actually thought it through.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

CH,
I don't think the need in translation modifier is eliminated by customized DRF concept.

Here is the example where the CRDF won't do the job, but the translation modifier will:
A typical flat plate - bottom surface is primary datum feature A, a pattern of two holes is secondary datum feature B, and other features are controlled wrt A and B. Primary datum plane stops 3 degrees of freedom (as usual). Then I want the two holes to stop the two remaining translations and the one remaining rotation. But the one rotational degree of freedom that those two holes together can stop I want stopped by the two holes no matter how far apart the actual as-produced holes are. Why? Because the mating part has two pins that are dove-tailed into a plate such that they stay perpendicular to the plate and parallel to each other, but they can slide back and forth freely.

In this application DOFs are not customized (so the CRDF concept is useless), but the default basic locational relationship between secondary datum feature simulator pins will be negated by specifying translation modifier after datum feature B reference in feature control frames applied to other features of the part.

 

[CheckerHater]

Ah, the magical sliding pins... No more questions.