Datum set-up and referencing

[SpaciouS]

Say you start with a square plate with 2 holes. The surface of the plate is identified as datum A, 1 hole is datum B and the other hole is datum C. Then lets say datum features B & C are not parallel (horizontal or vertical) to the edge of the plate.

Q: Since datums B & C are both axis, would they be an adequate secondary and tertiary datum reference for position and clocking hole patterns?

Q: If so, the clocking orientation is relative to a theoretical line from one datum axis to another, in this case not parallel to the plates edge?

Q: If so, and you want a hole pattern clocked parallel to the plate edge, a basic angular dimension from the theoretical line between B & C datum axis and the hole pattern is required?




Thanks,
Sean

 

[pmarc]

My short answer to all 3 questions would be YES, however for question 3 if a pattern needs to be oriented (clocked) parallel to one of the side faces, I do not see a reason why that face assigned as datum feature D should not be referenced as a tertiary datum in the positional callout for the pattern.

 

[SpaciouS]

Agreed. I'm dealing with a different geometry, but just want to simplify my question.

Thanks,
Sean

 

[PaulJackson]

Spacious,

Since B is not perpendicular to A it will likely constrain the one remaining rotation as well as the two remaining translations rendering C without any degrees-of-freedom to constrain.

If you want B to only constrain the two translations then you will have to say so. You could do it in a note if dimensioning to ASME pre-2009 or with a modified DRF according to the 2009 Standard.

Paul

 

[PaulJackson]

OPs you didn't say that B was not perpendicular to A. I had an errant picture that I was responding to.

Paul

 

[SpaciouS]

Paul,

Datums B & C are perpendicular to datum A. What I mean is, datums B & C, relative to each other, are not parallel to the vertical and horizontal edges of the plate.

Thanks,
Sean

 

[fsincox]

The hole pattern orientation to the edge is not important if it is not important to you for the design. It may be important to manufacturing to get started but that is their concern.
Frank