Datum holes and frame of reference

[hygear]

I have a question about using holes as datums and how a frame of reference is defined (see attachment).

It is my understanding that holes can be used as datums even if they aren't aligned along the same line as shown in the orientation labeled "1" in my drawing (for clarification, Datum A would be the surface of the plate, Datum B would be defined with perpendicularity in A and datum C would be defined with position in A and B). A few of my colleagues believe it is not correct to do this and that the orientation shown in "2" is the only correct interpretation. Which one is correct?

 

[SeasonLee]

No attachment.

 

[hygear]

Sorry...

 

[Tarator]

There is no attachment.. and also what version of Y14.5 are you using?

 

[hygear]

The attachment is above. We are using Y14.5M-2009.

 

[Tarator]

By default, Datum B is the Origin, and X-axis goes through Datum C. However, in 2009 version you can override it and define your own coordinate system since everything is in BASIC dimensions and as long as you use cartesian system.

 

[Tarator]

I forgot to mention: The origin is the Secondary Datum (I assumed it is Datum B)...

 

[pmarc]

The text in following tip may also help you:

http://www.tec-ease.com/gdt-tips-view.php?q=243

Just try to ignore the fact that the drawing does not fully define true position of hole C relative to hole B

 

[hygear]

Ok, this is sort of a follow question that I can't seem to find any good references on...

I have an assembly composed of several welded plates and I would like to use the plate I used in my example (see previous attachment) as the base. At the child part level, I can attach the tolerance to the hole dimensions and use these to define a datum. At the assembly level, there is some disagreement in my office on the correct way to setup the datums. Here are the three possible arrangements:

1. At the assembly level, the surface of the plate defines datum A and the datums B and C are defined by the holes. Since this is on the assembly level, reference dimensions are used on the holes and the same tolerances used in the original child part are attached to the reference dimension. The datums are then oriented relative to each other with basic dimensions.
2. At the assembly level, the surface of the plate defines datum A and the datums B and C are defined by the holes. All of the information that was on the child part for defining the holes/tolerances/datums is copied to the assembly level.
3. At the assembly level, the surface of the plate defines datum A and the datums B and C are defined by the holes. Datum tags are added to the holes and basic dimensions are added to orient the holes to each other without tolerances.

Personally I think #1 is the correct method, but it seems like adding redundant information could cause issues down the road. What is the best method for setting up the datums on the assembly level?

 

[powerhound]

The correct interpretation is #2 but I think you're missing a huge piece of the puzzle. If all you are asking about is the centerline spacing of the holes then it really doesn't matter. It seems that you are somehow associating the datum structure with the outside of the part. Are you setting this up to check the outside contour? Also, what condition are the datums specified at? Are B and C called out at MMC (or MMB if 2009)? This is critical information we need to know before advising on how to set up the datum reference frame.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
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