DATUM from the hole pattern 1

[Vimalmechs]

Hi all,

PFA.
The figure is an assembly came for review. Which is having 3 pads welded on a sheet metal base.Here i am not able to understand that how DATUM J is deriving from the 5X holes. I have seen that datum deriving from 2, 3 and 4 holes. but i'm ended up nowhere to understand the datum from 5 holes. Anyone please shed some light .

Regards
Vimal VV

 

[fsincox]

Datums from hole patterns are very "functional" though not popular with all manufacturing and inspection people if they do not use functional gauging. I love the way it has freed me from the 'hold it to the edges" kind of thinking and am in BIG favor of it as expressing the functional requirement (assuming it does). Manufacturing can still use the edges, if they want, it does not mean they can not!
Frank

 

[Belanger]

Agreed, Frank!
Vimal, the key is that when datum J is referenced in the feature control frame, it is modified with the "M." This side-steps the issue of finding the true center of each hole.

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

 

[fsincox]

JP,
Good Point!!!, Thank you.
Frank

 

[pmarc]

J-P,
Could you please elaborate on: "This side-steps the issue of finding the true center of each hole"?

 

[Belanger]

Well, if the MMB modifier weren't there, we'd have to have 5 expanding pins at the perfect locations. Tricky, yes?

But with MMB, we can use a functional gage, which need not find the center of each hole within the pattern.

This is a perennial discussion, so I sense that you are getting at something else...

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

 

[pmarc]

Well, if the MMB modifier weren't there, we'd have to have 5 expanding pins at the perfect locations. Tricky, yes?

Of course agree that the pins would have to be expandable and that this procedure isn't clearly defined by the standard, but wouldn't the datum be established from datum feature simulators centers, regardless of their size?

 

[Belanger]

Yes, the centers of the simulators would create the datum. But if any one of the holes is slightly out of position itself, then as you stated it is not very clear on how to "equalize" them.

I agree that it's perfectly legal to have the pattern referenced RMB, but it does have some drawbacks. That's why I said that MMB "side-steps" the issue -- functional gaging is the easy answer.

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

 

[Vimalmechs]

Hi JP,

Will you please explain little bit more about "Yes, the centers of the simulators would create the datum. But if any one of the holes is slightly out of position itself, then as you stated it is not very clear on how to "equalize" them."

I am not able to relate the reply from you with the below,
1) Can we derive secondary and tertiary datum from the combination of any 2 holes in the "5X Holes pattern".When we are inspecting by functional gage, "combination of any two holes only workout" .Am i right ?

Or we should derive the datums only by considering the entire hole pattern ?.

Regards
Vimal VV

 

[Vimalmechs]

Hi JP,

Your explaining that the functional gauging pins will be having dimension equal to "MMC size of the hole". By implementing the "MMC" modifier in the FCF call out, we can check the assembly whether "accepted" or "rejected" by functional gauging.


The hole pattern with "J datum" itself acts like secondary and tertiary datum. Right?.
Please clarify me.

Regards
Vimal VV

 

[fsincox]

The pins will actually be at the virtual condition of the hole this is not necessarily its MMC.
Frank

 

[Belanger]

Right -- I don't think I ever said MMC, but rather "MMB," which is essentially the virtual condition.

It's true that the pattern datum J serves as a secondary and tertiary combined. Thus, all degrees of freedom are now controlled by J along with the primary datum.

I would be careful when saying that the datum can be formed from any 2 holes in the pattern, because we don't know which 2 holes will hit the gage first. Think about holes that are made at the largest size; there might be a bit of slop or shift around them as they sit on the gage.

Two holes are the minimum necessary to stop the rotation, but depending on the size and location of those 5 holes, it could be a different situation each time.

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

 

[Vimalmechs]

Understood.

Thank you JP, PMARC & FRANK.