Datum reference frame for position on thin parts...

[Tenkan]

Lets say we have a simple part that is a thin round disk with one or more hole patterns. Lets call the bottom surface Datum A and the major O.D. Datum B.
Using position to locate the holes, is it necessary to use Datum A in the FCF?


lightweight, cheap, strong... pick 2

 

[powerhound]

Yes

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

 

[CheckerHater]

Yes

 

[Tenkan]

Is the reason to control perpendicularity to the flat surface and to show what surface (datum) the perpendicularity (and position) is defined from?

lightweight, cheap, strong... pick 2

 

[Belanger]

The perpendicularity aspect alone might seem negligible to you, but the real reason is that datum A helps establish the "datum reference frame."
If you omitted datum A, then B would be primary. Think of how that might change the way a fixture or gage grips onto the part.

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

 

[CheckerHater]

If you like formal approach, you need datum A to constrain degrees of freedom.

If you like more practical point of view, your “thin flat part” is most likely some sort of lid or gasket.
Functionally it is leaning against flat mating surface and your datum framework simulates that condition: lay flat first, make sure bolts are fitting thru holes second.

All of the aspects of this were already discussed here so many times, I am out of convincing examples.

 

[Tenkan]

thanks for the replies, the resounding yes is confident enough and the brief explanations made sense (although I've always struggled to understand datum precedence but that's another subject).

lightweight, cheap, strong... pick 2

 

[pmarc]

Koda94,
For datum precedence concept, you may want to take a look to this thread for example:

http://www.eng-tips.com/viewthread.cfm?qid=354287

Just keep in mind that the discussion was about ISO, so don't let the concentricity symbol (shown in my picture posted 29 Oct 13 17:45) scare you.

 

[Tarator]

Datum A is especially important for a thin part!! (for repeatability, consistency, etc.)
And how does the part assemble with the mating parts? Does it not sit flat on Datum A (full area contact)?