Datums - common vs system

[pernilla]

Hi.

I've been reading in ISO 5459:2011 about datums and datum systems. It's very informative but somehow leaves me with more questions than I had before reading it...

I'll just post one of those for now.

So the standard explains establishing a datum system from a cylinder and a perpendicular plane, where the cylinder is the primary datum (A), perpendicular to the plane (secondary datum B).

Next,

the standard explains establishing a common datum from a plane and a perpendicular cylinder, where the plane is the primary datum (A), perpendicular to the cylinder (B). Only in this example it's a common datum (A-B).

Why? Why does one use common datums in some cases? What would the difference be if the second example was also a datum system?

As far as I can see the end result is a straight line and a point in both examples. So what is the difference?

Anyone?

[NinjaBadger]

combined_pane_cyl_datum.JPG

Are you referring to this figure? Or does this answer your question? (Actually I can't quite remember where I found this but I think it's ISO)

It depends on the design intent and how the part will assemble & function.

Same physical part, but depending how you fit the DRF to it, will dictate where any tolerance zones (which relate to that datum system) will be located.

[NinjaBadger]

Yes. So in A|B Dat A takes precedence.

In B|A Dat B takes precedence.

In A-B the 'error' is equalised (see where figure '4' is shown in the third example).

[pernilla]

Right. Figure 4 represents the same difference in both places...
Thank you! You've just made my reading of ISO 5459:2011 much more fun!

[NinjaBadger]

When you start reading standards it's a bit like 'what the actual $%[email protected]???'

But once you get into them and they start to make sense it's great!

(I am such a nerd!)


GD&T is awesome though (when you understand it).

[pernilla]

Here are the examples...

system.png







common.png