Basic Dim on Series of Holes

[franhd]

Hi everyone,

I'm pretty sure I know the answer to this, but just want to double check here.
Take the example image above, and you're asked to measure true position of all three holes, with respect to datums A | B | C.

The first hole from left edge will have position measured with .500 as the target.
The next two holes, I would assume you use 1.500 and 2.500 as the target to calculate true position since datum C is the reference datum.
What I don't want to happen is inspectors stacking tolerances from hole to hole and for ex. calculate true position with 1.000 from the adjacent hole.

I don't see too many resources explaining this. Let me know if I'm correct about this or not.
Thanks!

 

[jassco]

Hi, franhd:

There won't be any tolerance stacks as the holes are positioned to datum A, B, C. And that is the beauty part of BASIC dimensions.

Best regards,

Alex

 

[Mech1595]

Jassco's correct. You could also change the basic dimensions to all originate from [C] on the drawing, if you're not confident that your inspector will interpret the original correctly.

 

[franhd]

Thanks Jassco!

Mech1595: I could also do it that way, the problem is when it gets to 10 holes or even 20 holes in a row.

But I'm glad to know that my beliefs are confirmed.

 

[CTengIS]

Hi franhd,
You can refer to this figure from ASME Y14.5 to point out there is no tolerance stack on a chain of basic dimensions:

 

[Garland23]

It shouldn't be an issue, really. This is a foundational idea in GD&T and there are a lot of resources to back up the idea that there's no accumulation of tolerances across those three.

 

[CheckerHater]

Is anyone concerned by the fact that there is no target to meet and no tolerances to stack without FCF anyway?

 

[e1baz_]

If you want to make your drawing needlessly ambiguous and confusing (in practice hardly any inspector cares about GD&T) then go ahead and do it like you've shown. I tend to prefer simple and obvious.

 

[Garland23]

CheckerHater, there's no concern about that because the OP stated that we are "asked to measure true position of all three holes, with respect to datums A | B | C."
No number given, but we get the intent.