Interpretation of projected position tolerances at MMC

[pylfrm]

Imagine a drawing that defines a part with two holes, each having a length of 8 mm, specified as follows:

hole 1:

diameter 6 +/- 0.1 THRU
[box]position[/box][box]diameter 0.4 (M) (P) 32[/box][box]A[/box][box]B[/box][box]C[/box]​

hole 2:

M6 X 1 - 6H THRU (pitch diameter 5.350 - 5.500)
[box]position[/box][box]diameter 0.4 (M) (P) 32[/box][box]A[/box][box]B[/box][box]C[/box]​

Drawing interpretation is per ASME Y14.5-2009. Units are millimeters. The datum reference frame is fully constrained with respect to the part. The true position of each hole is fully defined with respect to the datum reference frame. The projection direction for the position tolerances is defined.

What is the meaning of each position tolerance?


pylfrm

 

[Belanger]

That brings up a good point... The standard mentions two interpretations for a position tolerance: Axis interpretation (particularly for RFS) and then a boundary interpretation (particularly for MMC).
So we often resort to "boundary" for MMC, but with the projected tolerance zone it's best to stick with the axis idea.

 

[pmarc]

So we often resort to "boundary" for MMC, but with the projected tolerance zone it's best to stick with the axis idea.

But this does not mean that hard gages could not be used.

 

[Belanger]

Right -- MMC is the ticket to using fixed functional gaging.
I was just pointing out that we often speak of boundary with MMC, yet with a projected tolerance we still want to speak of the axis (in terms of the theory).

John-Paul

 

[pmarc]

Well, this is how the committee chose to explain the concept in the standard, but technically they could use surface interpretation too.

Maybe the reason why they did not decide to explain it in terms of surface is that in some/many(?) cases the boundary generated by position @MMC with projected tolerance zone requirement would not really mimic true functional worst-case boundary.

 

[greenimi]

pmarc,
How ISO is using the projected tolerance concept?
Since in ISO the axis is defined as "derived median line" (more or less closely related with it) then how this concept works in ISO? Just curious.

 

[pmarc]

greenimi,

It is true that in ISO the axis of a cylindrical feature is more like the Derived Median Line in ASME (they call it the Extracted Median Line), but when it comes to the projected tolerance zone concept the feature that has to fall within the projected tolerance zone is actually the extended portion of a perfectly straight axis derived from an associated cylinder. That associated cylinder is pretty much the same entity as the Unrelated Actual Mating Envelope in ASME.

 

[pylfrm]

Just for completeness, let's add another feature to the imaginary part described in the original post. I will also repeat the existing feature specifications for reference:

hole 1:

diameter 6 +/- 0.1 THRU
[box]position[/box][box]diameter 0.4 (M) (P) 32[/box][box]A[/box][box]B[/box][box]C[/box]​

hole 2:

M6 X 1 - 6H THRU (pitch diameter 5.350 - 5.500)
[box]position[/box][box]diameter 0.4 (M) (P) 32[/box][box]A[/box][box]B[/box][box]C[/box]​

hole 3:

M6 X 1 - 6H THRU (pitch diameter 5.350 - 5.500)
[box]position[/box][box]diameter 0.4 (M)[/box][box]A[/box][box]B[/box][box]C[/box]​

What would the design look like for a hard gage intended to check each of these position tolerances?

Well, this is how the committee chose to explain the concept in the standard, but technically they could use surface interpretation too.

How do you imagine a surface interpretation for projected tolerances might be defined?


3DDave,

Good point about the degenerate cases. An axis interpretation for MMB or LMB datum feature references is something I haven't really considered before, so I'll have to give that some more thought.


pylfrm

 

[pmarc]

How do you imagine a surface interpretation for projected tolerances might be defined?

Similar to regular position at MMC callouts with the exception that the VC boundary is located outside of the toleranced feature, not inside, meaning that it is not the surface of the feature that can never violate the VC boundary, but the surface of a mating part's feature, if you will. Verification of this requirement creates some practical difficulties as far as use of hard gages is considered, but it is doable. If you have access to ASME Y14.43-2011, take a look at fig. B-20. It shows how to use hard gages to verify projected position tolerance (at MMC basis) applied to threaded holes.

 

[pylfrm]

Similar to regular position at MMC callouts with the exception that the VC boundary is located outside of the toleranced feature, not inside, meaning that it is not the surface of the feature that can never violate the VC boundary, but the surface of a mating part's feature, if you will.

I have a guess at the general concept you're describing here, but I don't really understand in detail.

I probably won't have access to ASME Y14.43-2011 in the near future, but I'll see what I can do to change that. Thank you for the suggestion.


pylfrm