Tapered wedge as a datum feature

[gabimo]

If a tapered wedge IS defined with ± angle and direct toleranced dimension on one end, can its middle plane be used as a datum feature?
How to generate the centerplane / middle plane as a datum from the non-parallel but planar adjacent surfaces?

Details: A part has its left and right sides tapered and has its primary datum the feature that connect these two sides.
Secondary datum is intended to be the middle / center plane of the tapered sides.
There are three holes on the part (normal/perpendicular to the primary) that should be centered (within some tolerance) to the secondary (middle plane).
Is the middle plane created by this tapered side walls a valid datum?

 

[chez311]

pylfrm,

You bring up an interesting point, which in my mind also creates more questions than answers - but that may be because I do not have as solid a grip on the concepts as you do. I understand and agree now how your example shows that #1 and #2/#3 are indeed not equivalent but I'm not sure I grasp exactly how - as in exactly how the datum feature simulators would behave in this case. As I said before the different combinations of datum feature geometry, precedence, and interactions with higher precedence datum features has a large impact in how exactly each is considered.

For example in reference to 4-29 (a) I think my statement about the portion of unusable zone being determined only after measurement seems to hold true, which would mean that in this case the result might be the same with or without the self-referencing secondary datum feature*. Wouldn't this mean that because the primary datum feature constrains translation the zone and true profile for the secondary datum feature is fixed at basic location (28) and size (R15) - therefore the feature can come in anywhere within that fixed zone even if its self-referencing or not, right?

Now considering my tapered examples #2/#3 as I said I agree now that it is not equivalent to #1, however how do we know exactly how the simulators would behave in relation to the true profile? I'm having a hard time reconciling this in my mind - I tried to draw out a few figures but I see several different possibilities and I'm not sure which one is valid, could you show me how you're thinking it looks like? The way I'm interpreting it seems to suggest that for one feature the tolerance zone is halved (in this case B) and for the other the entire zone is usable (for C) but is that the only solution, could it not be the other way around - and how do we know which one is correct? It seems ambiguous to me, but clarification on the behavior of the simulators might bring that into focus.. I also don't see where your concept of the available tolerance zone being any ratio between 1 and 0 fitting in here.

*Edit: by this I mean with a self-referencing secondary datum feature vs. with a standard non self-referencing datum feature

 

[pylfrm]

For example in reference to 4-29 (a) I think my statement about the portion of unusable zone being determined only after measurement seems to hold true, which would mean that in this case the result might be the same with or without the self-referencing secondary datum feature*. Wouldn't this mean that because the primary datum feature constrains translation the zone and true profile for the secondary datum feature is fixed at basic location (28) and size (R15) - therefore the feature can come in anywhere within that fixed zone even if its self-referencing or not, right?

The primary datum feature in ASME Y14.5-2009 Fig. 4-29 leaves a rotational degree of freedom unconstrained, so the concept of my previous counterexample can be applied here as well.

Now considering my tapered examples #2/#3 as I said I agree now that it is not equivalent to #1, however how do we know exactly how the simulators would behave in relation to the true profile? I'm having a hard time reconciling this in my mind - I tried to draw out a few figures but I see several different possibilities and I'm not sure which one is valid, could you show me how you're thinking it looks like?

I think of this in terms of a single secondary datum feature which happens to be composed of two surfaces. The datum feature simulator would have a uniform offset from the true profile, and that offset would be as far as possible in the direction toward the material of the part.

I also don't see where your concept of the available tolerance zone being any ratio between 1 and 0 fitting in here.

If the datum feature simulator ends up at the MMB of the profile tolerance, then the entire tolerance zone is usable. If the simulator ends up at the LMB, then none of the zone is usable. Any intermediate result is possible as well.


pylfrm