unusual application of datum target areas
 

unusual application of datum target areas
not sure i'll be able to explain this in words but here goes.
i have a part i'm looking at that is a component of a moving stage and has lots of surfaces holes etc for bearing, switches, motors & the like. datums a, b & c have been set up based on how it mates to other parts in the assy. datum d is a precision bore housing some bearings & a motor from which various mounting holes are related.
it has a couple of patterns of holes for limit switches, these parts are small without much tolerance allowance and need placing precisely. the holes are related to b for primary datum, & a for secondary based on function & orientation etc.
for their tertiary datums they relate functionally to only a small area of larger surfaces either side of the part which themselves don't need to be tightly controlled as they are mostly in free air. however, a small area of each surface will hit a small stop if the assembly over travels. so essentially for the tertiary datum only a small area of the face is relevant.
the drawing uses datum target areas to simulate this and uses them for the tertiary datum for the hole paterns.
is this legal per asme y14.5m-1994, i've looked in section 4.6 and don't see anything saying it isn't legal but it's not something i've really seen before?

kenat,
have you reminded yourself of
imo, definite yes, assuming they target areas are properly indicated.

thanks ringster, seemed like a very good application for datum target areas, just wasn't like anything i'd seen before. i believe they are properly indicated but that will be part of my detail drawing check.
kenat,
being familiar with the parts you're talking about, i would also say this is a good application; especially since 14.5 para 4.6 says that "datum targets and datum features may be combined to establish a datum reference frame."
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