yang686526

can a feature be controlled by two different position callou
i have one subassembly. it contains one welded rectangular frame boxe and one floor pan/bracket. for the welded frame, there are three holes on bottom surface of frame to be used to mound to floor pan and there are four holes on top face of frame which will be used as mounting hole for another bracket. the floor pan has five holes. three small holes for welded frame and two large holes for locating pin of floor. for this sub-assembly, i have to set floor surface as datum a and two large holes on floor pan as b and c. i also set up another datum system for four holes on the top surface of frame. i choose top surface as datum d and two holes as e and f. datum e has position tolerance to a,b and c.
can i assign rest holes on top surface of frame two position tolerance callout (one respect to a,b and c and one respect to d,e,f)?
can one hole or feature be controlled by two position tolerance back to different datum system such as a,b,c and d,e,f?
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ye368,
i am not sure what you are trying to do here. are you subcontracting the whole sub-assembly, or are you getting the parts fabricated and assembling it yourself?
if you subcontract the assembly and you have no plans to interchange the parts, you should let the sub-contractor locate the three small holes in the bottom.
on the top, you can use a composite tolerance for your holes, meaning that your tolerance block has two sets of positional tolerances. take a look at figure 5-19 in asme y14.5m-1994.
you need to visualize your datums as fixturing points rather than as things to dimension from. i would be reluctant to accept a completely different set of datums at the top.
how about you specify your top surface as datum_d and do the following composite positional tolerance?
pos 2mm dia a b c
pos 0.5mm dia d
i think this would make sense in the asme standard.
jhg