【转帖】indicating concentricity on a over-molded par

yang686526

indicating concentricity on a over-molded part
imagine a 4" dia. x .100" thickness part "part1".  now this part has a ring around it over-molded "part 2".  "part 1" has a 1" hole right on the center.
i want to indicate on the print that the id of "part 1" must be within .010" tol. from the i.d. of "part 2"
i added a datum callout on the i.d. of "part 2"
am i on the right path?  i am redoing this print and the way it is called out now is with a long paragraph and leaders pointing at the edges of the parts that are supposed to be concentric.
i think you have it backwards.  since part1 is the parent part that gets overmolded with part2.  thus the id of part1 should be the datum and the id of part2 is controlled to that datum.
but i could see it being done your way but it depends how the tool is set up.  overmolds can be tricky.  i am doing an overmold right now it's a 17-4 stainless steel exoskeleton with abs overmold.
heckler
sr. mechanical engineer
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(in reference to david beckham) "he can't kick with his left foot, he can't tackle, he can't head the ball and he doesn't score many goals. apart from that, he's all right."  -- george best
i'm inclined to agree with heckler, i'd use the hole in a as my datum and go from there if i'm understanding correctly.  assuming that hole exists before the mold step.
also what type of tolerance are you planning to apply, or is this your question?  concentricity is rarely the correct tolerance to use, even if simplistically that is what you are trying to achieve.  positional, runout or total runout are usually more suitable.  if you explain the function of your part more one of the more experienced guys here may be able to give some good advice.
yeah i think the datum should be on the 1st part, you guys are correct.
tolerance i need of concentricity is .010"
it is a subwoofer (speaker) spider so it has to be pretty well centered.
how familiar are you with gd&t?  concentricity rarely ends up being the right option.
one of the more experienced guys may be able to explain better but concentricity is essentially applied to the 'theoretical' centerline and can be difficult to verify.  
simplistically i'm wondering if positional may be better but i'm not an expert and don't fully understand your application.
depending on your software and how it reads theese type of relationships , in this point contricity, just depends how it is shown thru your dwg.