【转帖】one million datum targets

yang686526

one million datum targets?
i have a plastic part that attaches to another part by using several hoop snaps.  the coworker i am working with is insisting on using a datum target at each hoop snap location to establish datum a since the snaps are functional features.  the resulting drawing ends up having 12 datum a targets.  has anyone heard of doing it that way instead of using three target and profile of a surface control frames on each hoop snap?
well, unless i'm missing something you definitely can't have 12 datum a.  each datum identifier should only be used on one datum (though it can be shown in more than one view).
can the snaps not be tied back to the main datums?
kenat, probably the least qualified checker you'll ever meet...
i agree with kenat.
having 12 datum a's will cause issues with the mfr/shop and inspection. it is not standard practice.
chris
solidworks/pdmworks 08 3.1
autocad 08
you need batter form control to ensure your latch points are in place.
oh no, it's one datum a as the main datum established by 12 datum targets;  a1, a2,... a12.  he says inspection will clamp down each of the 12 datum targets then measure the part.  the part is abs plasic.
ikrayg,
   if your primary datum is a plane, it is defined by three datum targets.  i do not think a snap is very good for this.  a snap would define a centre would it not?  twelve datum targets have no geometric meaning to me.
   you should use some other feature as your primary datum.  probably, you can use snaps as your secondary and tertiary datums.  locate your snaps with positional tolerances.
                       jhg

does this help?
so mis read the op, when using datum targets simplistically you have 3 datum targest to create your primary datum plane, 2 points to create your secondary datum plane & 1 point to create your tertiary.  it's all about fixing all 6 degrees of freedom
take a look at ctophers links.
i tend to agree with drawoh look at using another feature if appropriate, or just pick 3 of the snaps if you must use them.
kenat, probably the least qualified checker you'll ever meet...
as the others have noted, more than three points to define a datum is overkill.
when the people fear their government, there is tyranny; when the government fears the people, there is liberty.fff"> - thomas jefferson

far be it from me to get in over my head on a gd&t analysis, but if the part is plastic and can deform then would it not be reasonable to "overdefine" your datum a by specifying each hoop snap as part of that datum?  maybe the part will be differently warped if only 11/12 points are in contact.
well said jabberwocky!