【转帖】datum definition and datum usage

yang686526

datum definition and datum usage
if an id is defined as datum b with rfs on a drawing, can the datum b be used as a secondary datum for a different feature on the same drawing with mmc call out for datum b?  
find a job or post a job opening
weisusu, i'm not sure what you mean by 'defined as datum b with rfs'.  is datum b placed on an fcf in which rfs is stated?
certainly once you have datum b defined then other fcf which relate to it may or may not use mmc as required.
kenat,
yes rfs is defined in datum b fcf. the drawing define a feature with its fcf relates to datum b, but the modifier for datum b calls out mmc. just curious if that is a correct call out.  
ok, now i'm more confused, can you put a sketch of what you mean.   
kenat,
the datum is what it is. if you call out a feature relative to datum b with datum b at rfs in one place on a drawing and another feature relative to datum b with datum b at mmc somewhere else on the same drawing, that is legal but it is a separate requirement and need not meet it's tolerance simultaneously with the first feature.
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
powerhound is correct.  the rfs locating datum b is for just that, locating datum b.  datum b will still exhibit a feature size, and can be used as a secondary datum at lmc, rfs, or mmc.
the edge... there is no honest way to explain it because the only people who really know where it is are the ones who have gone over.fff"> - hunter s. thompson

weisusu,
   when you identify your feature as a datum, there is no capability in the standard to call it rfs, mmc or lmc.  when you attach a feature control frame calling up your datum, then you can call it rfs, mmc or lmc.  
   your datum should be defined by a square with a letter in it.  there should be no additional boxes.
   like everyone else here, i am trying to visualize what you are looking at.
                        jhg
just for the heck of it, suppose we stated that 'the inside diameter of the part is defined as datum feature b.'  would not this be more technically correct and to a degree clarify the situation?  
agreed the fcf could specify the simulated datum  at lmc rfs or mmc condition.
there are certain methods shown in the standard for defining datum features and this is not one of those methods. the idea of gd&t is to reduce the notes on a print, not add notes. having a box with a datum letter inside of it with an extension and a "sucker" on the end of it attached to the exact datum feature referred to, should be clear to even a rookie that it is a datum feature.
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
weisusu,
a picture would help.
chris
solidworks/pdmworks 08 3.1
autocad 08
i agree that some sort of picture would help.
purely speculation, but i can think of one possibility for what weisusu might be referring to.  i've seen something in a very popular gd&t book called an "implied self-datum".  it's put forth as an alternative for controlling mutual coaxiality.  for example, a pattern of coaxial holes will have a position fcf that references datum a, and also has a datum feature label a attached to the fcf.  something like this:
3x dia 1.000 +/- .005
|pos|dia .010(m)|a(m)|
-a-
i'm not a fan of this technique at all, but i have seen it used.
evan janeshewski
axymetrix quality engineering inc.