【转帖】confusingodd looking datums

yang686526

confusing/odd looking datums
take a look at the attached sketch, it's a much simplified & incomplete (so invoke 1.1.4 of asme y14.5m-1994 etc.) sketch of an actual case our contract checker came across on one of our parts today.
as far as we can tell looking at the standard b & d are valid, d is basically just used to locate the top radius centrally on the width of the item.
however, it just don't quite look right.
anyone care to opine?
  
kenat, probably the least qualified checker you'll ever meet...
where is a?  it's referenced in the fcfs but not identified that i can see.
i think there needs to be something to define and control the arc lengths of the r.75 bits.
"... it's a much simplified & incomplete (so invoke 1.1.4 of asme y14.5m-1994 etc.) sketch ..."
just assume a is on a surface in the orientation of the sketch, i.e. perpendicular to b & c.
kenat, probably the least qualified checker you'll ever meet...
there is no location for the dim "r 2.000". there is position (fcf) tol, but no dim to locate it.
chris
solidworks/pdmworks 08 3.1
autocad 06/08
sorry i didn't feel that was necessary for the point of my post.
maybe i was unclear.
the issue in question was that having both b, as an edge, and d, as the width looks odd.
kenat, probably the least qualified checker you'll ever meet...
quote:
much simplified & incomplete
yes, but we don't know what things are missing, and what things just aren't there.
anyway,  it is "interpretable", although maybe not intelligent.
any feature located wrt d is dependant on the actual location of d subject to whatever tolerance is allowed on the 3.000 width dimension.  so it has the effect of making the tolerance zones of such features an oblong, rather than a circle.

kenat,
how can a datum be confusing or odd?  firstly they are only theoretical, secondly they are points, lines and planes.  
i think you really meant 'datum features'.
please accept as it is intended.

as to the sketch,  i do not see a problem other than incompletness.  all that is in place appears to me to be legit per y14.5.
it seems legit to me also.  while they may appear odd as used, datums b and d do serve different purposes.
it is more important to locate the ?.500 hole relative to the edge and the top radius relative to the width, for whatever reason.
when the people fear their government, there is tyranny; when the government fears the people, there is liberty.fff"> - thomas jefferson

unusual but perfectly legit.
b is fine.  d is fine.  there is nothing that says one can't have both.  just can't use both at same time.
i agree that the presence of both b and d is unusual, but i don't see anything wrong with it.
i do have a problem with the position tolerance on the 2.000 radius though.  it's an example of a "partial feature of size" which is a classic pain in the cmm world.  the partial arc (in this case it's around 1/4 of a circle) has no opposed points anywhere, and is technically not a feature of size.  finding its actual "axis" is highly uncertain if the feature has any form error.  i am also highly suspicious of the mmc reference on the considered feature.  what function would justify letting the arc center drift further away from true position if the arc was a smaller radius?  
i've found that in most cases, partial arc features like this don't function as features of size.  they are just surfaces that happen to be nominally cylindrical.  in my opinion, they should be controlled using surface profile.
sorry for the off-topic rant - in past lives i spent many hours measuring the "size" and "location" of partial arc features and arguing over the results.
evan janeshewski
axymetrix quality engineering inc.