【转帖】does a centerline datum follow a deflected shape

yang686526

does a centerline datum follow a deflected shape?
for long slender rolls, according to asme y14.5, does the centerline "datum" follow the natural deflection of the roll due to gravity or is it a theroretical straight line?  for example, in a free state a 2" diameter roll 40" long sags about 2-3 thou under its own weight.  a cylindricity req'mt on the drawing of say .001" will be violated by the sag itself unless the centerline datum goes with the shape.  any insight?
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it is a theroretical straight line, if you intend it to be a straight line. if the part sags beyond it, you need to either redesign or ... if ok change the tolerance to show it.
chris
systems analyst, i.s.
solidworks/pdmworks 05
autocad 05
castermaster,
   your term "centerline datum" makes me nervous.  you cannot apply a datum to a centreline.  you must apply it to a feature.  this feature may be symmetrical, implying a centreline.
   section 6.8 of asme y14.5m-1994 discusses the specification, fixturing and inspection of flexible parts.  in general, you can state how your part is to be clamped in place before your datum is inspected for.
   in the case of your long, slender roll, you might want to specify that your fixture is a 40" v-block.  defining your outside diameter as a datum would implicitly specify this, but an additional note would make things clearer.
   alternately, you could define a pair of datum targets for small v-blocks, and account for the resulting sag in your tolerances.  i do not know why you would want to do this.  i am merely noting the possibility.
                          jhg
thanks drawoh and ctopher ...
you're right, "centerline datum" is misleading, i meant it to be based on a surface feature (diameter), poor choice of words perhaps.  also thanks for the reference to section 6.8 ... i missed that one previously and it's helpful.
cd
a cylindricity requirement has no datum reference. runout requirements, circular or total, do have datum references. a datum axis is a theoretical line that is perfectly straight. you could call out a circularity requirement, which has no datum reference, and a straightness, which has no datum reference. you also could call out straightness over a given length. something like: circularity of .001, straightness of .008 overall and .001/12". this would control the shape of the bar and allow the sag over it's entire length free-state.