【转帖】how to specify a cone as a datum

yang686526

how to specify a cone as a datum
how would i call out the centerline of a cone as a datum??
you don't call out the centerline of anything as a datum.  you use feature surfaces for datums.
attach the datum to the cone surface, and the centerline will be used as the reference for certain calouts (location, runout, concentricity).
well,
one way would be to estabish 3 datum points at 2 different elevations a and b on the cone.  these could then be used to establish axis a-b.  then you could use the surface at the base as c.  a-b would be the primary and c the secondary.  tiertary could be added if needed.
the tick,
   i've seen datum axises called out as the only datum for a part. you see this especially on shafts with concentricity and perpendicularity callouts in reference to the datum axis a. an example is shown in asme y14.5m-1994 5.12.2.
theedudenator,
    grab or draw the dimension of the smallest or largest cross-section of the cone, either end will do. insert your datum inline with the dimension arrows.
failure is a prerequisite of successful design
i. did not say there could only be one axis.  what i said was that to reference the axis of a feature, the feature's surface must be used as a datum.  it is incorrect to place a datum flag on the centerline of a feature.
fintomfff">:
in the figure you mention above, is the datum flag placed on a centerline or on a surface?
fintom,
that reference isn't using the "axis" or centerline as the datum. the datum is the cylinder which has a cylindrical tolerance zone about an axis, but the centerline is not in fact the datum. a datum must be an instpectable feature, centerlines do not qualify as features.
the rest of your advice however is sound.
david
could you add an arbitrary distance from the base of the cone to a "datum diameter" and then use that dia like you would a cylinder to est to perpendicular datum plans coincident w/ the axis?
the portion of my previous post that reads "to est to perpendicular datum plans" should read "to establish two perpendicual datum planes"...
rltwfff">:
yes, you can specify a specific distance and area to use for datum.  this is a datum target.
is that a datum target - even if it's really a zero area target? (i'm not being critical - i'm new to using gd&t beyond basic shop use...)
that would be a datum pt if area isn't specified.