【转帖】cylinder surface as a datum

yang686526

cylinder surface as a datum
i have an old drawing showing a cylinder in profile and specifying the side of the cylinder as a datum. that datum is referenced by another cylinder elsewhere in the weldment, making a side of that cylinder parallel to the datum.
is this an allowable call out? i'm thinking the datum and feature control should be referencing the axis, not the sides.
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you are correct that the datum is referencing the axis.  the location of the callout is also a correct one, if it is simply pointing to or connected to the profile of the cylinder.
cwdaniel,
   your datum must be a real feature that can be fixtured to.  with asme y14.5m-1994, a datum on an axis is a mistake.
   your drawing probably is correct.
   apparently, an iso standard somewhere allows you to place datums on axes, but i have not seen it explained.  i have no idea what it would mean.
                   jhg
these cylindracal features are actually tubes so i'd say the id of a tube can be fixtured to if it's important.
can't holes or other id's be used as a datum and that applies to the axis of the feature?
in this case though the outside is probably more useful for fixturing.
yes
one can have one side of a cylindrical feature as a datum but that is unusual. if the designer really wanted this, then there would be an end view with a phantom line tangent to the diameter and the datum designation symbol attached.
i belive that your designer wanted the c/l of the cylindrical feature and should have reflected this a bit differently.
holes and other ids are used as datums and should be. it depends upon how the part assembles.
if the part assembles with bolts protruding throught some holes, then one of the holes is the secondary datum and all dimensions come from that hole. there would be a 3rd datum or teriary that is used to orient (rotate) the part.
dave d.
while it may be poor practice, i don't think the design intent was to designate one side of the cylinder as a datum, but was used as a shortcut rather than tying the datum to a diameter dimension.  dimensions are sometimes (again not preferred) placed such that the dimension lines are within the part boundary, which would be a similar case.
i realize i am looking at an older book, but i rely on my drafting book from college which is engineering design graphics, seventh ed., by james h. earle.  it states on page 358 that the "cylindrical datum feature" is the axis of a true cylinder.  in other words, two of the three required planes are represented as being the perpindicular centerlines intersecting at the axis of the shaft.  this should be noted as a datum surface symbol in the circular view of the shaft. this practice may have changed with newer standards.
as explained to me recently during a course on bs8888 gd&t, you can only use the following as datums...
points, edges, axes' and surfaces...
any part of a surface of a cylinder would be classed as a profile and would not qualify as a datum.
the axis of the cylinder is the datum and the other cylinder on the weldment should be referenced to it. the form of the cylinders should be controlled by additional tolerances such as cylindricity and run out etc...
hyd'
draftsmandon,
time to get the actual standard.  the datum may be the axis of a cylinder, but the datum feature is the cylinder itself.  as far a locating the datum callout, that is more a matter of preference, as long as it does not violate y14.5.  it is just as correct to link the callout to a cylindrical dimension in a side view as it is to a circular dimension in an end view (of a cylinder).
the best way to think of this is, where would you dimension the feature from? if you wouldn't attach the dimension to the centerline, you wouldn't attach the datum identifier to the centerline. the centerline is the datum but in order to make it a datum you must have a verifiable feature to measure from, hence attaching the datum indentifier to either the extension line of a dimension, a diameter callout or the edge of the feature.
read section 4 of the asme y14.5m-1994 standard.
david