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.
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.
FinTom:
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.
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?
Can you put bell-centers on the end of your shaft to use for centering for machining and measuring runout? I would recommend this over using a conic ecternal surface in a chuck.
I had a contract job designing gearboxes for machine tools. All gears and spindles got bell centers for consistent location.
This post is shining example of how Y14.5 still has a ways to go on the definitions and interpretations of datums. Real-life parts can't always rely on cylindrical holes for datums, as in this case with a cone. Greg Hetland if IIGDT (Intl Institute of GD&T) has a great handout showing what other features (cone included) may be used to establish datums. He may be willing to send you one? He's near Minneapolis, MN.
There are a number of ways to use a cone as a datum feature; how you call out the datum(s) depends on how you "chuck" the part in practice. For a mortise taper or tooling taper, you'd use the entire surface of the cone, or at least the portion that mates with the tool holder. Put the datum feature callout on the edge profile of the cone, and indicate the limits of the "zone" if needed, using basic dimensions. That's all you need to do.
Now, for the part that most people tend to miss. If the cone is used to generate the Primary Datum, it actually generates all three mutually perpendicular datum planes. The first two, as expected, are through the axis of the cone. The third is at the apex of the cone. The problem there is that this typically is not a real location, or one that is easily used. So, if you are using the cone to generate the Primary Datum, but you don't want to use the apex to generate that third plane, you will need to indicate that in a note.
Jim Sykes, P.Eng, GDTP-S
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