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Near Flat Conical Surface as primary Datum

Near Flat Conical Surface as primary Datum

Near Flat Conical Surface as primary Datum

(OP)
Please see attached drawing:

I have a part with a near flat conical surface (labeled as A), which I want to use as my primary datum (1° draft angle due to injection mold tool design), how do I go about this?

I know that a cone produces three mutually perpendicular planes but in this instance I only want the one, the same as if this feature was flat, I then want to use the other cylindrical feature (1° draft angle again) to produce the other two planes.

I know it seems simple enough but I have confused myself and any help would be gratefully appreciated.

Please note that the part is not the actual part but a simplified version to illustrate the principal.

Thank you,

Darren.
 

RE: Near Flat Conical Surface as primary Datum

RunningFree,

   Your datums are nothing more than fixturing points.  It is legal as per the standard to use the conical feature as a datum.  How to you plan to fixture to it?  Are you going to design inspection tools?

   How to you plan to inspect this thing?

               JHG

RE: Near Flat Conical Surface as primary Datum

I would use datum targets.  I have no idea what you will do.

RE: Near Flat Conical Surface as primary Datum

(OP)
Hello drawoh,

The part in question is a sealing cap similar to an engine oil filler cap, with an 1/8 turn locking action; the main feature I am worried about is the flatness (profile of a surface?) of the near flat conical surface as this is the seal mating face. The part will be made by a sub-contract injection molding company and produced in one operation.

Regarding checking fixtures I am not sure but I believe they have a CMM so I guess they would use this to verify the part, I'm not sure how they would mount it though.

The main reason I ask is using the near flat conical surface feature as a datum will in theory produce all three mutually perpendicular planes, but as the cone is near flat any slight variation in the form could throw the axis off by a fair amount. So I would like it to only set the plane and leave the cylindrical surface to set the axis. Does this make sense?

Thanks,

Darren.
 

RE: Near Flat Conical Surface as primary Datum

RunningFree,

   I would apply an accurate profile tolerance to your cone, and use something else as the datum.  This is a good case for a composite profile, since your form is critical, and your location to other features is less so.

   In the CMM, I would want to mount the part using my datum features, then inspect the cone.  The cone is a bad datum feature, since I would want unrestricted access to it.

   While desirable, it is not absolutely necessary for your datums to be orthogonal to each other.  Look at figure 4.4 in ASME Y14.5M-1994 (fig 4.7 in the 2009 version).  Look also at fig 4.39 (2009:fig 4.54).

   Your locking faces would make an excellent and appropriate set of datums.

               JHG

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