Near Flat Conical Surface as primary Datum

[RunningFree]

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.

 

[drawoh]

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

 

[TheTick]

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

 

[RunningFree]

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.

 

[PeterStock]

Attached is what I would do with the simplified model you posted. Given the need for a clocking feature as indicated by the 1/8 turn locking feature you mentioned in your last post, some changes would be needed.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net

 

[drawoh]

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