Lofted Surfaces and datums referenced in FCF for Surface Profile

[RLD89]

I have seen some drawings for parts that have surface profile call outs of a lofted surface. The FCFs have profiles with no Datum references in the FCFs. It has been suggested that the datum references in the FCF of the profile are not required since, for surface profile of .020, the surface cannot be outside a +/-.010 zone from the nominal loft.
Profiles without a Datum reference only control form. Location, orientation and size are not controlled relative to the DRF without them. From a tolerance stack up perspective, most parts absolutely require that relation to the DRF.
Is there any way to reason that because a lofted, mathematically controlled surface never requires Datums to be referenced in the FCF? Please correct and/or comment and thanks.

 

[CheckerHater]

No and no.

It is wrong that mathematically controlled surface never requires datum, and it is wrong not to use datums just because the part has mathematically controlled surface.

It is possible however that with SOME parts you can get away with that.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[RLD89]

Thank you much for the response.

 

[axym]

RLD89,

I agree with CH. There is no relationship between mathematical control and datum feature references.

It is true that the surface cannot be outside a +/- .010 zone from the nominal loft. But without datum feature references nothing is constraining the part to the nominal loft, so location and orientation are not controlled. The only correction I would make is that surface profile is capable of controlling form and size (if the lofted feature has a "size" aspect).

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[RLD89]

Axym,
Thanks for the input. Profile of a surface does control location to the datums referenced. Correct?

 

[CheckerHater]

Not necessarily.

Imagine "part" like the one on the enclosed picture.

Profile will control form, location, orientation and size - all without using datum(s).

So, like I said, SOMEtimes SOME things are possible, but they don't make the rule. (i like to play devil's advocate)

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future