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Best fit profile of a surface

Best fit profile of a surface

Best fit profile of a surface

I have a question on the meaning of a profile symbol with no feature control frame,  does this mean best fit? The drawing has multiple profile specs on it all with control frames attached, but has one with no control frame just the tolerance and a leader coming out from the bottom of the profile box
calling out this profile as datum -A-
        Thanks for the Help

RE: Best fit profile of a surface

I am a little confused by the question.  You say there is no control frame but then you say there is a leader coming from the box.  The box is the feature contol frame.  Do you mean there isn't a datum reference?  It is common, where two or more flat surfaces shown in-line on the drawing are to be used as a datum feature, to apply a profile of a surface tolerance to keep the surfaces coplanar and then to callout the pattern of surfaces as a datum feature to establish a datum plane.  Might this be the case?
In any case, profile of a surface is a total tolerance meaning all points of the considered feature(s) must fit inside the profile tolerance zone. You might check out the Tip found at:  http://www.tec-ease.com/tips/july-98.htm

RE: Best fit profile of a surface

In addition to what Coach posted, a surface profile without a datum reference can also be a form control, though that is not well understood/appreciated by most.  Which form control it indicates depends on the nominal shape of the surface (circular indicates circularity or cylindricity; flat indicates straightness or flatness), and may therefore be used as a form control for a datum surface.  Is it possible to post a graphic for us to see?

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

RE: Best fit profile of a surface

Thanks for the help GDT.  Yes my error there are no datum references and it is on 1 continus surface.  

RE: Best fit profile of a surface

Terry, I've posted your graphic & an explanation on my site; use the following link to view it.


In this case, without the datum reference, the FCF prescribes a tolerance zone symmetrically offset from the BASIC (ideal or theoretical design) surface, without respect to orientation wrt any datums.  

Per ASME Y14.5M-1994, - Mathematically Defined Surface; It is sometimes necessary to identify a compound curve or a contoured surface as a datum feature.  Such a feature can be used as a datum feature only when it can be mathematically defined and can be related to a three-plane datum reference frame.  In such cases, the theoretically true geometric counterpart of the shape is used to establish the datum.
In the explanation, I've elected to orient datum-A so that datum-B (targets) can be easily established horizontally; it could have been oriented otherwise wrt the inspection table as preferred.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

RE: Best fit profile of a surface

Good explanation, MechNorth.  I've had to "re-interpret" many customer drawings that use a profile surface as the primary datum.  First thing we do is to establish our 3 primary datums, then refer their datum to those.  They don't ask any more why their primary datum is our datum D.

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