True Position of Datums

[TAC45]

I have a Datum -C- (Slot Feature) with a true position callout of .4|A|B. The part is basically a flat sheet metal part with -A- being a plane and -B- a circle. What is the best method to dimension this as the datum location should be theoretically perfect as established in the alignment correct? The datum -B- circle also has a position callout of .1 to -A-. Why would the -A- have a position tolerance when it resides on the -A- plane? Thanks.

[JacobCheverie]

If the hole (circle) is perpendicular to the plane, the Position callout on B to A doesn't make much sense. It should be a cylinder, albeit short, called out perpendicular to A.

Datum C position to A|B - Measure datum A, Level Z+ to A, measure datum B, measure datum C, construct a line from B to C, rotate X or Y to the constructed line, set X&Y origin to B, Z origin to A. Output the position.

[VinniUSMC]

TP to A (if A is a plane) is perpendicularity (if the feature is nominally perp to A). They're completely interchangeable. Changing the callout is unnecessary, and probably better to be TP as TP is a much stronger and more versatile feature control.

[Wes Cisco]

Best practice is to control the datum features to the extent possible. As they are the foundation of the DRF, the amount of control possible increases as the DRF is constructed.

For the primary datum feature the only control possible is form. In most cases, flatness for a plan or cylindricity for a cylinder.

For the secondary datum feature, it is usually possible to control the orientation relative to the primary, and in doing so, one also controls the form of the secondary datum feature.

For the tertiary datum feature it is usually possible to control it's orientation, form, and location relative to the primary and secondary, with the caveat that the location will usually be a single axis control as the tertiary itself establishes the other axis, (as is the case with the plane-hole-slot DRF you describe.)

Position of the secondary to primary is not prohibited by ASME Y14.5, but the geometric reality is the only thing it controls is the orientation relative to the primary. The more proper and elegant choice for the designer here would be Perpendicularity rather than Position, but again, the resultant control is the same.

Finally the instructions JacobCheverie provided are spot on. The Position of C will be a single axis position, controlling the location of C relative to B and the orientation of C relative to A & B.

HTH

[VinniUSMC]

If the hole (circle) is perpendicular to the plane, the Position callout on B to A doesn't make much sense. It should be a cylinder, albeit short, called out perpendicular to A.

Datum C position to A|B - Measure datum A, Level Z+ to A, measure datum B, measure datum C, construct a line from B to C, rotate X or Y to the constructed line, set X&Y origin to B, Z origin to A. Output the position.

Yeah, I hate TP of datum B circle to datum A in sheet metal. It's just dumb.

For slot as datum C, there should be a callout for datum C being the opposed sides of the slot width, and a separate callout for the slot length (which usually has most tolerance). The TP of C to A|B of a slot in sheet metal is also dumb, because the slot is the origin, and it should be a single axis TP (perp to the line B to C). Then the slot length should have a TP to A|B|C, which allows the slot to drift along the line B-C.

[TAC45]

Thank you all for the feedback, I really appreciate it. As the material thickness for this part is .8 MM, the true position callout of -B- (Circle) to -A-(Plane) makes no sense to me.