Partial Datum - Area

dthom0425 · Nov 4, 2022

Hi all - see attached simplified model.

I have a plate with some clearance holes used for mounting. The datum reference frame will be A|B where A is two partial areas as detailed. Datum feature B will be the pattern of 2X holes and all other features that I redacted from this model will be profile to A|B(M). The edges of the plate mean nothing to me so I am opting to not do a 3-2-1 style set up.

My question is regarding the two partial areas. I understand that the partial area size is basic but how would I locate the partial areas given my set up above? I assume there should be a basic location between the two partial areas (is that correct?) but then what sets the initial location of the partial areas?

Thanks for the help

pmarc · Nov 4, 2022

dthom0425,

I assume the definition is going to have a basic dimension between the centers of the two holes. This basic dimension will also indicate how far the two square areas must be from each other. Nothing else needs to be defined.

Burunduk · Nov 4, 2022

EDIT: Initial thought - Since you need to somehow relate the location of the holes used as datum feature B to the limited areas defined as datum feature A, you will need datum references that can impose the constraints needed to locate the 'B' holes and provide a basis for the location of the 'A' areas in the relevant directions. Even though you don't want to use the edges as datum references for the other controls, You could specify datum targets C and D on the edges, then relate the areas by basic dimensions to them acc. to the same principle as in fig. 9-16 attached below. Then also control datum feature B (the 2X holes) by position (preferably at MMC) with ref. to A, C, D. The other features can be controlled with ref. to A B(M) as you want.

Second thought, based on pmarc's comment - the most reasonable interpretation of Y14.5 is that the center of each cylinder of datum feature B's true geometric counterpart has to be centered to each planar area in datum feature A's true geometric counterpart. Therefore, the datum targets may be unnecessary.

3DDave · Nov 4, 2022

They do not fall under the definition of partial surface as a datum feature. They are datum target areas.

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Partial Datum - Area

dthom0425

Mechanical

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pmarc

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Burunduk

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3DDave

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