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We have a part which has 2 openings where 2 clips will go and snap into semicircles marked in red. Centers of those 2 clips (or red semicircles) will be our 2 main datums. What is the correct GD&T drawing callout to show those 2 semicircles as datums?
A radius such as that is technically not a "feature of size," so the only option may be datum targets.
[QUOTE=Belanger;11829]A radius such as that is technically not a "feature of size," so the only option may be datum targets.[/QUOTE]
I would concur with Belanger, these features are not true features of size, so that would eliminate using anything other than a profile tolerance, with datum targets on the surface. In doing this, you would create only a datum point at each location, assuming that these are thin parts (3 datum targets), if it has some depth to it, then you could create a datum axis with 6 points. You could create a co-datum with either the points or the axes for datum references.
Since clips are being installed in this location, and with such small features, it makes me wonder if these features are the most stable features to be datum's. Understand that I say this without knowing the function of that part, but I wonder. Is your intention of referencing datum's on other feature's of the part? Or are you just going to reference these features to each other?
Not trying to be nosey, but want to help you out.
Here is a little more info. Part is plastic, overall dimensions as shown, approx. 3 mm thick. We need to order checking fixture but all we have is CAD file and 2D drawing which does not have any positional callouts so it is useless for gauge maker. All the 2D drawing has are overall dimensions but customer still wants locations of B, C, D and E checked on the fixture. B and C are main datums, B is 6.0 +/- 0.5, C is 6.0 +/- 0.1. D is only specified in Y as 5.0 +/- 0.1 and E is diameter of 5.0 +/- 0.5. Distances between B and C are approx 270 mm in X and 110 mm in Y, D and E are as shown and are almost in line with B (10 mm difference in Y). We need to decide without customer what the position tolerances will be and that's what I need help with so they are not too tight and not too big.
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Check fixture will have MMC + gage tolerance pins or features located at nominal distance dimension. These features will be designed to translate +/- X and Y relative to reference feature agreed upon and defined by the engineering drawing.
LMC - gage tolerance no-go gages will also be incorporated to verify dimensional conformance.