Hole Pattern Position With Central Hole as Datum 1

[SPRO]

Hello everyone,

I am fairly new to GD&T and am trying to get some practice in before staring a course in a few weeks. I have what seems to be a fairly basic question but can't seem to find a specific answer for it.

What I am trying to do is place a hole directly in the centre of a plate, and position a hole pattern relative to it. Figures 7-4 and 7-5 come the closest in ASME Y14.5-2009 seem to be the closest to what I'm looking for.

Instead of controlling the position of the 4 holes to the center plane datums B & C, I care more about positioning the holes relative to the central hole. Here is what I've come up with:

I have 2 questions:
[ol 1]
[li]Do I need the circled basic dimensions, or is having the hole-to-hole basic dimension with the centerlines enough?[/li]
[li]Are the datums I've used for the 4 x 0.250 holes okay? Do I need A? Do I need to add either B or C?[/li]
[/ol]

I appreciate any help!

 

[chez311]

1. The centerlines imply the the holes are symmetric on either side so it is not necessary, but does not hurt.
2. Use of A is not necessary with D as primary as it adds nothing to the tolerance zone definition, but may be included if it reflects functional/assembly requirements or is desired for simultaneous requirements with another feature/DRF.

Note that with |D| or |D|A| there is no rotational constraint around D. Since presumably its not okay for the holes to be in any orientation relative to B or C then use of either one, or some other clocking feature, would be recommended. Ideally you would want to again mimic how the part functions/assembles.

 

[SPRO]

Great, thank you for the great feedback!

 

[pmarc]

If D is controlled relative to A|B|C (in other words, A is of higher order of precedence than D), then D being a higher order of precedence datum than A in the position tolerance for the hole pattern looks suspiciously (at least for this kind of part).