Question on Equalizing Datums ASME Y14.5M-1994 section 4.6.6

[KENAT]

Question on equalizing datums for you, with reference to the attached sketch (I've embedded the image too - it's only 95k so hopefully won't kill anyone's download time etc.) and ASME Y14.5M-1994 (I don't have 2009 and we still work to -1994).

As shown in the bottom left view, I want the major & minor axis to be used to generate datums B & C - essentially getting center planes from the length & width. The ellipse shape is the interface at the assembly level as it fits in an ellipse cut out.

If it was a machined part I'd do it as bottom left view labelled 'conventional datum structure'.

However, the part is to be cast and the foundry we work with usually prefers datum targets which is typical for castings. However, due to the shape of the part and it's functional interface I think Equalizing Datums using V type equalizers make sense rather than simple datum targets.

Q. I was hoping someone with some experience in these matters might be able to tell me if I've got it right in View A-A. Is it OK to use the V-type equalizers on both ends? Figure 4-38 only has them at one end with conventional targets at the other - though of course "The absence of a figure illustrating the desired application is neither reason to assume inapplicability, nor basis for drawing rejection".

Also before any one brings up my labeling of the center lines as datums in this case I believe it's acceptable per the last sentences of 4.6.6:

It is permissible, in such a case, to use the datum feature symbol to identify the equalized theoretical planes of the datum reference frame. It should be noted however, that its is an exception, and is to be done only when necessary and in conjunction with datum targets.

Thanks,Ken.

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[SDETERS]

I would reconsider putting the datum "v" block on the edge of the casting as shown. I would put them .03"-.06" down from the edge. This way you have a nice surface to sit your v blocks on. Sometime on these edges you may have a gates or some flash and or die casting flaws.

 

[pmarc]

Kenat,
I see no major issues with view A-A. Only some small remarks:

- I would try not to forget about adding profile of surface callout wrt A to the ellipse contour (just like it was done on bottom left view);
- The view A-A alone does not clarify whether V-type targets are planes or lines;
- Datums B and C labelling -- I do not think that it really matters, but to me somehow more logical would be to switch them, i.e. to make vertical plane B and horizontal C.

 

[KENAT]

Thanks guys,

Interesting point on the flash/flaws. I'll discuss with the foundry but I don't think it will be an issue for this part depending where their riser is. I expect the mold split line will be along the C datum so any mismatch etc. will be away from where the V blocks touch.

On the right I was deliberately only showing the datum structure for clarity - yes I'd still need to dimension & tolerance the ellipse.

Per 4.6.6 "V-type planes may be indicated by only showing the lines in the top view" so if I don't show a height in another view they are assumed as planes correct?

Not that datum alphabetical order technically matters etc. but I was thinking the datum's made sense in the order I had them but I'm having trouble articulating my logic - care to expand on your thinking Pmarc?

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[SDETERS]

If that is the case on the die split line your "A" datum target points are going to be on a drafted surface. Which I assumed you needed to have flat per your sketch. Which, you are not showing this surface drafted in your sketch. So datum target A1 is going to be on the die split line? You will have some trim marks flash ECt. Just something to think about.

 

[KENAT]

Actually, the casting process our vendor uses allows 0° draft in many cases (Rubber Plaster Mold Casting) so I don't think draft will be an issue.

However, it occurs to me after your prompting that the parting line/mold half mismatch might be an issue.

In that case, could I make B or C my primary datum for later referencing, and change A to be a single datum point on the opposite side of the small ellipse (or similar) so as to allow face of what is currently datum A to be cleaned up by machining?

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[pmarc]

I am also having difficulties to express my point, but technically V-block B on view A-A constitutes two planes of datum reference frame - one passing through the center of V and second passing the apex of V and being perpendicular to the first. In order to contrain rotation of the planes better you use datum target C. When you do that, you are allowed to move horizonal datum plane (or even the vertical) anywhere with the use of basic dimensions. In your case, you set the horizontal datum plane in the middle of B and C V-blocks, thus my "logic" is -- vertical datum plane established by V-block B stays unchanged, so let it be B, horizontal datum plane is influenced by V-block C, so let it be C. Does it hold water?

BTW, I actually do not know which datum target V-block is secondary and which tertiary. There is no feature control frame on the drawing specifying datum features order of precedence .

BTW2, yes, if you do not show datum target lines in another view, the targets shown on A-A are assumed to be planes.