Multi Datums

[SeasonLee]

Actually, this thread arose from my earlier thread “Questions on drawing reading”, I would rather to start a new thread and focus on the specific topic “multi datum” here.

Everyone think it’s illegal to designate two different datum X and L(or Y and M) on the same feature on my earlier thread, but after further reading at Geo-Metrics III (by Lowell W. Foster), it states that the multi use datum targets is allowable, so I am thinking : Is it legal on the shackle drawing to have datum X and L (or Y and M) at same time, however I can’t find out this information on the standard Y14.5M, expect someone can give us more detailed information.

There are three pages on the attached file, 1st page is shackle drawing while 2nd and 3rd are quoted from the textbook “Geo-Metrics III” page 264 and 265

Thanks for your inputs

SeasonLee

 

[ringster]

I thinketh that you could possibly define an infinite number of datum points on a part. Then you could selectively use them as design requirements dictate. But to me it would mke no sense whatsoever to use the very same point, (or area) with a different identification.

 

[dingy2]

SeasonLee:

On your drawing, I noticed datum targets X1 and Y1 but I could not find any datum X or datum Y. I did find points X and Y but there are not datums. Datum targets reflect the set up on the datum so could one have a datum target X1 and Y1 when there are no datums X and Y?? I don't think so.

I reviewed the drawing from Lowell Foster but his workbook should also follow the ASME standard. While I did learn GD&T in the mid 80s from his books and still use his philosophy today, I don't know where he is basing the example.

Check out pages 74 & 75 of the ASMEY14.5M-94 standard.



Dave D.

http://www.qmsi.ca

 

[SeasonLee]

Dave

Thanks for the reply. Please note the profile callout
|profile|.006|X|L-M|N| X is primary datum
|profile|.006|Y|L-M|N| Y is primary datum
So, X and Y here is not just a datum targets.

SeasonLee

 

[dingy2]

Yes you have a feature control frame showing X is a primary datum and also another one where Y is a primary datum but you do not have datums X & Y. The drawing does show datum L, M and N but that is it.

Maybe my eyesight is getting weak, so please tell me which quadrant X & & are shown as a datum with the datum symbol? If they are really there and I can't see it, I will get my eyes tested.

A datum target does not reflect a datum except in cases where there isn't a datum feature. We do have many, many features on this drawing.

Dave D.

http://www.qmsi.ca

 

[KENAT]

SeasonLee, I raised several concerns over the profile call outs themselves in your previous thread but no one seemed to pick up on them. I did not explicitly say it was illegal since the drawing has other issues which make it difficult to determine.

Being there is only a single datum point from which to generate datums X & Y then I'm struggling with how they can be a meaningful primary datum, though I suppose if there is a meaningfull secondary & tertiary frame you could develop it.

Also, X & Y aren't fully defined, it isn't shown how far along the diameters they are located. I don’t believe X or Y are the diameters.

Not clear what L-M is meant to be. Looked at 4.5.7 of ASME Y14.5M-1994 and am still not quite clear. I think it's meant to be the common plane through the two diameters, however I'm not sure how correct this is as usually I’d expect it to have L and M as separate boxes in the FCF more like figure 4-8/4-9. Unless this is a ‘pattern of features’ for a datum deal like 4.5.8 talks about, however in this case you end up with effectively 4 datum’s for the profile.

I’ve linked my understanding of the datum scheme they are trying to set up, though I’m not sure what they appear to be trying to do is correct.

http://files.engineering.com/downlo...a-9455-fb108ed5e373&file=SeasonLee-datums.tif

If my guess is correct then I’d have thought L-M would be the primary datum, N secondary and X or Y tertiary though.



KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[SeasonLee]

Dave and KENAT :

Thanks for your inputs, now I understood where is the problem on the drawing.

SeasonLee