[en...] Posted December 21, 2018 Share Posted December 21, 2018 hi guys, when measure the linear dimension 60.25+-0.2, which plane should be selected as the datum? left or right? why ? thank you Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/ Share on other sites More sharing options...
[Ja...] Posted December 21, 2018 Share Posted December 21, 2018 I would assume that you would use the plane on the left of the drawing. It appears that you would be able to take more points on it, getting a more accurate representation of your datum. I may be wrong and I'm sure some of the guys on here will have better reasoning either way. Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/#findComment-110322 Share on other sites More sharing options...
[Er...] Posted December 21, 2018 Share Posted December 21, 2018 Wich plane? Both Why? If it's a linear distance you should actually measure in both directions, cause that's what the standard says. But if you go by the standard it takes 30 minutes or so to create all you need for one distance. So go for the one where you can collect the best data. The tolerance isn't very tight. Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/#findComment-110324 Share on other sites More sharing options...
[Aa...] Posted December 21, 2018 Share Posted December 21, 2018 Please sign in to view this quote. Not to be argumentative, but what standard says that? I've never read or heard that anywhere. Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/#findComment-110326 Share on other sites More sharing options...
[Er...] Posted December 21, 2018 Share Posted December 21, 2018 Terminology error. Should not have said "linear distance". Should have said "local size" or "when only a tolerance of size is applied". Or how ever you describe it in English. If my memory doesn't fail me I think it's in iso 14405-something. But I know you know you standards so feel free to correct me 😃 Or explain it your self, I never get my message thru so it's understandable. 🙄 When I look at the picture again I see SC, is that a modifier? Similar to SCS? Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/#findComment-110328 Share on other sites More sharing options...
[Da...] Posted December 21, 2018 Share Posted December 21, 2018 SC could be Significant Characteristic Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/#findComment-110330 Share on other sites More sharing options...
[Aa...] Posted December 21, 2018 Share Posted December 21, 2018 That's just it, Eric. When it comes to ISO standards, I don't know them as well as I'd like. That's why I asked the question. I haven't seen 14405-1, but I know 14405-2 is about dimensions other than linear sizes. It's basically a long-winded explanation for why limit dimensions are ambiguous in such cases (reference the OP's original question) and suggestions for how to use geometric tolerancing to avoid such ambiguity. That's why I was surprised when you said the standard said to measure both ways. Have a Merry Christmas! Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/#findComment-110332 Share on other sites More sharing options...
[en...] Posted December 24, 2018 Author Share Posted December 24, 2018 ASME 14.5 said that the longer surface could have been selected as the origin so what do you say ? Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/#findComment-110334 Share on other sites More sharing options...
[Da...] Posted December 24, 2018 Share Posted December 24, 2018 I don't get it guys. Where do you see any standard shown on the picture? @enhui li aren't there somewhere A and B at least on the drawing? Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/#findComment-110336 Share on other sites More sharing options...
[en...] Posted December 24, 2018 Author Share Posted December 24, 2018 Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/#findComment-110338 Share on other sites More sharing options...
[Aa...] Posted December 26, 2018 Share Posted December 26, 2018 Please sign in to view this quote. Nowhere--but then, we were only shown a small section of the print. On the other hand, using commas for decimal points strongly suggests the use of ISO instead of ASME as the standard. Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/#findComment-110340 Share on other sites More sharing options...
[Aa...] Posted December 26, 2018 Share Posted December 26, 2018 🤣 If you're in ASME, then it looks like you found the answer to your own question. I would like to draw attention to the last sentence of the note included below figure 2-5. "Locating features using directly toleranced dimensions is not recommended." Sounds like they're on the same page as the committee (or sub-committee) that put together 14405-2. Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/#findComment-110342 Share on other sites More sharing options...
[Me...] Posted December 27, 2018 Share Posted December 27, 2018 How exactly do we know what is ISO or ASME? I see the ASME Y14.5 note consistently. I do not believe I've ever come across an ISO note in the Title Block or on the drawing before... Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/#findComment-110344 Share on other sites More sharing options...
[Aa...] Posted December 28, 2018 Share Posted December 28, 2018 Please sign in to view this quote. Must be nice... ISO has an "Invocation Principle" in 8015, which says if any part of the ISO Geometric Product Specification (GPS) system is invoked on the print, all parts are invoked. Thus ensuring they can sell more standards. Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/#findComment-110346 Share on other sites More sharing options...
[Me...] Posted December 28, 2018 Share Posted December 28, 2018 ***Never noticed the note if there was one. I at least know to look now lol. I looked through my current company's drawing history and every drawing I've looked at so far is ASME. Please sign in to view this quote. Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/#findComment-110348 Share on other sites More sharing options...
[Er...] Posted January 2, 2019 Share Posted January 2, 2019 Well, in asme it seems to be the "envelope principle". But a picture says more then my words. This is what I was trying to say with "both directions". At least this is how I interprets it. But I love to be corrected if wrong. Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/#findComment-110350 Share on other sites More sharing options...
[Aa...] Posted January 2, 2019 Share Posted January 2, 2019 Ah, yes, the Envelope Principle applies to regular features of size, which, from what I can see of the OP's print, is not what is under discussion. (The two planes are opposing in direction, but it would appear that nowhere along the extent of the midplane are there two opposing surface points.) Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/#findComment-110352 Share on other sites More sharing options...
[Er...] Posted January 4, 2019 Share Posted January 4, 2019 Yes I know that's not what under discussion. But since it's a ambiguous I still withstand my statement that both surfaces should be datums. You can somewhat act as a functional gauge by measuring Cartesian distance and parallelism. With both the surfaces as datums, respectively. Or a true position.trpo.pngtrpo.png Link to comment https://qualityforum.zeiss.com/topic/19620-which-plane-should-be-selected-as-the-datum/#findComment-110354 Share on other sites More sharing options...
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