[Me...] Posted December 13, 2019 Share Posted December 13, 2019 We have a particular customer that consistently does not allow MMC for True Position on a toleranced bore/shaft either by negligence or mistake...Funny thing is they often call out MMB for the datums within the same True Position dimension... I know we 100% have to follow the print but it drives me nuts, specifically with TP callouts of .001 (inches) with no MMC on a ± .0005 diameter. In what cases does this make any sense if it is done on purpose? Link to comment Share on other sites More sharing options...
[Br...] Posted December 13, 2019 Share Posted December 13, 2019 All of these cases might be a good application for using MMB without MMC on the datum features geometric control. 1. 2. 3. Link to comment Share on other sites More sharing options...
[Me...] Posted December 13, 2019 Author Share Posted December 13, 2019 Please sign in to view this quote. I'm not seeing any of the cases bud. Link to comment Share on other sites More sharing options...
[Er...] Posted December 13, 2019 Share Posted December 13, 2019 There is cases where this is logical. What if your holes are really close to an edge, and you really need to make sure that the wall thinkness is maintained. Then MMC isn't the beast of ideás.... So it can make sense..... Link to comment Share on other sites More sharing options...
[Ja...] Posted December 13, 2019 Share Posted December 13, 2019 Please sign in to view this quote. Wouldn't you utilize LMC if you were trying to preserve wall thickness? Link to comment Share on other sites More sharing options...
[Br...] Posted December 13, 2019 Share Posted December 13, 2019 Please sign in to view this quote. Now you're catching on 😉 Link to comment Share on other sites More sharing options...
[Br...] Posted December 13, 2019 Share Posted December 13, 2019 Please sign in to view this quote. Exactly Link to comment Share on other sites More sharing options...
[Me...] Posted December 13, 2019 Author Share Posted December 13, 2019 I found case #2 to be quite compelling. Thanks for the info, Brett! Link to comment Share on other sites More sharing options...
[Ja...] Posted December 13, 2019 Share Posted December 13, 2019 Please sign in to view this quote. Shawn, Are you referring to MMC not on the feature but it (MMB) is on the datums? Ex: [Ø .002|A|B(M)|C(M)] but you'd rather see [Ø .002 (M)|A|B(M)|C(M)]? Or are you referring to a FCF called out using a datum at MMB but the datum feature itself is dimensioned not at MMC? Ex: [Ø .002|A|B(M)|C(M)] where datum B is a cylindrical shank dimensioned as [Straightness|.010] VS [Straightness|.010 (M)]? Link to comment Share on other sites More sharing options...
[Me...] Posted December 13, 2019 Author Share Posted December 13, 2019 The first example. Link to comment Share on other sites More sharing options...
[Er...] Posted December 13, 2019 Share Posted December 13, 2019 Please sign in to view this quote. Not neccisarly. You could, but you dont have to. So still, there are cases where it makes sense. Link to comment Share on other sites More sharing options...
[Br...] Posted December 13, 2019 Share Posted December 13, 2019 Please sign in to view this quote. I may actually have misunderstood Shawn's question then. I thought Shawn was talking about the 2nd example- Where the datum with MMB applied doesn't utilize any MMC in its own geometric tolerance. (That's what i get for being facetious.) One example would be for press fit holes. They apply at RFS because there is no clearance. No matter what size they are made, the pin will center itself in the hole, hence "Regardless of feature size", but the pattern of pins could still exist on a part where the datum features they reference allow for Datum Shift. Link to comment Share on other sites More sharing options...
[An...] Posted December 13, 2019 Share Posted December 13, 2019 Please sign in to view this quote. That's right. See attached. ___Contribution_14_12_2019.pdf Link to comment Share on other sites More sharing options...
[SH...] Posted December 15, 2019 Share Posted December 15, 2019 Please sign in to view this quote. I think for controlling the true position tolerance product designer deliberately element the MMC, in addition to that he is emphasising on datum shift. Link to comment Share on other sites More sharing options...
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