Profile with zero tolerance

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Profile with zero tolerance

    I have a new part thet I'm trying to program that has several profiles with a zero tolerance (ZERO TOL.jpg), but the have MMC on datums B & C. I use legacy dimensioning and would like to use variables to find the MMC to put into my profile dimension. HELP
    DeWain Hodge

  • #2
    I think.....

    MMC on B and C would be datum shift.

    i have no idea if PCDMIS can calculate that. Sorry.

    B&S CHAMELEON/PCDMIS CAD++ V2011

    There are no bugs, only "UNDOCUMENTED ENHANCEMENTS!"

    sigpic

    Comment


    • #3
      That is a very unusual callout.
      -B- and/or -C- will need to be somewhat undersize in order to allow any deviation at all on the surface that is dimensioned.
      Do -B- and -C- have a lot of tolerance?
      Lately, it occurs to me
      What a long, strange trip it's been.

      2017 R1 (Offline programming)

      Comment


      • #4
        Did you clarify with the engi-nerds?
        Eventhought modifiers are allowed they aren't recomended.
        Last edited by Roberto; 11-01-2014, 12:37 PM.
        sigpicIt's corona time!
        737 Xcel Cad++ v2009MR1....SE HABLA ESPAÑOL

        Comment


        • #5
          Sounds like human error. I would get a clarifaction of the print.
          sigpicSummer Time. Gotta Love it!

          Comment


          • #6
            Maybe a different approach.

            If you use all the profiles that reference this datum and perform a 3D best fit then check that datum -B- and -C- are still within their MMB. You could calculate the shift in the datum positions using variables.

            You will never show one of these parts as being in tolerance unless you have only one surface and you only measured a single point on the surface. According to the control frame the shape of the surface(s) must be perfect regardless of the bonus from the datum (the 'form' part of 'form and location').
            Last edited by Rondog; 11-01-2014, 08:47 AM.

            Comment


            • #7
              I would plug in whichever of the 2 datums has the smallest tolerance as my profile. At least that way, you are safe.
              I'm upping my standards........

              Up yours!

              Comment


              • #8
                Wish we could see a view from the print.

                Comment


                • #9
                  +.102/-0.00mm tolerance on the -B-/-C- diameters

                  Originally posted by John Riggins View Post
                  That is a very unusual callout.
                  -B- and/or -C- will need to be somewhat undersize in order to allow any deviation at all on the surface that is dimensioned.
                  Do -B- and -C- have a lot of tolerance?
                  DeWain Hodge

                  Comment


                  • #10
                    Could be that engineer needed 3 place decimal, and had parameters set for 2. As an example, .003 would be rounded to .00 (in short, a mistake)

                    Comment


                    • #11
                      Originally posted by DeWain Hodge View Post
                      I have a new part thet I'm trying to program that has several profiles with a zero tolerance ([ATTACH=CONFIG]9815[/ATTACH]), but the have MMC on datums B & C. I use legacy dimensioning and would like to use variables to find the MMC to put into my profile dimension. HELP
                      Originally posted by DeWain Hodge View Post
                      +.102/-0.00mm tolerance on the -B-/-C- diameters
                      Hi DeWain!

                      Step 1 is to send a flag up the chain asking:
                      If it's an in-house design, ask if the engineer has been under a lot of stress lately, or is fresh out of college, or thinks this is a space shuttle part.
                      If it's a customer design... do your company's people who quoted and bid on the job know what that callout means?
                      Maybe it is a space shuttle part.

                      Step 2 is to take a deep breath... and dive into XactMeasure mode because Legacy mode does NOT support material modifiers on datums correctly.
                      While you could screw around with variable alignment shifts, that has unpleasant side effects (like choosing between shifting nominals or dissociating completely from the CAD model) and even if it works - still will not work as well as XactMeasure to get the parts to pass. Save yourself 60 hours of headaches and just use XactMeasure mode.

                      MMC on datums means this: instead of getting "bonus tolerance" like MMC on inspected feature gives, instead we get to minimize the coordinate results of the measured data by shifting our measurement zero point from the center of the datum to a little bit over to one side. If we were checking the part with a hard gage, we would build the gage with datum posts sized to their minimum diameter - and then when checking a part with nominal or on-the-high-side sized holes, we could slide the smaller hard gage datum posts over to the side-wall limit of the bigger holes to get it to fit. It's called datum shift.

                      Here's a little get-started primer on XactMeasure:
                      In this mode, you need to Define the Datums. You can go Insert-> Dimension-> Datum Definition, or you can open your Profile dialog and there's a shortcut Datum Definition button in there. Either way each definition is a separate new line of code in the program.
                      Warning 1: avoid using Constructed Features as datums, there's a little bug we're working on.
                      Warning 2: never edit a datum letter if you've already made dimensions using it. Delete the dimension, delete the datum definition and re-create it (thanks to Wes Cisco who discovered this the hard way).

                      Next, simply build a replica of the callout. You pick the datums with little drop-down menus in the empty feature control frame.

                      Finally, click to the Advanced Tab. In there is a list of items, find the datums and enter their size limits there. This allows the datum shift calculations to work: for every bit that the datum is over it's minimum allowed size, you get that much data shift taken off the coordinate results of the data.

                      In this case, because both Secondary and Tertiary datums allow MMC datum shift, the software will essentially do a 2D best fit in the allowed confines of both possible shifts. There's where it beats any convoluted variable-shifting-alignment Legacy trickiness all day.

                      Good luck!

                      Comment

                      Related Topics

                      Collapse

                      Working...
                      X