True Position Formula

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

  • True Position Formula

    Some discussion here about how to calculate t/p using 3 datums.

    Any and all help is appreciated.

    thanks
    sigpic
    Screw It. Let's Ride

    2011 MR1 Cad ++

  • #2
    This site has some good examples.
    When in doubt, post code. A second set of eyes might see something you missed.
    sigpic

    Comment


    • #3
      Thanks for the quick reply CJ but IT has us locked out of most of the internet. I can't follow the link.
      sigpic
      Screw It. Let's Ride

      2011 MR1 Cad ++

      Comment


      • #4
        Maybe an example?
        The site that John pointed out is really good, you must get there

        Comment


        • #5
          I believe I got this information from Matt some time ago but the actual formulas are:
          x(dev)^2+y(dev)^2+z(dev)^2 or (x(dev)*x(vector))^2+(y(dev)*y(vector))^2+(z(dev)* z(vector)^2)
          sigpic
          Xcel 15-20-10 - PFXcel 7-6-5 - Merlin 11-11-7 - Romer Absolute 7525SI
          PCDMIS 2012
          Windows Office XP

          Comment


          • #6
            You forgot the sqrt of the formula

            SQRT(xdev^2+ydev^2+zdev^2)

            This will give the TP in 3 axis, but not TP using 3 'datums', which was the original question, and if the datums are with MMC, then you will need about 17 pages to print out all the math!
            sigpic
            Originally posted by AndersI
            I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.

            Comment


            • #7
              Ooops! Sorry to misquote you Matt. I thought I was missing something there.
              sigpic
              Xcel 15-20-10 - PFXcel 7-6-5 - Merlin 11-11-7 - Romer Absolute 7525SI
              PCDMIS 2012
              Windows Office XP

              Comment


              • #8
                The basic formula for True Position is 2 x the square root of the deviation in one axis squared + the deviation in the other axis squared.

                True Position = 2√ Deviation in X² + Deviation in Y²

                It's hard to convey a good example of the formula with the font limitation of this forum. However, the X & Y axis’ are for a feature measured in the Z axis plane. If you measure in the X axis plane, you would substitute the X² with Z². If you measure in the Y axis you would substitute the Y² with Z².

                Subsequently, if there is a MMC modifier in the call out you would be allowed bonus tolerance.
                Marty Dobson
                Director of Quality
                http://www.linkedin.com/in/mjdobson

                PC-DMIS CAD V4.2 MR1 & V2011
                PH10MQ
                TP200

                Comment


                • #9
                  On the wilcox site there is a document written by Rob Jensen about true position and others.
                  I used to be high on life but I built up a tolerance.

                  Brown & Sharpe Global Advantage
                  PCDMIS CAD++ v2011mr2
                  PH10MQ/SP600M


                  sigpic

                  Comment


                  • #10
                    As long as you do NOT have any modifiers (meaning the tolerance, as well as the datums are at RFS (Regardless of Feataure Size)), you can do the math by hand. Even a modifier on the tolerance is still quite easy to calculate.

                    The moment you at a modifier on the datums though, things get complicated immediately. I agree with Mat, it can take pages to figure it out by hand.

                    I have done many experiements using modifiers on datums using PC-DMIS V4.1, comparing it to hand estimates. Generally, I think that they calculate it correctly. You have to use the new Xactmeasure GD&T to get it right. I believe it does not work using the legacy dimensioning.


                    Jan.
                    ***************************
                    PC-DMIS/NC 2010MR3; 15 December 2010; running on 18 machine tools.
                    Romer Infinite; PC-DMIS 2010 MR3; 15 December 2010.

                    Comment


                    • #11
                      Originally posted by mdobb829 View Post
                      The basic formula for True Position is 2 x the square root of the deviation in one axis squared + the deviation in the other axis squared.

                      True Position = 2√ Deviation in X² + Deviation in Y²

                      It's hard to convey a good example of the formula with the font limitation of this forum. However, the X & Y axis’ are for a feature measured in the Z axis plane. If you measure in the X axis plane, you would substitute the X² with Z². If you measure in the Y axis you would substitute the Y² with Z².

                      Subsequently, if there is a MMC modifier in the call out you would be allowed bonus tolerance.
                      This is a good 2 dimensional true position calculator: http://www.efunda.com/DesignStandard...ion_single.gif
                      sigpic
                      Xcel 15-20-10 - PFXcel 7-6-5 - Merlin 11-11-7 - Romer Absolute 7525SI
                      PCDMIS 2012
                      Windows Office XP

                      Comment


                      • #12
                        Thanks for all the good replies guys. I screwed my knee up last Thursday and just got back to work today.

                        Wish I could post a pic but can't. I'll try it this way:

                        the call out is: t/p dia. .015 to f d & e
                        We make landing gear for large airplanes so everything is round and cyclindrical(sp).

                        The part is shaped like a cyclinder, open on one end (the face on that end happens to be datum -e-), closed on the other end with a lug w/an approx. 4.5" hole running perpendicular to the cyclinder. Clear as mud?

                        The C/L of the cyclinder is datum -d- with the face of the lug being designated as datum -f-. All datums are listed as RFS.

                        It was my understanding that you simply added the third datum to the equation as Matt has shown. My partner in crime here on the 2nd shift was under the impression that there was a lot more to it than that.

                        Thanks again. I can always count on you guys for the help.
                        sigpic
                        Screw It. Let's Ride

                        2011 MR1 Cad ++

                        Comment


                        • #13
                          Originally posted by rangerboat72 View Post
                          This is a good 2 dimensional true position calculator: http://www.efunda.com/DesignStandard...ion_single.gif
                          I have had a need for this many times. It makes life simplies.
                          sigpicSummer Time. Gotta Love it!

                          Comment


                          • #14
                            This is an excel file with a tp calc also, downloaded from:
                            http://www.geomeasure.com/documents.htm

                            HTH
                            Attached Files
                            Global S Blue 12.15.10 HH-AS-T2.5 / HP-S-X1h |PC-DMIS CAD++|Quindos7 CAD; Leitz Reference Xi 15.9.7

                            Comment


                            • #15
                              Ese leon...! Do you have the formula for True position with polar coordinates?

                              Comment

                              Related Topics

                              Collapse

                              • Guest's Avatar
                                True position Formula
                                by Guest
                                Can anyone tell me what the formula is for True Position? For whatever reason this engineer thinks that the true position should have a bilateral tolerance....
                                06-12-2008, 07:22 PM
                              • j.gonzalez
                                true position on angle
                                by j.gonzalez
                                so i have a sketch of what im doing i need to find true position in x direction from my datum(1.849 dia) need help keeps giving bad numbers checking manually...
                                06-04-2009, 07:43 PM
                              • Ken8282
                                True Position
                                by Ken8282
                                I recently did a true position and may 321 alignment to the datums and picked the feature to be true positioned but I get a smaller TP than if I do a...
                                04-30-2018, 03:38 PM
                              • G3CLB
                                True position problems
                                by G3CLB
                                Ok so I am figuring out a true position on a hole and its asking for max on the on the true position and on both the datum's . the problem I am having...
                                02-04-2016, 11:26 AM
                              • RSNICKER
                                True Position
                                by RSNICKER
                                Why is it that I get different numbers when I report true position in the legacy dimensions than what I get in the new true position report and if so...
                                01-23-2009, 01:41 PM
                              Working...
                              X